1.
Sphere
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A sphere is a perfectly round geometrical object in three-dimensional space that is the surface of a completely round ball. This distance r is the radius of the ball, and the point is the center of the mathematical ball. The longest straight line through the ball, connecting two points of the sphere, passes through the center and its length is twice the radius. While outside mathematics the terms sphere and ball are used interchangeably. The ball and the share the same radius, diameter. The surface area of a sphere is, A =4 π r 2, at any given radius r, the incremental volume equals the product of the surface area at radius r and the thickness of a shell, δ V ≈ A ⋅ δ r. The total volume is the summation of all volumes, V ≈ ∑ A ⋅ δ r. In the limit as δr approaches zero this equation becomes, V = ∫0 r A d r ′, substitute V,43 π r 3 = ∫0 r A d r ′. Differentiating both sides of equation with respect to r yields A as a function of r,4 π r 2 = A. Which is generally abbreviated as, A =4 π r 2, alternatively, the area element on the sphere is given in spherical coordinates by dA = r2 sin θ dθ dφ. In Cartesian coordinates, the element is d S = r r 2 − ∑ i ≠ k x i 2 ∏ i ≠ k d x i, ∀ k. For more generality, see area element, the total area can thus be obtained by integration, A = ∫02 π ∫0 π r 2 sin θ d θ d φ =4 π r 2. In three dimensions, the volume inside a sphere is derived to be V =43 π r 3 where r is the radius of the sphere, archimedes first derived this formula, which shows that the volume inside a sphere is 2/3 that of a circumscribed cylinder. In modern mathematics, this formula can be derived using integral calculus, at any given x, the incremental volume equals the product of the cross-sectional area of the disk at x and its thickness, δ V ≈ π y 2 ⋅ δ x. The total volume is the summation of all volumes, V ≈ ∑ π y 2 ⋅ δ x. In the limit as δx approaches zero this equation becomes, V = ∫ − r r π y 2 d x. At any given x, a right-angled triangle connects x, y and r to the origin, hence, applying the Pythagorean theorem yields, thus, substituting y with a function of x gives, V = ∫ − r r π d x. Which can now be evaluated as follows, V = π − r r = π − π =43 π r 3
2.
Circle
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A circle is a simple closed shape in Euclidean geometry. The distance between any of the points and the centre is called the radius, a circle is a simple closed curve which divides the plane into two regions, an interior and an exterior. Annulus, the object, the region bounded by two concentric circles. Arc, any connected part of the circle, centre, the point equidistant from the points on the circle. Chord, a segment whose endpoints lie on the circle. Circumference, the length of one circuit along the circle, or the distance around the circle and it is a special case of a chord, namely the longest chord, and it is twice the radius. Disc, the region of the bounded by a circle. Lens, the intersection of two discs, passant, a coplanar straight line that does not touch the circle. Radius, a line segment joining the centre of the circle to any point on the circle itself, or the length of such a segment, sector, a region bounded by two radii and an arc lying between the radii. Segment, a region, not containing the centre, bounded by a chord, secant, an extended chord, a coplanar straight line cutting the circle at two points. Semicircle, an arc that extends from one of a diameters endpoints to the other, in non-technical common usage it may mean the diameter, arc, and its interior, a two dimensional region, that is technically called a half-disc. A half-disc is a case of a segment, namely the largest one. Tangent, a straight line that touches the circle at a single point. The word circle derives from the Greek κίρκος/κύκλος, itself a metathesis of the Homeric Greek κρίκος, the origins of the words circus and circuit are closely related. The circle has been known since before the beginning of recorded history, natural circles would have been observed, such as the Moon, Sun, and a short plant stalk blowing in the wind on sand, which forms a circle shape in the sand. The circle is the basis for the wheel, which, with related inventions such as gears, in mathematics, the study of the circle has helped inspire the development of geometry, astronomy and calculus. Some highlights in the history of the circle are,1700 BCE – The Rhind papyrus gives a method to find the area of a circular field. The result corresponds to 256/81 as a value of π.300 BCE – Book 3 of Euclids Elements deals with the properties of circles
3.
Visual angle
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The visual angle is the angle a viewed object subtends at the eye, usually stated in degrees of arc. It also is called the angular size. The diagram on the shows a observers eye looking at a frontal extent that has a linear size S. The central line of the bundle represents the chief ray, the same holds for object point B and its retinal image at b. The visual angle V is the angle between the rays for A and B. The visual angle V can be measured using a theodolite placed at point O. Or, it can be calculated, in radians, using the formula. However, for visual angles smaller than about 10 degrees, this simpler formula provides very close approximations, as the above sketch shows, a real image of the object is formed on the retina between points a and b. For small angles, the size of this retinal image R is R n = tan V, where n is the distance from the nodal points to the retina, about 17 mm. If one looks at an object at a distance of one meter. Thus they have the same image size R ≈0.17 mm. That is just a bit larger than the image size for the moon, which is about 0.15 mm, because, with S =2160 miles. Therefore, if one is interested in the performance of the eye or the first processing steps in the visual cortex, what matters is the visual angle V which determines the size of the retinal image. In astronomy the term apparent size refers to the physical angle V or angular diameter, but in psychophysics and experimental psychology the adjective apparent refers to a persons subjective experience. So, apparent size has referred to how large an object looks, additional confusion has occurred because there are two qualitatively different size experiences for a viewed object. One is the visual angle V ′ which is the subjective correlate of V. The perceived visual angle is best defined as the difference between the directions of the objects endpoints from oneself. The other size experience is the perceived linear size S ′ which is the subjective correlate of S. Widespread use of the ambiguous terms apparent size and perceived size without specifying the units of measure has caused confusion, the brains primary visual cortex contains a spatially isomorphic representation of the retina
4.
Optics
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Optics is the branch of physics which involves the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviolet, and infrared light, because light is an electromagnetic wave, other forms of electromagnetic radiation such as X-rays, microwaves, and radio waves exhibit similar properties. Most optical phenomena can be accounted for using the classical description of light. Complete electromagnetic descriptions of light are, however, often difficult to apply in practice, practical optics is usually done using simplified models. The most common of these, geometric optics, treats light as a collection of rays that travel in straight lines, physical optics is a more comprehensive model of light, which includes wave effects such as diffraction and interference that cannot be accounted for in geometric optics. Historically, the model of light was developed first, followed by the wave model of light. Progress in electromagnetic theory in the 19th century led to the discovery that waves were in fact electromagnetic radiation. Some phenomena depend on the fact that light has both wave-like and particle-like properties, explanation of these effects requires quantum mechanics. When considering lights particle-like properties, the light is modelled as a collection of particles called photons, quantum optics deals with the application of quantum mechanics to optical systems. Optical science is relevant to and studied in many related disciplines including astronomy, various engineering fields, photography, practical applications of optics are found in a variety of technologies and everyday objects, including mirrors, lenses, telescopes, microscopes, lasers, and fibre optics. Optics began with the development of lenses by the ancient Egyptians and Mesopotamians, the earliest known lenses, made from polished crystal, often quartz, date from as early as 700 BC for Assyrian lenses such as the Layard/Nimrud lens. The ancient Romans and Greeks filled glass spheres with water to make lenses, the word optics comes from the ancient Greek word ὀπτική, meaning appearance, look. Greek philosophy on optics broke down into two opposing theories on how vision worked, the theory and the emission theory. The intro-mission approach saw vision as coming from objects casting off copies of themselves that were captured by the eye, plato first articulated the emission theory, the idea that visual perception is accomplished by rays emitted by the eyes. He also commented on the parity reversal of mirrors in Timaeus, some hundred years later, Euclid wrote a treatise entitled Optics where he linked vision to geometry, creating geometrical optics. Ptolemy, in his treatise Optics, held a theory of vision, the rays from the eye formed a cone, the vertex being within the eye. The rays were sensitive, and conveyed back to the observer’s intellect about the distance. He summarised much of Euclid and went on to describe a way to measure the angle of refraction, during the Middle Ages, Greek ideas about optics were resurrected and extended by writers in the Muslim world
5.
Lens (optics)
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A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a piece of transparent material, while a compound lens consists of several simple lenses. Lenses are made from such as glass or plastic, and are ground. A lens can focus light to form an image, unlike a prism, devices that similarly focus or disperse waves and radiation other than visible light are also called lenses, such as microwave lenses, electron lenses, acoustic lenses, or explosive lenses. The word lens comes from the Latin name of the lentil, the genus of the lentil plant is Lens, and the most commonly eaten species is Lens culinaris. The lentil plant also gives its name to a geometric figure, the variant spelling lense is sometimes seen. While it is listed as a spelling in some dictionaries. The oldest lens artifact is the Nimrud lens, dating back 2700 years to ancient Assyria, david Brewster proposed that it may have been used as a magnifying glass, or as a burning-glass to start fires by concentrating sunlight. Another early reference to magnification dates back to ancient Egyptian hieroglyphs in the 8th century BC, the earliest written records of lenses date to Ancient Greece, with Aristophanes play The Clouds mentioning a burning-glass. Some scholars argue that the evidence indicates that there was widespread use of lenses in antiquity. Such lenses were used by artisans for fine work, and for authenticating seal impressions, both Pliny and Seneca the Younger described the magnifying effect of a glass globe filled with water. The Viking lenses were capable of concentrating enough sunlight to ignite fires, between the 11th and 13th century reading stones were invented. Often used by monks to assist in illuminating manuscripts, these were primitive plano-convex lenses initially made by cutting a sphere in half. As the stones were experimented with, it was understood that shallower lenses magnified more effectively. Lenses came into use in Europe with the invention of spectacles. Spectacle makers created improved types of lenses for the correction of vision based more on knowledge gained from observing the effects of the lenses. With the invention of the telescope and microscope there was a deal of experimentation with lens shapes in the 17th. Opticians tried to construct lenses of varying forms of curvature, wrongly assuming errors arose from defects in the figure of their surfaces
6.
Radian
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The radian is the standard unit of angular measure, used in many areas of mathematics. The length of an arc of a circle is numerically equal to the measurement in radians of the angle that it subtends. The unit was formerly an SI supplementary unit, but this category was abolished in 1995, separately, the SI unit of solid angle measurement is the steradian. The radian is represented by the symbol rad, so for example, a value of 1.2 radians could be written as 1.2 rad,1.2 r,1. 2rad, or 1. 2c. Radian describes the angle subtended by a circular arc as the length of the arc divided by the radius of the arc. One radian is the angle subtended at the center of a circle by an arc that is equal in length to the radius of the circle. Conversely, the length of the arc is equal to the radius multiplied by the magnitude of the angle in radians. As the ratio of two lengths, the radian is a number that needs no unit symbol, and in mathematical writing the symbol rad is almost always omitted. When quantifying an angle in the absence of any symbol, radians are assumed, and it follows that the magnitude in radians of one complete revolution is the length of the entire circumference divided by the radius, or 2πr / r, or 2π. Thus 2π radians is equal to 360 degrees, meaning that one radian is equal to 180/π degrees, the concept of radian measure, as opposed to the degree of an angle, is normally credited to Roger Cotes in 1714. He described the radian in everything but name, and he recognized its naturalness as a unit of angular measure, the idea of measuring angles by the length of the arc was already in use by other mathematicians. For example, al-Kashi used so-called diameter parts as units where one part was 1/60 radian. The term radian first appeared in print on 5 June 1873, in examination questions set by James Thomson at Queens College, Belfast. He had used the term as early as 1871, while in 1869, Thomas Muir, then of the University of St Andrews, in 1874, after a consultation with James Thomson, Muir adopted radian. As stated, one radian is equal to 180/π degrees, thus, to convert from radians to degrees, multiply by 180/π. The length of circumference of a circle is given by 2 π r, so, to convert from radians to gradians multiply by 200 / π, and to convert from gradians to radians multiply by π /200. This is because radians have a mathematical naturalness that leads to a more elegant formulation of a number of important results, most notably, results in analysis involving trigonometric functions are simple and elegant when the functions arguments are expressed in radians. Because of these and other properties, the trigonometric functions appear in solutions to problems that are not obviously related to the functions geometrical meanings
7.
Small-angle approximation
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The small-angle approximation is a useful simplification of the basic trigonometric functions which is approximately true in the limit where the angle approaches zero. They are truncations of the Taylor series for the trigonometric functions to a second-order approximation. This truncation gives, sin θ ≈ θ cos θ ≈1 − θ22 tan θ ≈ θ, where θ is the angle in radians. The small angle approximation is useful in areas of engineering and physics, including mechanics, electromagnetics, optics, cartography, astronomy. The accuracy of the approximations can be seen below in Figure 1, as the angle approaches zero, it is clear that the gap between the approximation and the original function quickly vanishes. The red section on the right, d, is the difference between the lengths of the hypotenuse, H, and the adjacent side, A. As is shown, H and A are almost the length, meaning cos θ is close to 1. Cos θ ≈1 − θ22 The opposite leg, O, is equal to the length of the blue arc. Simplifying leaves, sin θ ≈ tan θ ≈ θ, the Maclaurin expansion of the relevant trigonometric function is sin θ = ∑ n =0 ∞ n. + ⋯ where θ is the angle in radians.01, Figure 3 shows the relative errors of the small angle approximations. The angles at which the error exceeds 1% are as follows. Sin θ ≈ θ at about 0.244 radians, cos θ ≈1 − θ2/2 at about 0.664 radians. In astronomy, the angle subtended by the image of a distant object is only a few arcseconds. The linear size is related to the size and the distance from the observer by the simple formula D = X d 206265 where X is measured in arcseconds. The number 7005206265000000000♠206265 is approximately equal to the number of arcseconds in a circle, the exact formula is D = d tan and the above approximation follows when tan X is replaced by X. The second-order cosine approximation is useful in calculating the potential energy of a pendulum. The small-angle approximation also appears in structural mechanics, especially in stability and this leads to significant simplifications, though at a cost in accuracy and insight into the true behavior. The 1 in 60 rule used in air navigation has its basis in the small-angle approximation, skinny triangle Infinitesimal oscillations of a pendulum Versine and haversine Exsecant and excosecant
8.
Milliradian
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A milliradian, often called a mil or mrad, is an SI derived unit for angular measurement which is defined as a thousandth of a radian. Mils are used in adjustment of firearm sights by adjusting the angle of the sight compared to the barrel, Mils are also used for comparing shot groupings, or to compare the difficulty of hitting different sized targets at different distances. Using optics with mil markings in the one can make a range estimation of a known size target, or vice versa to determine a target size if the distance is known. In such applications it is useful to use a unit for target size that is a thousandth of the unit for range, for instance by using the metric units millimeters for target size and meters for range. This coincides with the definition of the milliradian where the arc length is defined as 1/1000 of the radius, a common adjustment value in firearm sights is 1 cm at 100 meters which equals 10 mm/100 m = 1/10 mil. The true definition of a milliradian is based on a circle with a radius of one. There are other definitions used for mapping and artillery which are rounded to more easily be divided into smaller parts. The milliradian was first used in the mid nineteenth century by Charles-Marc Dapples, degrees and minutes were the usual units of angular measurement but others were being proposed, with grads under various names having considerable popularity in much of northern Europe. However, Imperial Russia used a different approach, dividing a circle into equilateral triangles, around the time of the start of World War I, France was experimenting with the use of milliemes for use with artillery sights instead of decigrades. The United Kingdom was also trialing them to replace degrees and minutes and they were adopted by France although decigrades also remained in use throughout World War I. The United States, which copied many French artillery practices, adopted mils, before 2007 the Swedish defence forces used streck which is closer to the milliradian but then changed to NATO mils. After the Bolshevik Revolution and the adoption of the system of measurement the Red Army expanded the 600 unit circle into a 6000 mil one. Hence the Russian mil has a different origin than those derived from French artillery practices. In the 1950s, NATO adopted metric units of measurement for land, Mils, meters, and kilograms became standard, although degrees remained in use for naval and air purposes, reflecting civil practices. The approximation error by using the linear formula will increase as the angle increases. New shooters are often explained the principle of subtensions in order to understand that a milliradian is an angular measurement, subtension is the physical amount of space covered by an angle and varies with distance. Thus, the corresponding to a mil varies with range. Subtensions always change with distance, but a mil is always a mil regardless of distance, therefore ballistic tables and shot corrections are given in mils thereby avoiding the need of mathematical calculations
9.
Astronomy
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Astronomy is a natural science that studies celestial objects and phenomena. It applies mathematics, physics, and chemistry, in an effort to explain the origin of those objects and phenomena and their evolution. Objects of interest include planets, moons, stars, galaxies, and comets, while the phenomena include supernovae explosions, gamma ray bursts, more generally, all astronomical phenomena that originate outside Earths atmosphere are within the purview of astronomy. A related but distinct subject, physical cosmology, is concerned with the study of the Universe as a whole, Astronomy is the oldest of the natural sciences. The early civilizations in recorded history, such as the Babylonians, Greeks, Indians, Egyptians, Nubians, Iranians, Chinese, during the 20th century, the field of professional astronomy split into observational and theoretical branches. Observational astronomy is focused on acquiring data from observations of astronomical objects, theoretical astronomy is oriented toward the development of computer or analytical models to describe astronomical objects and phenomena. The two fields complement each other, with theoretical astronomy seeking to explain the results and observations being used to confirm theoretical results. Astronomy is one of the few sciences where amateurs can play an active role, especially in the discovery. Amateur astronomers have made and contributed to many important astronomical discoveries, Astronomy means law of the stars. Astronomy should not be confused with astrology, the system which claims that human affairs are correlated with the positions of celestial objects. Although the two share a common origin, they are now entirely distinct. Generally, either the term astronomy or astrophysics may be used to refer to this subject, however, since most modern astronomical research deals with subjects related to physics, modern astronomy could actually be called astrophysics. Few fields, such as astrometry, are purely astronomy rather than also astrophysics, some titles of the leading scientific journals in this field includeThe Astronomical Journal, The Astrophysical Journal and Astronomy and Astrophysics. In early times, astronomy only comprised the observation and predictions of the motions of objects visible to the naked eye, in some locations, early cultures assembled massive artifacts that possibly had some astronomical purpose. Before tools such as the telescope were invented, early study of the stars was conducted using the naked eye, most of early astronomy actually consisted of mapping the positions of the stars and planets, a science now referred to as astrometry. From these observations, early ideas about the motions of the planets were formed, and the nature of the Sun, Moon, the Earth was believed to be the center of the Universe with the Sun, the Moon and the stars rotating around it. This is known as the model of the Universe, or the Ptolemaic system. The Babylonians discovered that lunar eclipses recurred in a cycle known as a saros
10.
Celestial object
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An astronomical object or celestial object is a naturally occurring physical entity, association, or structure that current astronomy has demonstrated to exist in the observable universe. In astronomy, the object and body are often used interchangeably. Examples for astronomical objects include planetary systems, star clusters, nebulae and galaxies, while asteroids, moons, planets, and stars are astronomical bodies. A comet may be identified as both body and object, It is a body when referring to the nucleus of ice and dust. The universe can be viewed as having a hierarchical structure, at the largest scales, the fundamental component of assembly is the galaxy. Galaxies are organized groups and clusters, often within larger superclusters. Disc galaxies encompass lenticular and spiral galaxies with features, such as spiral arms, at the core, most galaxies have a supermassive black hole, which may result in an active galactic nucleus. Galaxies can also have satellites in the form of dwarf galaxies, the constituents of a galaxy are formed out of gaseous matter that assembles through gravitational self-attraction in a hierarchical manner. At this level, the fundamental components are the stars. The great variety of forms are determined almost entirely by the mass, composition. Stars may be found in systems that orbit about each other in a hierarchical organization. A planetary system and various objects such as asteroids, comets and debris. The various distinctive types of stars are shown by the Hertzsprung–Russell diagram —a plot of stellar luminosity versus surface temperature. Each star follows a track across this diagram. If this track takes the star through a region containing a variable type. An example of this is the instability strip, a region of the H-R diagram that includes Delta Scuti, RR Lyrae, the table below lists the general categories of bodies and objects by their location or structure. International Astronomical Naming Commission List of light sources List of Solar System objects Lists of astronomical objects SkyChart, Sky & Telescope Monthly skymaps for every location on Earth
11.
Earth
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Earth, otherwise known as the World, or the Globe, is the third planet from the Sun and the only object in the Universe known to harbor life. It is the densest planet in the Solar System and the largest of the four terrestrial planets, according to radiometric dating and other sources of evidence, Earth formed about 4.54 billion years ago. Earths gravity interacts with objects in space, especially the Sun. During one orbit around the Sun, Earth rotates about its axis over 365 times, thus, Earths axis of rotation is tilted, producing seasonal variations on the planets surface. The gravitational interaction between the Earth and Moon causes ocean tides, stabilizes the Earths orientation on its axis, Earths lithosphere is divided into several rigid tectonic plates that migrate across the surface over periods of many millions of years. About 71% of Earths surface is covered with water, mostly by its oceans, the remaining 29% is land consisting of continents and islands that together have many lakes, rivers and other sources of water that contribute to the hydrosphere. The majority of Earths polar regions are covered in ice, including the Antarctic ice sheet, Earths interior remains active with a solid iron inner core, a liquid outer core that generates the Earths magnetic field, and a convecting mantle that drives plate tectonics. Within the first billion years of Earths history, life appeared in the oceans and began to affect the Earths atmosphere and surface, some geological evidence indicates that life may have arisen as much as 4.1 billion years ago. Since then, the combination of Earths distance from the Sun, physical properties, in the history of the Earth, biodiversity has gone through long periods of expansion, occasionally punctuated by mass extinction events. Over 99% of all species that lived on Earth are extinct. Estimates of the number of species on Earth today vary widely, over 7.4 billion humans live on Earth and depend on its biosphere and minerals for their survival. Humans have developed diverse societies and cultures, politically, the world has about 200 sovereign states, the modern English word Earth developed from a wide variety of Middle English forms, which derived from an Old English noun most often spelled eorðe. It has cognates in every Germanic language, and their proto-Germanic root has been reconstructed as *erþō, originally, earth was written in lowercase, and from early Middle English, its definite sense as the globe was expressed as the earth. By early Modern English, many nouns were capitalized, and the became the Earth. More recently, the name is simply given as Earth. House styles now vary, Oxford spelling recognizes the lowercase form as the most common, another convention capitalizes Earth when appearing as a name but writes it in lowercase when preceded by the. It almost always appears in lowercase in colloquial expressions such as what on earth are you doing, the oldest material found in the Solar System is dated to 4. 5672±0.0006 billion years ago. By 4. 54±0.04 Gya the primordial Earth had formed, the formation and evolution of Solar System bodies occurred along with the Sun
12.
Minute and second of arc
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A minute of arc, arcminute, arc minute, or minute arc is a unit of angular measurement equal to 1/60 of one degree. Since one degree is 1/360 of a turn, one minute of arc is 1/21600 of a turn, a second of arc, arcsecond, or arc second is 1/60 of an arcminute, 1/3600 of a degree, 1/1296000 of a turn, and π/648000 of a radian. To express even smaller angles, standard SI prefixes can be employed, the number of square arcminutes in a complete sphere is 4 π2 =466560000 π ≈148510660 square arcminutes. The standard symbol for marking the arcminute is the prime, though a single quote is used where only ASCII characters are permitted. One arcminute is thus written 1′ and it is also abbreviated as arcmin or amin or, less commonly, the prime with a circumflex over it. The standard symbol for the arcsecond is the prime, though a double quote is commonly used where only ASCII characters are permitted. One arcsecond is thus written 1″ and it is also abbreviated as arcsec or asec. In celestial navigation, seconds of arc are used in calculations. This notation has been carried over into marine GPS receivers, which normally display latitude and longitude in the format by default. An arcsecond is approximately the angle subtended by a U. S. dime coin at a distance of 4 kilometres, a milliarcsecond is about the size of a dime atop the Eiffel Tower as seen from New York City. A microarcsecond is about the size of a period at the end of a sentence in the Apollo mission manuals left on the Moon as seen from Earth, since antiquity the arcminute and arcsecond have been used in astronomy. The principal exception is Right ascension in equatorial coordinates, which is measured in units of hours, minutes. These small angles may also be written in milliarcseconds, or thousandths of an arcsecond, the unit of distance, the parsec, named from the parallax of one arcsecond, was developed for such parallax measurements. It is the distance at which the radius of the Earths orbit would subtend an angle of one arcsecond. The ESA astrometric space probe Gaia is hoped to measure star positions to 20 microarcseconds when it begins producing catalog positions sometime after 2016, there are about 1.3 trillion µas in a turn. Currently the best catalog positions of stars actually measured are in terms of milliarcseconds, apart from the Sun, the star with the largest angular diameter from Earth is R Doradus, a red supergiant with a diameter of 0.05 arcsecond. The dwarf planet Pluto has proven difficult to resolve because its angular diameter is about 0.1 arcsecond, space telescopes are not affected by the Earths atmosphere but are diffraction limited. For example, the Hubble space telescope can reach a size of stars down to about 0. 1″
13.
Degree (angle)
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A degree, usually denoted by °, is a measurement of a plane angle, defined so that a full rotation is 360 degrees. It is not an SI unit, as the SI unit of measure is the radian. Because a full rotation equals 2π radians, one degree is equivalent to π/180 radians, the original motivation for choosing the degree as a unit of rotations and angles is unknown. One theory states that it is related to the fact that 360 is approximately the number of days in a year. Ancient astronomers noticed that the sun, which follows through the path over the course of the year. Some ancient calendars, such as the Persian calendar, used 360 days for a year, the use of a calendar with 360 days may be related to the use of sexagesimal numbers. The earliest trigonometry, used by the Babylonian astronomers and their Greek successors, was based on chords of a circle, a chord of length equal to the radius made a natural base quantity. One sixtieth of this, using their standard sexagesimal divisions, was a degree, Aristarchus of Samos and Hipparchus seem to have been among the first Greek scientists to exploit Babylonian astronomical knowledge and techniques systematically. Timocharis, Aristarchus, Aristillus, Archimedes, and Hipparchus were the first Greeks known to divide the circle in 360 degrees of 60 arc minutes, eratosthenes used a simpler sexagesimal system dividing a circle into 60 parts. Furthermore, it is divisible by every number from 1 to 10 except 7 and this property has many useful applications, such as dividing the world into 24 time zones, each of which is nominally 15° of longitude, to correlate with the established 24-hour day convention. Finally, it may be the case more than one of these factors has come into play. For many practical purposes, a degree is a small enough angle that whole degrees provide sufficient precision. When this is not the case, as in astronomy or for geographic coordinates, degree measurements may be written using decimal degrees, with the symbol behind the decimals. Alternatively, the sexagesimal unit subdivisions can be used. One degree is divided into 60 minutes, and one minute into 60 seconds, use of degrees-minutes-seconds is also called DMS notation. These subdivisions, also called the arcminute and arcsecond, are represented by a single and double prime. For example,40. 1875° = 40° 11′ 15″, or, using quotation mark characters, additional precision can be provided using decimals for the arcseconds component. The older system of thirds, fourths, etc. which continues the sexagesimal unit subdivision, was used by al-Kashi and other ancient astronomers, but is rarely used today
14.
Astronomical unit
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The astronomical unit is a unit of length, roughly the distance from Earth to the Sun. However, that varies as Earth orbits the Sun, from a maximum to a minimum. Originally conceived as the average of Earths aphelion and perihelion, it is now defined as exactly 149597870700 metres, the astronomical unit is used primarily as a convenient yardstick for measuring distances within the Solar System or around other stars. However, it is also a component in the definition of another unit of astronomical length. A variety of symbols and abbreviations have been in use for the astronomical unit. In a 1976 resolution, the International Astronomical Union used the symbol A for the astronomical unit, in 2006, the International Bureau of Weights and Measures recommended ua as the symbol for the unit. In 2012, the IAU, noting that various symbols are presently in use for the astronomical unit, in the 2014 revision of the SI Brochure, the BIPM used the unit symbol au. In ISO 80000-3, the symbol of the unit is ua. Earths orbit around the Sun is an ellipse, the semi-major axis of this ellipse is defined to be half of the straight line segment that joins the aphelion and perihelion. The centre of the sun lies on this line segment. In addition, it mapped out exactly the largest straight-line distance that Earth traverses over the course of a year, knowing Earths shift and a stars shift enabled the stars distance to be calculated. But all measurements are subject to some degree of error or uncertainty, improvements in precision have always been a key to improving astronomical understanding. Improving measurements were continually checked and cross-checked by means of our understanding of the laws of celestial mechanics, the expected positions and distances of objects at an established time are calculated from these laws, and assembled into a collection of data called an ephemeris. NASAs Jet Propulsion Laboratory provides one of several ephemeris computation services, in 1976, in order to establish a yet more precise measure for the astronomical unit, the IAU formally adopted a new definition. Equivalently, by definition, one AU is the radius of an unperturbed circular Newtonian orbit about the sun of a particle having infinitesimal mass. As with all measurements, these rely on measuring the time taken for photons to be reflected from an object. However, for precision the calculations require adjustment for such as the motions of the probe. In addition, the measurement of the time itself must be translated to a scale that accounts for relativistic time dilation
15.
Parsec
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The parsec is a unit of length used to measure large distances to objects outside the Solar System. One parsec is the distance at which one astronomical unit subtends an angle of one arcsecond, a parsec is equal to about 3.26 light-years in length. The nearest star, Proxima Centauri, is about 1.3 parsecs from the Sun, most of the stars visible to the unaided eye in the nighttime sky are within 500 parsecs of the Sun. The parsec unit was likely first suggested in 1913 by the British astronomer Herbert Hall Turner, named from an abbreviation of the parallax of one arcsecond, it was defined so as to make calculations of astronomical distances quick and easy for astronomers from only their raw observational data. Partly for this reason, it is still the unit preferred in astronomy and astrophysics, though the light-year remains prominent in science texts. This corresponds to the definition of the parsec found in many contemporary astronomical references. Derivation, create a triangle with one leg being from the Earth to the Sun. As that point in space away, the angle between the Sun and Earth decreases. A parsec is the length of that leg when the angle between the Sun and Earth is one arc-second. One of the oldest methods used by astronomers to calculate the distance to a star is to record the difference in angle between two measurements of the position of the star in the sky. The first measurement is taken from the Earth on one side of the Sun, and the second is approximately half a year later. The distance between the two positions of the Earth when the two measurements were taken is twice the distance between the Earth and the Sun. The difference in angle between the two measurements is twice the angle, which is formed by lines from the Sun. Then the distance to the star could be calculated using trigonometry. 5-parsec distance of 61 Cygni, the parallax of a star is defined as half of the angular distance that a star appears to move relative to the celestial sphere as Earth orbits the Sun. Equivalently, it is the angle, from that stars perspective. The star, the Sun and the Earth form the corners of a right triangle in space, the right angle is the corner at the Sun. Therefore, given a measurement of the angle, along with the rules of trigonometry. A parsec is defined as the length of the adjacent to the vertex occupied by a star whose parallax angle is one arcsecond
16.
Earth's orbit
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Earths orbit is the path through which the Earth travels around the Sun. The average distance between the Earth and the Sun is 149.60 million kilometers, and a complete orbit occurs every 365.256 days, Earths orbit has an eccentricity of 0.0167. Earths orbital motion gives an apparent movement of the Sun with respect to other stars at a rate of about 1° per day eastward as seen from Earth. Earths orbital speed averages about 30 km/s, which is fast enough to cover the planets diameter in seven minutes, viewed from a vantage point above the north poles of both the Sun and the Earth, the Earth would appear to revolve in a counterclockwise direction about the Sun. From the same point, both the Earth and the Sun would appear to rotate in a counterclockwise direction about their respective axes. Heliocentrism is the model that first placed the Sun at the center of the Solar System and put the planets, including Earth. Historically, heliocentrism is opposed to geocentrism, which placed the Earth at the center, aristarchus of Samos already proposed a heliocentric model in the 3rd century BC. This Copernican revolution resolved the issue of planetary motion by arguing that such motion was only perceived. Although Copernicuss groundbreaking book. had been over a century earlier, because of Earths axial tilt, the inclination of the Suns trajectory in the sky varies over the course of the year. For an observer at a northern latitude, when the pole is tilted toward the Sun the day lasts longer. This results in average temperatures, as additional solar radiation reaches the surface. When the north pole is tilted away from the Sun, the reverse is true, above the Arctic Circle and below the Antarctic Circle, an extreme case is reached in which there is no daylight at all for part of the year. This is called a polar night and this variation in the weather results in the seasons. The solstices and equinoxes divide the year up into four equal parts. In the northern winter solstice occurs on or about December 21, summer solstice is near June 21, spring equinox is around March 20. In modern times, Earths perihelion occurs around January 3, the changing Earth–Sun distance results in an increase of about 6. 9% in total solar energy reaching the Earth at perihelion relative to aphelion. The Hill sphere of the Earth is about 1,500,000 kilometers in radius and this is the maximal distance at which the Earths gravitational influence is stronger than the more distant Sun and planets. Objects orbiting the Earth must be within this radius, otherwise they can become unbound by the perturbation of the Sun
17.
Astronomical object
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An astronomical object or celestial object is a naturally occurring physical entity, association, or structure that current astronomy has demonstrated to exist in the observable universe. In astronomy, the object and body are often used interchangeably. Examples for astronomical objects include planetary systems, star clusters, nebulae and galaxies, while asteroids, moons, planets, and stars are astronomical bodies. A comet may be identified as both body and object, It is a body when referring to the nucleus of ice and dust. The universe can be viewed as having a hierarchical structure, at the largest scales, the fundamental component of assembly is the galaxy. Galaxies are organized groups and clusters, often within larger superclusters. Disc galaxies encompass lenticular and spiral galaxies with features, such as spiral arms, at the core, most galaxies have a supermassive black hole, which may result in an active galactic nucleus. Galaxies can also have satellites in the form of dwarf galaxies, the constituents of a galaxy are formed out of gaseous matter that assembles through gravitational self-attraction in a hierarchical manner. At this level, the fundamental components are the stars. The great variety of forms are determined almost entirely by the mass, composition. Stars may be found in systems that orbit about each other in a hierarchical organization. A planetary system and various objects such as asteroids, comets and debris. The various distinctive types of stars are shown by the Hertzsprung–Russell diagram —a plot of stellar luminosity versus surface temperature. Each star follows a track across this diagram. If this track takes the star through a region containing a variable type. An example of this is the instability strip, a region of the H-R diagram that includes Delta Scuti, RR Lyrae, the table below lists the general categories of bodies and objects by their location or structure. International Astronomical Naming Commission List of light sources List of Solar System objects Lists of astronomical objects SkyChart, Sky & Telescope Monthly skymaps for every location on Earth
18.
Sun
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The Sun is the star at the center of the Solar System. It is a perfect sphere of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process. It is by far the most important source of energy for life on Earth. Its diameter is about 109 times that of Earth, and its mass is about 330,000 times that of Earth, accounting for about 99. 86% of the total mass of the Solar System. About three quarters of the Suns mass consists of hydrogen, the rest is mostly helium, with smaller quantities of heavier elements, including oxygen, carbon, neon. The Sun is a G-type main-sequence star based on its spectral class and it formed approximately 4.6 billion years ago from the gravitational collapse of matter within a region of a large molecular cloud. Most of this matter gathered in the center, whereas the rest flattened into a disk that became the Solar System. The central mass became so hot and dense that it eventually initiated nuclear fusion in its core and it is thought that almost all stars form by this process. The Sun is roughly middle-aged, it has not changed dramatically for more than four billion years and it is calculated that the Sun will become sufficiently large enough to engulf the current orbits of Mercury, Venus, and probably Earth. The enormous effect of the Sun on Earth has been recognized since prehistoric times, the synodic rotation of Earth and its orbit around the Sun are the basis of the solar calendar, which is the predominant calendar in use today. The English proper name Sun developed from Old English sunne and may be related to south, all Germanic terms for the Sun stem from Proto-Germanic *sunnōn. The English weekday name Sunday stems from Old English and is ultimately a result of a Germanic interpretation of Latin dies solis, the Latin name for the Sun, Sol, is not common in general English language use, the adjectival form is the related word solar. The term sol is used by planetary astronomers to refer to the duration of a solar day on another planet. A mean Earth solar day is approximately 24 hours, whereas a mean Martian sol is 24 hours,39 minutes, and 35.244 seconds. From at least the 4th Dynasty of Ancient Egypt, the Sun was worshipped as the god Ra, portrayed as a falcon-headed divinity surmounted by the solar disk, and surrounded by a serpent. In the New Empire period, the Sun became identified with the dung beetle, in the form of the Sun disc Aten, the Sun had a brief resurgence during the Amarna Period when it again became the preeminent, if not only, divinity for the Pharaoh Akhenaton. The Sun is viewed as a goddess in Germanic paganism, Sól/Sunna, in ancient Roman culture, Sunday was the day of the Sun god. It was adopted as the Sabbath day by Christians who did not have a Jewish background, the symbol of light was a pagan device adopted by Christians, and perhaps the most important one that did not come from Jewish traditions
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Moon
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The Moon is an astronomical body that orbits planet Earth, being Earths only permanent natural satellite. It is the fifth-largest natural satellite in the Solar System, following Jupiters satellite Io, the Moon is second-densest satellite among those whose densities are known. The average distance of the Moon from the Earth is 384,400 km, the Moon is thought to have formed about 4.51 billion years ago, not long after Earth. It is the second-brightest regularly visible celestial object in Earths sky, after the Sun and its surface is actually dark, although compared to the night sky it appears very bright, with a reflectance just slightly higher than that of worn asphalt. Its prominence in the sky and its cycle of phases have made the Moon an important cultural influence since ancient times on language, calendars, art. The Moons gravitational influence produces the ocean tides, body tides, and this matching of apparent visual size will not continue in the far future. The Moons linear distance from Earth is currently increasing at a rate of 3.82 ±0.07 centimetres per year, since the Apollo 17 mission in 1972, the Moon has been visited only by uncrewed spacecraft. The usual English proper name for Earths natural satellite is the Moon, the noun moon is derived from moone, which developed from mone, which is derived from Old English mōna, which ultimately stems from Proto-Germanic *mǣnōn, like all Germanic language cognates. Occasionally, the name Luna is used, in literature, especially science fiction, Luna is used to distinguish it from other moons, while in poetry, the name has been used to denote personification of our moon. The principal modern English adjective pertaining to the Moon is lunar, a less common adjective is selenic, derived from the Ancient Greek Selene, from which is derived the prefix seleno-. Both the Greek Selene and the Roman goddess Diana were alternatively called Cynthia, the names Luna, Cynthia, and Selene are reflected in terminology for lunar orbits in words such as apolune, pericynthion, and selenocentric. The name Diana is connected to dies meaning day, several mechanisms have been proposed for the Moons formation 4.51 billion years ago, and some 60 million years after the origin of the Solar System. These hypotheses also cannot account for the angular momentum of the Earth–Moon system. This hypothesis, although not perfect, perhaps best explains the evidence, eighteen months prior to an October 1984 conference on lunar origins, Bill Hartmann, Roger Phillips, and Jeff Taylor challenged fellow lunar scientists, You have eighteen months. Go back to your Apollo data, go back to computer, do whatever you have to. Dont come to our conference unless you have something to say about the Moons birth, at the 1984 conference at Kona, Hawaii, the giant impact hypothesis emerged as the most popular. Afterward there were only two groups, the giant impact camp and the agnostics. Giant impacts are thought to have been common in the early Solar System, computer simulations of a giant impact have produced results that are consistent with the mass of the lunar core and the present angular momentum of the Earth–Moon system
20.
Helix Nebula
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The Helix Nebula, also known as The Helix, NGC7293, is a large planetary nebula located in the constellation Aquarius. Discovered by Karl Ludwig Harding, probably before 1824, this object is one of the closest to the Earth of all the planetary nebulae. The estimated distance is about 215 parsecs, the Helix Nebula has sometimes been referred to as the Eye of God in pop culture, as well as the Eye of Sauron. The Helix Nebula is an example of a nebula, formed by an intermediate to low-mass star. Gases from the star in the space appear, from our vantage point. The remnant central stellar core, known as a planetary nebula nucleus or PNN, is destined to become a white dwarf star, the observed glow of the central star is so energetic that it causes the previously expelled gases to brightly fluoresce. The Helix Nebula in the constellation of Aquarius lies about 700 light-years away, currently, the age is estimated to be 10600+2300 −1200 years, based solely upon a measured expansion rate of 31 km·s−1. The size of the disk is 8×19 arcmin in diameter, the outer torus is 12×22 arcmin in diameter. We see the outer-most ring as flattened on one due to its colliding with the ambient interstellar medium. Expansion of the planetary nebula structure is estimated to have occurred in the last 6,560 years. Spectroscopically, the outer rings expansion rate is 40 km·s−1, the Helix Nebula was the first planetary nebula discovered to contain cometary knots. Its main ring contains knots of nebulosity, which have now been detected in many nearby planetaries. These knots are highly symmetric and are described as cometary, each centered on a core of neutral molecular gas and containing bright cusps towards the central star. All tails extend away from the PNN in a radial direction, excluding the tails, they are the size of the Solar system, while each of the cusp knots are optically thick due to Lyc photons from the PNN. There are more than 20,000 cometary knots estimated to be in the Helix Nebula, the excitation temperature varies across the Helix nebula
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Eagle Nebula
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The Eagle Nebula is a young open cluster of stars in the constellation Serpens, discovered by Jean-Philippe de Chéseaux in 1745–46. The nebula contains several active star-forming gas and dust regions, including the Pillars of Creation, the Eagle Nebula is part of a diffuse emission nebula, or H II region, which is catalogued as IC4703. This region of current star formation is about 7000 light-years distant. A spire of gas that can be seen coming off the nebula in the part is approximately 9.5 light-years or about 90 trillion kilometers long. The cluster associated with the nebula has approximately 8100 stars, which are concentrated in a gap in the molecular cloud to the north-west of the Pillars. The brightest star has an apparent magnitude of +8.24 and it is actually a binary star formed of an O3. 5V star plus an O7. 5V companion. This star has a mass of roughly 80 solar masses, the clusters age has been estimated to be 1–2 million years. The descriptive names reflect impressions of the shape of the central pillar rising from the southeast into the central luminous area, the name Star Queen Nebula was introduced by Robert Burnham, Jr. reflecting his characterization of the central pillar as the Star Queen shown in silhouette. Images taken by Jeff Hester and Paul Scowen using the Hubble Space Telescope in 1995 greatly improved understanding of processes inside the nebula. One of these photographs became famous as the Pillars of Creation, the small dark areas in the photograph are believed to be protostars. These columns – which resemble stalagmites protruding from the floor of a cavern – are composed of hydrogen gas and dust. Inside the columns and on their surface astronomers have found knots or globules of denser gas, stars are being formed inside some of these EGGs. X-ray images from the Chandra observatory compared with Hubbles Pillars image have shown that X-ray sources do not coincide with the pillars, any protostars in the pillars EGGs are not yet hot enough to emit X-rays. Evidence from the Spitzer Telescope suggests that the pillars in M16 may already have been destroyed by a supernova explosion, hot gas observed by Spitzer in 2007 suggests that the area was disturbed by a supernova that exploded some 8000 to 9000 years ago
22.
Venus
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Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. It has the longest rotation period of any planet in the Solar System and it is named after the Roman goddess of love and beauty. It is the second-brightest natural object in the sky after the Moon, reaching an apparent magnitude of −4.6. Because Venus orbits within Earths orbit it is a planet and never appears to venture far from the Sun. Venus is a planet and is sometimes called Earths sister planet because of their similar size, mass, proximity to the Sun. It is radically different from Earth in other respects and it has the densest atmosphere of the four terrestrial planets, consisting of more than 96% carbon dioxide. The atmospheric pressure at the surface is 92 times that of Earth. Venus is by far the hottest planet in the Solar System, with a surface temperature of 735 K. Venus is shrouded by an layer of highly reflective clouds of sulfuric acid. It may have had water oceans in the past, but these would have vaporized as the temperature rose due to a greenhouse effect. The water has probably photodissociated, and the hydrogen has been swept into interplanetary space by the solar wind because of the lack of a planetary magnetic field. Venuss surface is a dry desertscape interspersed with rocks and is periodically resurfaced by volcanism. As one of the brightest objects in the sky, Venus has been a fixture in human culture for as long as records have existed. It has been sacred to gods of many cultures, and has been a prime inspiration for writers and poets as the morning star. Venus was the first planet to have its motions plotted across the sky, as the closest planet to Earth, Venus has been a prime target for early interplanetary exploration. It was the first planet beyond Earth visited by a spacecraft, Venuss thick clouds render observation of its surface impossible in visible light, and the first detailed maps did not emerge until the arrival of the Magellan orbiter in 1991. Plans have been proposed for rovers or more missions. Venus is one of the four planets in the Solar System
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Jupiter
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Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a giant planet with a mass one-thousandth that of the Sun, Jupiter and Saturn are gas giants, the other two giant planets, Uranus and Neptune are ice giants. Jupiter has been known to astronomers since antiquity, the Romans named it after their god Jupiter. Jupiter is primarily composed of hydrogen with a quarter of its mass being helium and it may also have a rocky core of heavier elements, but like the other giant planets, Jupiter lacks a well-defined solid surface. Because of its rotation, the planets shape is that of an oblate spheroid. The outer atmosphere is visibly segregated into several bands at different latitudes, resulting in turbulence, a prominent result is the Great Red Spot, a giant storm that is known to have existed since at least the 17th century when it was first seen by telescope. Surrounding Jupiter is a faint planetary ring system and a powerful magnetosphere, Jupiter has at least 67 moons, including the four large Galilean moons discovered by Galileo Galilei in 1610. Ganymede, the largest of these, has a greater than that of the planet Mercury. Jupiter has been explored on several occasions by robotic spacecraft, most notably during the early Pioneer and Voyager flyby missions and later by the Galileo orbiter. In late February 2007, Jupiter was visited by the New Horizons probe, the latest probe to visit the planet is Juno, which entered into orbit around Jupiter on July 4,2016. Future targets for exploration in the Jupiter system include the probable ice-covered liquid ocean of its moon Europa, Earth and its neighbor planets may have formed from fragments of planets after collisions with Jupiter destroyed those super-Earths near the Sun. Astronomers have discovered nearly 500 planetary systems with multiple planets, Jupiter moving out of the inner Solar System would have allowed the formation of inner planets, including Earth. Jupiter is composed primarily of gaseous and liquid matter and it is the largest of the four giant planets in the Solar System and hence its largest planet. It has a diameter of 142,984 km at its equator, the average density of Jupiter,1.326 g/cm3, is the second highest of the giant planets, but lower than those of the four terrestrial planets. Jupiters upper atmosphere is about 88–92% hydrogen and 8–12% helium by percent volume of gas molecules, a helium atom has about four times as much mass as a hydrogen atom, so the composition changes when described as the proportion of mass contributed by different atoms. Thus, Jupiters atmosphere is approximately 75% hydrogen and 24% helium by mass, the atmosphere contains trace amounts of methane, water vapor, ammonia, and silicon-based compounds. There are also traces of carbon, ethane, hydrogen sulfide, neon, oxygen, phosphine, the outermost layer of the atmosphere contains crystals of frozen ammonia. The interior contains denser materials - by mass it is roughly 71% hydrogen, 24% helium, through infrared and ultraviolet measurements, trace amounts of benzene and other hydrocarbons have also been found
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Saturn
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Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with a radius about nine times that of Earth. Although it has only one-eighth the average density of Earth, with its larger volume Saturn is just over 95 times more massive, Saturn is named after the Roman god of agriculture, its astronomical symbol represents the gods sickle. Saturns interior is composed of a core of iron–nickel and rock. This core is surrounded by a layer of metallic hydrogen, an intermediate layer of liquid hydrogen and liquid helium. Saturn has a yellow hue due to ammonia crystals in its upper atmosphere. Saturns magnetic field strength is around one-twentieth of Jupiters, the outer atmosphere is generally bland and lacking in contrast, although long-lived features can appear. Wind speeds on Saturn can reach 1,800 km/h, higher than on Jupiter, sixty-two moons are known to orbit Saturn, of which fifty-three are officially named. This does not include the hundreds of moonlets comprising the rings, Saturn is a gas giant because it is predominantly composed of hydrogen and helium. It lacks a definite surface, though it may have a solid core, Saturns rotation causes it to have the shape of an oblate spheroid, that is, it is flattened at the poles and bulges at its equator. Its equatorial and polar radii differ by almost 10%,60,268 km versus 54,364 km, Jupiter, Uranus, and Neptune, the other giant planets in the Solar System, are also oblate but to a lesser extent. Saturn is the planet of the Solar System that is less dense than water—about 30% less. Although Saturns core is considerably denser than water, the specific density of the planet is 0.69 g/cm3 due to the atmosphere. Jupiter has 318 times the Earths mass, while Saturn is 95 times the mass of the Earth, together, Jupiter and Saturn hold 92% of the total planetary mass in the Solar System. On 8 January 2015, NASA reported determining the center of the planet Saturn, the temperature, pressure, and density inside Saturn all rise steadily toward the core, which causes hydrogen to transition into a metal in the deeper layers. Standard planetary models suggest that the interior of Saturn is similar to that of Jupiter, having a rocky core surrounded by hydrogen. This core is similar in composition to the Earth, but more dense, in 2004, they estimated that the core must be 9–22 times the mass of the Earth, which corresponds to a diameter of about 25,000 km. This is surrounded by a liquid metallic hydrogen layer, followed by a liquid layer of helium-saturated molecular hydrogen that gradually transitions to a gas with increasing altitude
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Mars
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Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, after Mercury. Named after the Roman god of war, it is referred to as the Red Planet because the iron oxide prevalent on its surface gives it a reddish appearance. Mars is a planet with a thin atmosphere, having surface features reminiscent both of the impact craters of the Moon and the valleys, deserts, and polar ice caps of Earth. The rotational period and seasonal cycles of Mars are likewise similar to those of Earth, Mars is the site of Olympus Mons, the largest volcano and second-highest known mountain in the Solar System, and of Valles Marineris, one of the largest canyons in the Solar System. The smooth Borealis basin in the northern hemisphere covers 40% of the planet, Mars has two moons, Phobos and Deimos, which are small and irregularly shaped. These may be captured asteroids, similar to 5261 Eureka, a Mars trojan, there are ongoing investigations assessing the past habitability potential of Mars, as well as the possibility of extant life. Future astrobiology missions are planned, including the Mars 2020 and ExoMars rovers, liquid water cannot exist on the surface of Mars due to low atmospheric pressure, which is about 6⁄1000 that of the Earths, except at the lowest elevations for short periods. The two polar ice caps appear to be largely of water. The volume of ice in the south polar ice cap, if melted. On November 22,2016, NASA reported finding a large amount of ice in the Utopia Planitia region of Mars. The volume of water detected has been estimated to be equivalent to the volume of water in Lake Superior, Mars can easily be seen from Earth with the naked eye, as can its reddish coloring. Its apparent magnitude reaches −2.91, which is surpassed only by Jupiter, Venus, the Moon, optical ground-based telescopes are typically limited to resolving features about 300 kilometers across when Earth and Mars are closest because of Earths atmosphere. Mars is approximately half the diameter of Earth with an area only slightly less than the total area of Earths dry land. Mars is less dense than Earth, having about 15% of Earths volume and 11% of Earths mass, the red-orange appearance of the Martian surface is caused by iron oxide, or rust. It can look like butterscotch, other common colors include golden, brown, tan. Like Earth, Mars has differentiated into a metallic core overlaid by less dense materials. Current models of its interior imply a core with a radius of about 1,794 ±65 kilometers, consisting primarily of iron and this iron sulfide core is thought to be twice as rich in lighter elements than Earths. The core is surrounded by a mantle that formed many of the tectonic and volcanic features on the planet
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Mercury (planet)
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Mercury is the smallest and innermost planet in the Solar System. Its orbital period around the Sun of 88 days is the shortest of all the planets in the Solar System and it is named after the Roman deity Mercury, the messenger to the gods. Like Venus, Mercury orbits the Sun within Earths orbit as a planet, so it can only be seen visually in the morning or the evening sky. Also, like Venus and the Moon, the displays the complete range of phases as it moves around its orbit relative to Earth. Seen from Earth, this cycle of phases reoccurs approximately every 116 days, although Mercury can appear as a bright star-like object when viewed from Earth, its proximity to the Sun often makes it more difficult to see than Venus. Mercury is tidally or gravitationally locked with the Sun in a 3,2 resonance, as seen relative to the fixed stars, it rotates on its axis exactly three times for every two revolutions it makes around the Sun. As seen from the Sun, in a frame of reference that rotates with the orbital motion, an observer on Mercury would therefore see only one day every two years. Mercurys axis has the smallest tilt of any of the Solar Systems planets, at aphelion, Mercury is about 1.5 times as far from the Sun as it is at perihelion. Mercurys surface appears heavily cratered and is similar in appearance to the Moons, the polar regions are constantly below 180 K. The planet has no natural satellites. Mercury is one of four planets in the Solar System. It is the smallest planet in the Solar System, with a radius of 2,439.7 kilometres. Mercury is also smaller—albeit more massive—than the largest natural satellites in the Solar System, Ganymede, Mercury consists of approximately 70% metallic and 30% silicate material. Mercurys density is the second highest in the Solar System at 5.427 g/cm3, Mercurys density can be used to infer details of its inner structure. Although Earths high density results appreciably from gravitational compression, particularly at the core, Mercury is much smaller, therefore, for it to have such a high density, its core must be large and rich in iron. Geologists estimate that Mercurys core occupies about 55% of its volume, Research published in 2007 suggests that Mercury has a molten core. Surrounding the core is a 500–700 km mantle consisting of silicates, based on data from the Mariner 10 mission and Earth-based observation, Mercurys crust is estimated to be 35 km thick. One distinctive feature of Mercurys surface is the presence of narrow ridges
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Uranus
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Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System, Uranus is similar in composition to Neptune, and both have different bulk chemical composition from that of the larger gas giants Jupiter and Saturn. For this reason, scientists often classify Uranus and Neptune as ice giants to distinguish them from the gas giants, the interior of Uranus is mainly composed of ices and rock. Uranus is the planet whose name is derived from a figure from Greek mythology. Like the other giant planets, Uranus has a system, a magnetosphere. The Uranian system has a unique configuration among those of the planets because its axis of rotation is tilted sideways and its north and south poles, therefore, lie where most other planets have their equators. In 1986, images from Voyager 2 showed Uranus as an almost featureless planet in visible light, observations from Earth have shown seasonal change and increased weather activity as Uranus approached its equinox in 2007. Wind speeds can reach 250 metres per second, like the classical planets, Uranus is visible to the naked eye, but it was never recognised as a planet by ancient observers because of its dimness and slow orbit. Uranus had been observed on many occasions before its recognition as a planet, possibly the earliest known observation was by Hipparchos, who in 128 BCE might have recorded it as a star for his star catalogue that was later incorporated into Ptolemys Almagest. The earliest definite sighting was in 1690 when John Flamsteed observed it at least six times, the French astronomer Pierre Lemonnier observed Uranus at least twelve times between 1750 and 1769, including on four consecutive nights. Sir William Herschel observed Uranus on March 13,1781 from the garden of his house at 19 New King Street in Bath, Somerset, England, Herschel engaged in a series of observations on the parallax of the fixed stars, using a telescope of his own design. Herschel recorded in his journal, In the quartile near ζ Tauri, either Nebulous star or perhaps a comet. On March 17 he noted, I looked for the Comet or Nebulous Star and found that it is a Comet, the sequel has shown that my surmises were well-founded, this proving to be the Comet we have lately observed. Herschel notified the Astronomer Royal, Nevil Maskelyne, of his discovery and received this flummoxed reply from him on April 23,1781, I dont know what to call it. It is as likely to be a planet moving in an orbit nearly circular to the sun as a Comet moving in a very eccentric ellipsis. I have not yet seen any coma or tail to it, although Herschel continued to describe his new object as a comet, other astronomers had already begun to suspect otherwise. Finnish-Swedish astronomer Anders Johan Lexell, working in Russia, was the first to compute the orbit of the new object and its nearly circular orbit led him to a conclusion that it was a planet rather than a comet. Berlin astronomer Johann Elert Bode described Herschels discovery as a star that can be deemed a hitherto unknown planet-like object circulating beyond the orbit of Saturn
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Neptune
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Neptune is the eighth and farthest known planet from the Sun in the Solar System. In the Solar System, it is the fourth-largest planet by diameter, the planet. Neptune is 17 times the mass of Earth and is more massive than its near-twin Uranus. Neptune orbits the Sun once every 164.8 years at a distance of 30.1 astronomical units. It is named after the Roman god of the sea and has the astronomical symbol ♆, Neptune is not visible to the unaided eye and is the only planet in the Solar System found by mathematical prediction rather than by empirical observation. Unexpected changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbit was subject to perturbation by an unknown planet. Neptune was subsequently observed with a telescope on 23 September 1846 by Johann Galle within a degree of the predicted by Urbain Le Verrier. Its largest moon, Triton, was discovered shortly thereafter, though none of the remaining known 14 moons were located telescopically until the 20th century. The planets distance from Earth gives it a small apparent size. Neptune was visited by Voyager 2, when it flew by the planet on 25 August 1989, the advent of the Hubble Space Telescope and large ground-based telescopes with adaptive optics has recently allowed for additional detailed observations from afar. Neptunes composition can be compared and contrasted with the Solar Systems other giant planets, however, its interior, like that of Uranus, is primarily composed of ices and rock, which is why Uranus and Neptune are normally considered ice giants to emphasise this distinction. Traces of methane in the outermost regions in part account for the blue appearance. In contrast to the hazy, relatively featureless atmosphere of Uranus, Neptunes atmosphere has active, for example, at the time of the Voyager 2 flyby in 1989, the planets southern hemisphere had a Great Dark Spot comparable to the Great Red Spot on Jupiter. These weather patterns are driven by the strongest sustained winds of any planet in the Solar System, because of its great distance from the Sun, Neptunes outer atmosphere is one of the coldest places in the Solar System, with temperatures at its cloud tops approaching 55 K. Temperatures at the centre are approximately 5,400 K. Neptune has a faint and fragmented ring system. On both occasions, Galileo seems to have mistaken Neptune for a star when it appeared close—in conjunction—to Jupiter in the night sky, hence. At his first observation in December 1612, Neptune was almost stationary in the sky because it had just turned retrograde that day and this apparent backward motion is created when Earths orbit takes it past an outer planet. Because Neptune was only beginning its yearly cycle, the motion of the planet was far too slight to be detected with Galileos small telescope
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Ceres (dwarf planet)
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Ceres is the largest object in the asteroid belt that lies between the orbits of Mars and Jupiter. Its diameter is approximately 945 kilometers, making it the largest of the planets within the orbit of Neptune. The 33rd-largest known body in the Solar System, it is the dwarf planet within the orbit of Neptune. Composed of rock and ice, Ceres is estimated to approximately one third of the mass of the entire asteroid belt. Ceres is the object in the asteroid belt known to be rounded by its own gravity. From Earth, the apparent magnitude of Ceres ranges from 6.7 to 9.3, Ceres was the first asteroid discovered, by Giuseppe Piazzi at Palermo on 1 January 1801. It was originally considered a planet, but was reclassified as an asteroid in the 1850s after many other objects in similar orbits were discovered. Ceres appears to be differentiated into a core and icy mantle. The surface is probably a mixture of ice and various hydrated minerals such as carbonates. In January 2014, emissions of water vapor were detected from several regions of Ceres and this was unexpected, because large bodies in the asteroid belt typically do not emit vapor, a hallmark of comets. The robotic NASA spacecraft Dawn entered orbit around Ceres on 6 March 2015, pictures with a resolution previously unattained were taken during imaging sessions starting in January 2015 as Dawn approached Ceres, showing a cratered surface. Two distinct bright spots inside a crater were seen in a 19 February 2015 image, on 11 May 2015, NASA released a higher-resolution image showing that, instead of one or two spots, there are actually several. In October 2015, NASA released a true portrait of Ceres made by Dawn. In February 2017, organics were reported to have been detected on Ceres in Ernutet crater, Johann Elert Bode, in 1772, first suggested that an undiscovered planet could exist between the orbits of Mars and Jupiter. Kepler had already noticed the gap between Mars and Jupiter in 1596, the pattern predicted that the missing planet ought to have an orbit with a semi-major axis near 2.8 astronomical units. Although they did not discover Ceres, they found several large asteroids. One of the selected for the search was Giuseppe Piazzi. Before receiving his invitation to join the group, Piazzi discovered Ceres on 1 January 1801 and he was searching for the 87th of the Catalogue of the Zodiacal stars of Mr la Caille, but found that it was preceded by another
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4 Vesta
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Vesta, minor-planet designation 4 Vesta, is one of the largest objects in the asteroid belt, with a mean diameter of 525 kilometres. It was discovered by the German astronomer Heinrich Wilhelm Olbers on 29 March 1807 and is named after Vesta, Vesta is the second-most-massive and second-largest body in the asteroid belt after the dwarf planet Ceres, and it contributes an estimated 9% of the mass of the asteroid belt. It is slightly larger than Pallas, though more massive. Vesta is the last remaining rocky protoplanet of the kind that formed the terrestrial planets, numerous fragments of Vesta were ejected by collisions one and two billion years ago that left two enormous craters occupying much of Vestas southern hemisphere. Debris from these events has fallen to Earth as howardite–eucrite–diogenite meteorites, Vesta is the brightest asteroid visible from Earth. Its maximum distance from the Sun is slightly greater than the distance of Ceres from the Sun. NASAs Dawn spacecraft entered orbit around Vesta on 16 July 2011 for an exploration and left orbit on 5 September 2012 en route to its final destination. Researchers continue to examine data collected by Dawn for additional insights into the formation, Heinrich Olbers discovered Pallas in 1802, the year after the discovery of Ceres. He proposed that the two objects were the remnants of a destroyed planet and these orbital intersections were located in the constellations of Cetus and Virgo. Olbers commenced his search in 1802, and on 29 March 1807 he discovered Vesta in the constellation Virgo—a coincidence, because Ceres, Pallas, and Vesta are not fragments of a larger body. Because the asteroid Juno had been discovered in 1804, this made Vesta the fourth object to be identified in the region that is now known as the asteroid belt, the discovery was announced in a letter addressed to German astronomer Johann H. Schröter dated 31 March. Gauss decided on the Roman virgin goddess of home and hearth, Vesta was the fourth asteroid to be discovered, hence the number 4 in its formal designation. The name Vesta, or national variants thereof, is in use with two exceptions, Greece and China. In Greek, the name adopted was the Hellenic equivalent of Vesta, Hestia, in English, in Chinese, Vesta is called the hearth-god star, 灶神星 zàoshénxīng, in contrast to the goddess Vesta, who goes by her Latin name. Upon its discovery, Vesta was, like Ceres, Pallas, the symbol representing the altar of Vesta with its sacred fire and was designed by Gauss. In Gausss conception, this was drawn, in its modern form, after the discovery of Vesta, no further objects were discovered for 38 years, and the Solar System was thought to have eleven planets. However, in 1845, new asteroids started being discovered at a rapid pace and it soon became clear that it would be impractical to continue inventing new planetary symbols indefinitely, and some of the existing ones proved difficult to draw quickly. That year, the problem was addressed by Benjamin Apthorp Gould, who suggested numbering asteroids in their order of discovery, thus, the fourth asteroid, Vesta, acquired the generic symbol ④
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Pluto
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Pluto is a dwarf planet in the Kuiper belt, a ring of bodies beyond Neptune. It was the first Kuiper belt object to be discovered, Pluto was discovered by Clyde Tombaugh in 1930 and was originally considered to be the ninth planet from the Sun. After 1992, its planethood was questioned following the discovery of objects of similar size in the Kuiper belt. In 2005, Eris, which is 27% more massive than Pluto, was discovered and this led the International Astronomical Union to define the term planet formally in 2006, during their 26th General Assembly. That definition excluded Pluto and reclassified it as a dwarf planet, Pluto is the largest and second-most-massive known dwarf planet in the Solar System and the ninth-largest and tenth-most-massive known object directly orbiting the Sun. It is the largest known trans-Neptunian object by volume but is less massive than Eris, like other Kuiper belt objects, Pluto is primarily made of ice and rock and is relatively small—about one-sixth the mass of the Moon and one-third its volume. It has an eccentric and inclined orbit during which it ranges from 30 to 49 astronomical units or AU from the Sun. This means that Pluto periodically comes closer to the Sun than Neptune, light from the Sun takes about 5.5 hours to reach Pluto at its average distance. Pluto has five moons, Charon, Styx, Nix, Kerberos. Pluto and Charon are sometimes considered a system because the barycenter of their orbits does not lie within either body. The IAU has not formalized a definition for binary dwarf planets, on July 14,2015, the New Horizons spacecraft became the first spacecraft to fly by Pluto. During its brief flyby, New Horizons made detailed measurements and observations of Pluto, on October 25,2016, at 05,48 pm ET, the last bit of data was received from New Horizons from its close encounter with Pluto on July 14,2015. In the 1840s, Urbain Le Verrier used Newtonian mechanics to predict the position of the then-undiscovered planet Neptune after analysing perturbations in the orbit of Uranus. Subsequent observations of Neptune in the late 19th century led astronomers to speculate that Uranuss orbit was being disturbed by another planet besides Neptune, by 1909, Lowell and William H. Pickering had suggested several possible celestial coordinates for such a planet. Lowell and his observatory conducted his search until his death in 1916, unknown to Lowell, his surveys had captured two faint images of Pluto on March 19 and April 7,1915, but they were not recognized for what they were. There are fourteen other known prediscovery observations, with the oldest made by the Yerkes Observatory on August 20,1909. Percivals widow, Constance Lowell, entered into a legal battle with the Lowell Observatory over her late husbands legacy. Tombaughs task was to image the night sky in pairs of photographs, then examine each pair
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Planet Nine
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Planet Nine does not have an official name, and it will not receive one unless its existence is confirmed, typically through optical imaging. Once confirmed, the IAU will certify a name, with priority given to a name proposed by its discoverers. It will likely be a name chosen from Roman or Greek mythology, in their original paper, Batygin and Brown simply referred to the object as perturber, and only in later press releases did they use Planet Nine. They have also used the names Jehoshaphat and George for Planet Nine, Brown has stated, We actually call it Phattie when were just talking to each other. Planet Nine is hypothesized to follow an elliptical orbit around the Sun, with an orbital period of 10. The period with which it goes around the sun is a multiple of the periods of all the furthest Kuiper Belt objects. The high eccentricity of Planet Nines orbit could take it as far away as 1,200 AU at its aphelion. Brown thinks that if Planet Nine is confirmed to exist, a probe could fly by it in as little as 20 years, the planet is estimated to have 10 times the mass and two to four times the diameter of Earth. An object with the diameter as Neptune has not been excluded by previous surveys. An infrared survey by the Wide-field Infrared Survey Explorer in 2009 allowed for a Neptune-sized object beyond 700 AU. Using a metric based on work by Jean-Luc Margot, Brown calculated that only at the smallest size and farthest distance was it on the border of being called a dwarf planet. Brown speculates that the planet is most likely an ejected ice giant, similar in composition to Uranus and Neptune. Attempts to detect planets beyond Neptune by indirect means such as orbital perturbation date back beyond the discovery of Pluto. That could have been another planet, it could have been a star that came close to the Sun, later in 2015, they also proposed that the Centaurs with large semi-major axis are also being pushed around by the putative planet. Gomes argued that a new planet was the probable of the possible explanations. The initial argument for the existence of a planet beyond Neptune was published in 2014 by astronomers Chad Trujillo, in a later work announcing the discovery of several more distant objects Trujillo and Sheppard noted a correlation between the longitude of perihelion and the argument of perihelion of these objects. Those with a longitude of perihelion of 0–120° have arguments of perihelion between 280–360°, and those with longitude of perihelion of 180–340° have argument of perihelion 0–40° and they found a statistical significance of this correlation of 99. 99%. It was a proof of concept simulation that did not obtain a unique orbit for the planet as they state there are many possible orbital configurations the planet could have
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R Doradus
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R Doradus is the name of a red giant Mira variable star in the far-southern constellation Dorado, although visually it appears more closely associated with the constellation Reticulum. Its distance from Earth is 178 ±10 light-years, having a uniform disk diameter of 0.057 ±0.005 arcsec, it is currently believed to be the extrasolar star with the largest apparent size as viewed from Earth. The estimated diameter of R Doradus is 515 ±70 million km or 370 ±50 times the diameter of the Sun, if placed at the centre of the Solar System, the orbit of Mars and most of the main asteroid belt would be contained within the star. With a near-infrared J band magnitude of −2.6, only Betelgeuse at −2.9 is brighter, swinburne Astronomy Online, information about R Doradus Variáveis Binoculares The 3µ spectrum of R Doradus observed with the ISO-SWS
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Betelgeuse
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Betelgeuse, also designated Alpha Orionis, is the ninth-brightest star in the night sky and second-brightest in the constellation of Orion. Distinctly reddish, it is a variable star whose apparent magnitude varies between 0.0 and 1.3, the widest range of any first-magnitude star. Betelgeuse is one of three stars make up the Winter Triangle asterism, and it marks the center of the Winter Hexagon. It would be the brightest star in the sky if the human eye could view all wavelengths of radiation. The star is classified as a red supergiant of spectral type M1-2 and is one of the largest and most luminous stars visible to the naked eye. If Betelgeuse were at the center of the Solar System, its surface would extend past the asteroid belt, wholly engulfing the orbits of Mercury, Venus, Earth, calculations of its mass range from slightly under ten to a little over twenty times that of the Sun. It is calculated to be 640 light-years away, yielding an absolute magnitude of about −6, less than 10 million years old, Betelgeuse has evolved rapidly because of its high mass. Currently in a stage of stellar evolution, the supergiant is expected to explode as a supernova within the next million years. In 1920, Betelgeuse became the first extrasolar star to have the size of its photosphere measured. It is also surrounded by a complex, asymmetric envelope roughly 250 times the size of the star, the angular diameter of Betelgeuse is only exceeded by R Doradus. α Orionis is the stars Bayer designation, the traditional name Betelgeuse is derived from the Arabic إبط الجوزاء Ibṭ al-Jauzā’, meaning the axilla of Orion, or يد الجوزاء Yad al-Jauzā’, meaning the hand of Orion. In 2016, the International Astronomical Union organized a Working Group on Star Names to catalog, the WGSNs first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN, which included Betelgeuse for this star. It is now so entered in the IAU Catalog of Star Names, in the nineteenth century, before modern systems of stellar classification, Angelo Secchi included Betelgeuse as one of the prototypes for his Class III stars. The variation in Betelgeuses brightness was first described in 1836 by Sir John Herschel, from 1836 to 1840, he noticed significant changes in magnitude when Betelgeuse outshone Rigel in October 1837 and again in November 1839. A 10-year quiescent period followed, then in 1849, Herschel noted another short cycle of variability, later observers recorded unusually high maxima with an interval of years, but only small variations from 1957 to 1967. The records of the American Association of Variable Star Observers show a maximum brightness of 0.2 in 1933 and 1942, and a minimum of 1.2, observed in 1927 and 1941. This variability in brightness may explain why Johann Bayer, with the publication of his Uranometria in 1603, from Arctic latitudes, Betelgeuses red colour and higher location in the sky than Rigel meant the Inuit regarded it as brighter, and one local name was Ulluriajjuaq large star. In 1920, Albert Michelson and Francis Pease mounted a 6-meter interferometer on the front of the 2. 5-meter telescope at Mount Wilson Observatory
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Eris (dwarf planet)
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Eris is the most massive and second-largest dwarf planet known in the Solar System. It is also the known body directly orbiting the Sun. It is measured to be 2,326 ±12 kilometers in diameter, Eris is 27% more massive than dwarf planet Pluto, though Pluto is slightly larger by volume. Eris mass is about 0. 27% of the Earths mass, Eris was discovered in January 2005 by a Palomar Observatory-based team led by Mike Brown, and its identity was verified later that year. It is an object and a member of a high-eccentricity population known as the scattered disk. It has one moon, Dysnomia. As of February 2016, its distance from the Sun is 96.3 astronomical units, because Eris appeared to be larger than Pluto, NASA initially described it as the Solar Systems tenth planet. This, along with the prospect of other objects of similar size being discovered in the future, motivated the International Astronomical Union to define the term planet for the first time. Observations of an occultation by Eris in 2010 showed that its diameter was 2,326 ±12 kilometers, very slightly less than Pluto. Eris was discovered by the team of Mike Brown, Chad Trujillo, the discovery was announced on July 29,2005, the same day as Makemake and two days after Haumea, due in part to events that would later lead to controversy about Haumea. Routine observations were taken by the team on October 21,2003, in January 2005, the re-analysis revealed Eriss slow motion against the background stars. Follow-up observations were carried out to make a preliminary determination of Eriss orbit. More observations released in October 2005 revealed that Eris has a moon, observations of Dysnomias orbit permitted scientists to determine the mass of Eris, which in June 2007 they calculated to be ×1022 kg, 27%±2% greater than Plutos. Eris is named after the Greek goddess Eris, a personification of strife, the regular adjectival form of Eris is Eridian. As a result, for a time the object known to the wider public as Xena. Xena was a name used internally by the discovery team. It was inspired by the character of the television series Xena. The discovery team had saved the nickname Xena for the first body they discovered that was larger than Pluto
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Alphard
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Alphard, also designated Alpha Hydrae is the brightest star in the constellation of Hydra. α Hydrae is the stars Bayer designation, the traditional name Alphard is from the Arabic الفرد, the solitary one, there being no other bright stars near it. It was also known as the backbone of the Serpent to the Arabs, in the catalogue of stars in the Calendarium of Al Achsasi Al Mouakket, it was designated Soheil al Fard, which was translated into Latin as Soheil Solitarius, meaning the bright solitary one. In 2016, the International Astronomical Union organized a Working Group on Star Names to catalog, the WGSNs first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN, which included Alphard for this star. It is now so entered in the IAU Catalog of Star Names, the European astronomer Tycho Brahe dubbed it Cor Hydræ, Latin for the heart of Hydra. In Chinese, 星宿, meaning Star, refers to an asterism consisting of Alphard, τ1 Hydrae, τ2 Hydrae, ι Hydrae,26 Hydrae,27 Hydrae, HD82477, consequently, Alphard itself is known as 星宿一, the First Star of Star. In ancient China it formed part of an asterism called the red bird, Alphard has three times the mass of the Sun. Its estimated age is 420 million years and it has evolved away from the sequence to become a giant star with a spectral classification of K3. The angular diameter has been measured using long baseline interferometry, yielding a value of 9.09 ±0.09 milliarcseconds, only beaten in it by Betelgeuse and it has expanded to 50 times the radius of the Sun. Alphards spectrum shows an excess of barium, an element that is normally produced by the s-process of nucleosynthesis. Typically a barium star belongs to a system and the anomalies in abundances are explained by mass transfer from a companion white dwarf star. Precise radial velocity measurements have shown variations in the radial velocities. The oscillations are multi-periodic with periods from several hours up to several days, the short-term oscillations were assumed to be a result of stellar pulsations, similar to the solar ones. A correlation between the variations in the asymmetry of the line profile and the radial velocity has also been found. The multi-periodic oscillations make HD81797 an object of interest for asteroseismologic investigations, Alphard appears on the flag of Brazil, symbolising the state of Mato Grosso do Sul
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Alpha Centauri
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Alpha Centauri is a star system in the Orion Arm of the Milky Way galaxy, and is the closest star system to the Solar System, being 4.37 light-years from the Sun. It consists of three stars, the pair Alpha Centauri A and Alpha Centauri B together with a small and faint red dwarf, Alpha Centauri C, that may be gravitationally bound to the other two. Alpha Centauri A has 1.1 times the mass and 1.519 times the luminosity of the Sun, while Alpha Centauri B is smaller and cooler, at 0.907 times the Suns mass and 0.445 times its visual luminosity. During the pairs 79. 91-year orbit about a common centre, Proxima Centauri is at the slightly smaller distance of 4.24 light-years from the Sun, making it the closest star to the Sun, even though it is not visible to the naked eye. The separation of Proxima from Alpha Centauri AB is about 15,000 astronomical units, α Centauri is the systems Bayer designation. It bore the traditional name Rigil Kentaurus, which is a latinisation of the Arabic name رجل القنطورس Rijl al-Qanṭūris, meaning Foot of the Centaur. Alpha Centauri C was discovered in 1915 by the Scottish astronomer Robert Innes, Director of the Union Observatory in Johannesburg, South Africa, the name is from Latin, meaning nearest of Centaurus. In 2016, the International Astronomical Union organized a Working Group on Star Names to catalog, the WGSN states that in the case of multiple stars the name should be understood to be attributed to the brightest component by visual brightness. The WGSN approved the name Proxima Centauri for Alpha Centauri C on 21 August 2016 and they are now both so entered in the IAU Catalog of Star Names. Alpha Centauri is the given to what appears as a single star to the naked eye. At −0.27 apparent visual magnitude, it is fainter only than Sirius and Canopus, the next-brightest star in the night sky is Arcturus. Alpha Centauri is a system, with its two main stars being Alpha Centauri A and Alpha Centauri B, usually defined to identify them as the different components of the binary α Cen AB. A third companion—Proxima Centauri —has a distance greater than the observed separation between stars A and B and is probably gravitationally associated with the AB system. As viewed from Earth, it is located at a separation of 2. 2° from the two main stars. Proxima Centauri would appear to the eye as a separate star from α Cen AB if it were bright enough to be seen without a telescope. Alpha Centauri AB and Proxima Centauri form a double star. Direct evidence that Proxima Centauri has an orbit typical of binary stars has yet to be found. Together, the three make a triple star system, referred to by double-star observers as the triple star
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Canopus
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Canopus, also designated Alpha Carinae, is the brightest star in the southern constellation of Carina, and the second brightest star in the night-time sky, after Sirius. Canopuss visual magnitude is −0.74, and it has a magnitude of −5.71. Canopus is a giant of spectral type A9, so it is essentially white when seen with the naked eye. It is located in the far southern sky, at a year 2000 declination of −52° 42′ and its name is generally considered to originate from the mythological Canopus, who was a navigator for Menelaus, king of Sparta. In Indian Vedic literature, the star Canopus is associated with the sage Agastya, Agastya, the star, is said to be the cleanser of waters and its rising coincides with the calming of the waters of the Indian Ocean. It is considered the son of Pulasthya, son of Brahma, Canopus was not visible to the mainland ancient Greeks and Romans, it was, however, visible to the ancient Egyptians. Hence Aratus did not write of the star as it remained below the horizon, while Eratosthenes and Ptolemy—observing from Alexandria—did, the Navajo named it Ma’ii Bizò‘. The Bedouin people of the Negev and Sinai also knew it as Suhayl, due to the fact that it disappears below the horizon in those regions, it became associated with a changeable nature, as opposed to always-visible Polaris, which was circumpolar and hence steadfast. It is also referred to by its Arabic name, سهيل, called the Old Man of the South Pole in Chinese, Canopus appears on the medieval Chinese manuscript the Dunhuang star chart, despite not being visible from the Chinese capital of Changan. The Chinese astronomer Yi Xing had journeyed south to chart Canopus, however, it was already mentioned by Sima Qian in the second century BC, drawing on sources from the Warring States period, as the southern counterpart of Sirius. Bright stars were important to the ancient Polynesians for navigation between the islands and atolls of the Pacific Ocean. Low on the horizon, they acted as stellar compasses to assist mariners in charting courses to particular destinations. The Hawaiian people called Canopus Ke Alii-o-kona-i-ka-lewa, The chief of the expanse, it was one of the stars used by Hawaii-loa. The Māori people of New Zealand/Aotearoa had several different names for Canopus, ariki, was known as a solitary star that appeared in the east, prompting people to weep and chant. They also named it Atutahi, Aotahi or Atuatahi, Stand Alone and its solitary nature indicates it is a tapu star, as tapu people are often solitary. Its appearance at the beginning of the Maruaroa season foretells the coming winter, light rays to the south indicate a cold wet winter, food was offered to the star on its appearance. This name has several different mythologies attached to it as well, one story tells of how Atutahi was left outside of the basket representing the Milky Way when Tāne wove it. Another related myth surrounding the star says that Atutahi was the child of Rangi
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Sirius
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Sirius is a star system and the brightest star in the Earths night sky. With a visual apparent magnitude of −1.46, it is almost twice as bright as Canopus, the system has the Bayer designation Alpha Canis Majoris. The distance separating Sirius A from its companion varies between 8.2 and 31.5 AU, Sirius appears bright because of its intrinsic luminosity and its proximity to Earth. At a distance of 2.6 parsecs, as determined by the Hipparcos astrometry satellite, Sirius is gradually moving closer to the Solar System, so it will slightly increase in brightness over the next 60,000 years. After that time its distance will begin to increase and it will become fainter, Sirius A is about twice as massive as the Sun and has an absolute visual magnitude of 1.42. It is 25 times more luminous than the Sun but has a lower luminosity than other bright stars such as Canopus or Rigel. The system is between 200 and 300 million years old and it was originally composed of two bright bluish stars. Sirius is also known colloquially as the Dog Star, reflecting its prominence in its constellation, the brightest star in the night sky, Sirius is recorded in the earliest astronomical records. Every year, it disappears for seventy days before returning to the sky just before sunrise. This occurs at Cairo on 19 July, placing it just prior to the summer solstice, the ancient Greeks observed that the appearance of Sirius heralded the hot and dry summer and feared that it caused plants to wilt, men to weaken, and women to become aroused. Due to its brightness, Sirius would have been noted to twinkle more in the weather conditions of early summer. To Greek observers, this signified certain emanations which caused its malignant influence, anyone suffering its effects was said to be star-struck. It was described as burning or flaming in literature, the season following the stars reappearance came to be known as the dog days. The inhabitants of the island of Ceos in the Aegean Sea would offer sacrifices to Sirius and Zeus to bring cooling breezes, if it rose clear, it would portend good fortune, if it was misty or faint then it foretold pestilence. Coins retrieved from the island from the 3rd century BC feature dogs or stars with emanating rays, ptolemy of Alexandria mapped the stars in Books VII and VIII of his Almagest, in which he used Sirius as the location for the globes central meridian. He depicted it as one of six red-coloured stars, the other five are class M and K stars, such as Arcturus and Betelgeuse. Bright stars were important to the ancient Polynesians for navigation between the islands and atolls of the Pacific Ocean. Low on the horizon, they acted as stellar compasses and they also served as latitude markers, the declination of Sirius matches the latitude of the archipelago of Fiji at 17°S and thus passes directly over the islands each night
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Altair
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Altair, also designated Alpha Aquilae, is the brightest star in the constellation of Aquila and the twelfth brightest star in the night sky. It is currently in the G-cloud—a nearby accumulation of gas and dust known as an interstellar cloud, Altair is an A-type main sequence star with an apparent visual magnitude of 0.77 and is one of the vertices of the asterism known as the Summer Triangle. It is 16.7 light-years from the Sun and is one of the closest stars visible to the naked eye, Altair rotates rapidly, with a velocity at the equator of approximately 286 km/s. This is a significant fraction of the estimated breakup speed of 400 km/s. A study with the Palomar Testbed Interferometer revealed that Altair is not spherical, other interferometric studies with multiple telescopes, operating in the infrared, have imaged and confirmed this phenomenon. α Aquilae is the stars Bayer designation, the traditional name Altair has been used since medieval times. It is an abbreviation of the Arabic phrase النسر الطائر, al-nesr al-ṭā’ir, in 2016, the International Astronomical Union organized a Working Group on Star Names to catalog and standardize proper names for stars. The WGSNs first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN, which included Altair for this star and it is now so entered in the IAU Catalog of Star Names. Along with Beta Aquilae and Gamma Aquilae, Altair forms the line of stars sometimes referred to as the Family of Aquila or Shaft of Aquila. Altair is a main sequence star with approximately 1.8 times the mass of the Sun and 11 times its luminosity. Altair possesses an extremely rapid rate of rotation, it has a period of approximately 9 hours. For comparison, the equator of the Sun requires a more than 25 days for a complete rotation. This rapid rotation forces Altair to be oblate, its diameter is over 20 percent greater than its polar diameter. As a result, it was identified in 2005 as a Delta Scuti variable star and its light curve can be approximated by adding together a number of sine waves, with periods that range between 0.8 and 1.5 hours. It is a source of coronal X-ray emission, with the most active sources of emission being located near the stars equator. This activity may be due to convection cells forming at the cooler equator, the angular diameter of Altair was measured interferometrically by R. Hanbury Brown and his co-workers at Narrabri Observatory in the 1960s. They found a diameter of 3 milliarcseconds, although Hanbury Brown et al. realized that Altair would be rotationally flattened, they had insufficient data to experimentally observe its oblateness. Altair was later observed to be flattened by infrared interferometric measurements made by the Palomar Testbed Interferometer in 1999 and 2000 and this work was published by G. T. van Belle, David R. Ciardi and their co-authors in 2001
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Deneb
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Deneb, also designated Alpha Cygni, is the brightest star in the constellation of Cygnus. It is one of the vertices of the known as the Summer Triangle. It is the 19th brightest star in the sky, with an apparent magnitude of 1.25. A blue-white supergiant, Deneb is also one of the most luminous stars, however, its exact distance has been difficult to calculate, it is estimated to be somewhere between 55,000 and 196,000 times as luminous as the Sun. α Cygni is the stars Bayer designation, the traditional name Deneb is derived from dhaneb, Arabic for tail, from the phrase ذنب الدجاجة Dhanab ad-Dajājah, or tail of the hen. In 2016, the International Astronomical Union organized a Working Group on Star Names to catalog, the WGSNs first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN, which included Deneb for this star. It is now so entered in the IAU Catalog of Star Names, Deneb lies at one vertex of a widely spaced asterism called the Summer Triangle, the other two members of which are the zero-magnitude stars Vega in the constellation Lyra and Altair in Aquila. This formation is the shape of a right triangle, with Deneb located at one of the acute angles. The Summer Triangle is recognizable in the skies for there are few other bright stars in its vicinity. Deneb is also spotted as the tip of the Northern Cross asterism made up of the brightest stars in Cygnus, the others being Beta, Gamma, Delta. It never dips below the horizon at or above 45° north latitude, just grazing the northern horizon at its lowest point at such locations as Minneapolis, Montréal, in the northern hemisphere Deneb is high in the sky during summer evenings. In the southern hemisphere, Deneb is not at all visible south of 45° south parallel, so it just barely rises above the horizon in Tasmania, due to the Earths axial precession, Deneb will become the Pole star at around 9800 AD. Denebs exact distance from the Earth is still rather uncertain, the Gaia satellite should provide distance measurements at least two orders of magnitude more reliable than Hipparcos and resolve many such questions, although it will not measure Deneb itself. Denebs absolute magnitude is estimated as −8.4, placing it among the most luminous stars known. This is towards the end of various published values over the last few decades. Deneb is the most luminous of the stars apparent magnitude brighter than 1.5. It is one of the largest known white stars, Deneb is a bluish-white star of spectral type A2Ia, with a surface temperature of 8,500 kelvin. Since 1943, its spectrum has served as one of the anchor points by which other stars are classified
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Proxima Centauri
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Proxima Centauri or Alpha Centauri C is a red dwarf, a small low-mass star, about 4.25 light-years from the Sun in the constellation of Centaurus. It was discovered in 1915 by the Scottish astronomer Robert Innes, the Director of the Union Observatory in South Africa, with an apparent magnitude of 11.05, it is too faint to be seen with the naked eye. Proxima Centauri forms a component of the Alpha Centauri binary star system, currently with a separation of about 13000+300 −100 AU. Because of Proxima Centauris proximity to Earth, its diameter can be measured directly. It is about one-seventh the diameter of the Sun and it has a mass about an eighth of the Suns mass, and its average density is about 40 times that of the Sun. Although it has a low average luminosity, Proxima is a flare star that undergoes random dramatic increases in brightness because of magnetic activity. The stars magnetic field is created by convection throughout the body. In 2016, the European Southern Observatory announced the discovery of Proxima b and its estimated mass is at least 1.3 times that of the Earth. Previous searches for orbiting companions had ruled out the presence of brown dwarfs, in 1915, the Scottish astronomer Robert Innes, Director of the Union Observatory in Johannesburg, South Africa, discovered a star that had the same proper motion as Alpha Centauri. He suggested that it be named Proxima Centauri and it was also found to be the lowest-luminosity star known at the time. An equally accurate parallax determination of Proxima Centauri was made by American astronomer Harold L. Alden in 1928, in 1951, American astronomer Harlow Shapley announced that Proxima Centauri is a flare star. Examination of past photographic records showed that the star displayed an increase in magnitude on about 8% of the images. The proximity of the star allows for detailed observation of its flare activity, in 1980, the Einstein Observatory produced a detailed X-ray energy curve of a stellar flare on Proxima Centauri. Further observations of flare activity were made with the EXOSAT and ROSAT satellites, Proxima Centauri has since been the subject of study by most X-ray observatories, including XMM-Newton and Chandra. In 2016, the International Astronomical Union organized a Working Group on Star Names to catalogue, the WGSN approved the name Proxima Centauri for this star on 21 August 2016 and it is now so entered in the IAU Catalog of Star Names. Because of Proxima Centauris southern declination, it can only be viewed south of latitude 27° N, Red dwarfs such as Proxima Centauri are far too faint to be seen with the naked eye. Even from Alpha Centauri A or B, Proxima would only be seen as a fifth magnitude star, Proxima Centauri is a red dwarf, because it belongs to the main sequence on the Hertzsprung–Russell diagram and is of spectral class M6. M6 means that it falls in the end of M-type stars
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Alnitak
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Alnitak, designated Zeta Orionis and 50 Orionis, is a multiple star several hundred parsecs from the Sun in the constellation of Orion. It is part of Orions Belt along with Alnilam and Mintaka, the primary star is a hot blue supergiant with an absolute magnitude of -6.0 and is the brightest class O star in the night sky with a visual magnitude of +2.0. It has two bluish 4th magnitude companions, one finely resolved and one only detected interferometrically and spectroscopically, the stars are members of the Orion OB1 association and the Collinder 70 association. Alnitak has been known since antiquity and, as a component of Orions belt, has been of cultural significance. It was reported to be a star by amateur German astronomer George K. Kunowsky in 1819. Alnitak is a star system at the eastern end of Orions belt. The primary is itself a binary, comprising Alnitak Aa. Aa is estimated as being up to 33 times as massive as the Sun and it is some 21,000 times brighter than the sun, with a surface brightness some 50 times greater. It is the brightest star of class O in the night sky, Alnitak B is a 4th magnitude B-type star which orbits Alnitak A every 1500 years. A fourth star, 9th magnitude Alnitak C, has not been confirmed to be part of the Aa-Ab-B group, the Alnitak system is bathed in the nebulosity of IC434. Zeta Orionis is the stars Bayer designation and 50 Orionis its Flamsteed designation, the traditional name Alnitak, alternately spelled Al Nitak or Alnitah, is taken from the Arabic النطاق an-niṭāq, the girdle. In 2016, the International Astronomical Union organized a Working Group on Star Names to catalog, the WGSNs first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN, which included Alnitak for this star. It is now so entered in the IAU Catalog of Star Names, the three belt stars were collectively known by many names in many cultures. Arabic terms include النجاد Al Nijād the Belt, النسك Al Nasak the Line, العلقات Al Alkāt the Golden Grains or Nuts and, in modern Arabic, in Chinese mythology they were known as The Weighing Beam. The belt was also the Three Stars mansion, one of the Twenty-eight mansions of the Chinese constellations and it is one of the western mansions of the White Tiger. In pre-Christian Scandinavia, the belt was known as Friggs Distaff or Freyjas distaff, similarly Jacobs Staff and Peters Staff were European biblical derived terms, as were the Three Magi, or the Three Kings. Väinämöinens Scythe and Kalevan Sword are terms from Finnish mythology, the Seri people of northwestern Mexico call the three belt stars Hapj which consists of three stars, Hap, Haamoja, and Mojet. Hap is in the middle and has been shot by the hunter, in Latin America, this asterism is known as Las Tres Marías or As Três Marias which stand for The Three Marys in Spanish and Portuguese respectively
44.
Hubble Space Telescope
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The Hubble Space Telescope is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. Although not the first space telescope, Hubble is one of the largest and most versatile, with a 2. 4-meter mirror, Hubbles four main instruments observe in the near ultraviolet, visible, and near infrared spectra. Hubbles orbit outside the distortion of Earths atmosphere allows it to take extremely high-resolution images, Hubble has recorded some of the most detailed visible light images ever, allowing a deep view into space and time. Many Hubble observations have led to breakthroughs in astrophysics, such as determining the rate of expansion of the universe. The HST was built by the United States space agency NASA, the Space Telescope Science Institute selects Hubbles targets and processes the resulting data, while the Goddard Space Flight Center controls the spacecraft. Space telescopes were proposed as early as 1923, Hubble was funded in the 1970s, with a proposed launch in 1983, but the project was beset by technical delays, budget problems, and the Challenger disaster. When finally launched in 1990, Hubbles main mirror was found to have been ground incorrectly, the optics were corrected to their intended quality by a servicing mission in 1993. Hubble is the telescope designed to be serviced in space by astronauts. After launch by Space Shuttle Discovery in 1990, five subsequent Space Shuttle missions repaired, upgraded, the fifth mission was canceled on safety grounds following the Columbia disaster. However, after spirited public discussion, NASA administrator Mike Griffin approved the fifth servicing mission, the telescope is operating as of 2017, and could last until 2030–2040. Its scientific successor, the James Webb Space Telescope, is scheduled for launch in 2018, the history of the Hubble Space Telescope can be traced back as far as 1946, to the astronomer Lyman Spitzers paper Astronomical advantages of an extraterrestrial observatory. In it, he discussed the two advantages that a space-based observatory would have over ground-based telescopes. First, the resolution would be limited only by diffraction, rather than by the turbulence in the atmosphere. Second, a telescope could observe infrared and ultraviolet light. Spitzer devoted much of his career to pushing for the development of a space telescope, space-based astronomy had begun on a very small scale following World War II, as scientists made use of developments that had taken place in rocket technology. An orbiting solar telescope was launched in 1962 by the United Kingdom as part of the Ariel space program, oAO-1s battery failed after three days, terminating the mission. It was followed by OAO-2, which carried out observations of stars and galaxies from its launch in 1968 until 1972. The continuing success of the OAO program encouraged increasingly strong consensus within the community that the LST should be a major goal
45.
Galilean moons
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The Galilean moons are the four largest moons of Jupiter—Io, Europa, Ganymede, and Callisto. They were first seen by Galileo Galilei in January 1610, and they are the first objects found to orbit another planet. Their names derive from the lovers of Zeus and they are among the largest objects in the Solar System with the exception of the Sun and the eight planets, with radii larger than any of the dwarf planets. Ganymede is the largest moon in the Solar System, and is bigger than the planet Mercury. The three inner moons—Io, Europa, and Ganymede—are in a 4,2,1 orbital resonance with each other, because of their much smaller size, and therefore weaker self-gravitation, all of Jupiters remaining moons have irregular forms rather than a spherical shape. The Galilean moons were discovered in either 1609 or 1610 when Galileo made improvements to his telescope, galileos discovery showed the importance of the telescope as a tool for astronomers by proving that there were objects in space that cannot be seen by the naked eye. Galileo initially named his discovery the Cosmica Sidera, but the names that eventually prevailed were chosen by Simon Marius. Marius discovered the moons independently at the time as Galileo, and gave them their present names. As a result of improvements Galileo Galilei made to the telescope, with a capability of 20×. This allowed Galilei to discover in either December 1609 or January 1610 what came to be known as the Galilean moons, on January 7,1610, Galileo wrote a letter containing the first mention of Jupiters moons. At the time, he saw three of them, and he believed them to be fixed stars near Jupiter. He continued to observe these celestial orbs from January 8 to March 2,1610, in these observations, he discovered a fourth body, and also observed that the four were not fixed stars, but rather were orbiting Jupiter. Galileos discovery proved the importance of the telescope as a tool for astronomers by showing that there were objects in space to be discovered that until then had remained unseen by the naked eye, nevertheless, Galileo accepted the Copernican theory. In 1605, Galileo had been employed as a tutor for Cosimo de Medici. In 1609, Cosimo became Grand Duke Cosimo II of Tuscany, Galileo, seeking patronage from his now-wealthy former student and his powerful family, used the discovery of Jupiters moons to gain it. Galileo asked whether he should name the moons the Cosmian Stars, after Cosimo alone, or the Medician Stars, the secretary replied that the latter name would be best. On March 12,1610, Galileo wrote his letter to the Duke of Tuscany. Behold, therefore, four stars reserved for your illustrious name, make their journeys and orbits with a marvelous speed around the star of Jupiter
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Callisto (moon)
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Callisto /kəˈlɪstoʊ/ is the second-largest moon of Jupiter, after Ganymede. It is the third-largest moon in the Solar System and the largest object in the Solar System not to be properly differentiated, Callisto was discovered in 1610 by Galileo Galilei. At 4821 km in diameter, Callisto has about 99% the diameter of the planet Mercury and it is the fourth Galilean moon of Jupiter by distance, with an orbital radius of about 1883000 km. It is not in an orbital resonance like the three other Galilean satellites—Io, Europa, and Ganymede—and is thus not appreciably tidally heated. Callistos rotation is locked to its orbit around Jupiter, so that the same hemisphere always faces inward. It is less affected by Jupiters magnetosphere than the inner satellites because of its more remote orbit. Callisto is composed of equal amounts of rock and ices, with a density of about 1.83 g/cm3. Compounds detected spectroscopically on the surface water ice, carbon dioxide, silicates. Investigation by the Galileo spacecraft revealed that Callisto may have a small silicate core, the surface of Callisto is the oldest and most heavily cratered in the Solar System. Its surface is covered with impact craters. Prominent surface features include multi-ring structures, variously shaped impact craters, at a small scale, the surface is varied and made up of small, sparkly frost deposits at the tips of high spots, surrounded by a low-lying, smooth blanket of dark material. The absolute ages of the landforms are not known, Callisto is surrounded by an extremely thin atmosphere composed of carbon dioxide and probably molecular oxygen, as well as by a rather intense ionosphere. Callisto is thought to have formed by accretion from the disk of the gas. Callistos gradual accretion and the lack of tidal heating meant that not enough heat was available for rapid differentiation, the likely presence of an ocean within Callisto leaves open the possibility that it could harbor life. However, conditions are thought to be less favorable than on nearby Europa, various space probes from Pioneers 10 and 11 to Galileo and Cassini have studied Callisto. Because of its low levels, Callisto has long been considered the most suitable place for a human base for future exploration of the Jovian system. Callisto was discovered by Galileo in January 1610, along three other large Jovian moons—Ganymede, Io, and Europa. Callisto is named one of Zeuss many lovers in Greek mythology
47.
Elongation (astronomy)
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In astronomy, a planets elongation is the angle between the Sun and the planet, with Earth as the reference point. The greatest elongation of a given planet occurs when this inner planet’s position, when a planet is at its greatest elongation, it is farthest from the Sun as viewed from Earth, so its view is also best at that point. When an inferior planet is visible after sunset, it is near its greatest eastern elongation, when an inferior planet is visible before sunrise, it is near its greatest western elongation. The value of the greatest elongation, for Mercury, is between 18° and 28°, and for Venus between 45° and 47° and this value varies because the orbits of the planets are elliptical, rather than perfect circles. Another minor contributor to this inconsistency is orbital inclination, each orbit is in a slightly different plane. Refer to astronomical tables and websites such as heavens-above to see when the planets reach their next maximum elongations, greatest elongations of a planet happen periodically, with a greatest eastern elongation followed by a greatest western elongation, and vice versa. The period depends on the angular velocity of Earth and the planet. The time it takes to complete this period is the period of the planet. Let T be the period, ω be the angular velocity, ωe Earths angular velocity. For example, Venuss year is 225 days, and Earths is 365 days, thus Venuss synodic period, which gives the time between two subsequent eastern greatest elongations, is 584 days. These values are approximate, because the planets do not have circular, coplanar orbits. Superior planets, dwarf planets and asteroids undergo a different cycle, in other words, as seen from an observer on the superior planet at opposition, the Earth appears at inferior conjunction with the Sun. Technically, the moment of opposition is slightly different from the moment of maximum elongation. For example, Pluto, whose orbit is inclined to the Earths orbital plane. All superior planets are most easily visible at their oppositions because they are near their closest approach to Earth and are also above the horizon all night, the variation in magnitude caused by changes in elongation are greater the closer the planets orbit is to the Earths. As one moves out, the difference in magnitude caused by the difference in elongation gradually fall. Since asteroids travel in a not much larger than the Earths. Sometimes elongation may instead refer to the distance of a moon of another planet from its central planet
