1.
Magnitude (astronomy)
–
In astronomy, magnitude is a logarithmic measure of the brightness of an object, measured in a specific wavelength or passband, usually in the visible or near-infrared spectrum. An imprecise but systematic determination of the magnitude of objects was introduced in ancient times by Hipparchus, astronomers use two different definitions of magnitude, apparent magnitude and absolute magnitude. This distance is 10 parsecs for stars and 1 astronomical unit for planets, a minor planets size is typically estimated based on its absolute magnitude in combination with its presumed albedo. The brighter an object appears, the lower the value of its magnitude, with the brightest objects reaching negative values. The Sun has an apparent magnitude of −27, the full moon −13, the brightest planet Venus measures −5, and Sirius, an apparent magnitude can also be assigned to man-made objects in Earth orbit. The brightest satellite flares are ranked at −9, and the International Space Station, ISS, the scale is logarithmic, and defined such that each step of one magnitude changes the brightness by a factor of the fifth root of 100, or approximately 2.512. For example, a magnitude 1 star is exactly a hundred times brighter than a magnitude 6 star, the magnitude system dates back roughly 2000 years to the Greek astronomer Hipparchus who classified stars by their apparent brightness, which they saw as size. To the unaided eye, a prominent star such as Sirius or Arcturus appears larger than a less prominent star such as Mizar. For all the other Stars, which are seen by the Help of a Telescope. Note that the brighter the star, the smaller the magnitude, Bright first magnitude stars are 1st-class stars, the system was a simple delineation of stellar brightness into six distinct groups but made no allowance for the variations in brightness within a group. He concluded that first magnitude stars measured 2 arc minutes in apparent diameter, with second through sixth magnitude stars measuring 1 1⁄2′, 1 1⁄12′, 3⁄4′, 1⁄2′, the development of the telescope showed that these large sizes were illusory—stars appeared much smaller through the telescope. However, early telescopes produced a spurious disk-like image of a star that was larger for brighter stars, early photometric measurements demonstrated that first magnitude stars are about 100 times brighter than sixth magnitude stars. Thus in 1856 Norman Pogson of Oxford proposed that a scale of 5√100 ≈2.512 be adopted between magnitudes, so five magnitude steps corresponded precisely to a factor of 100 in brightness. Every interval of one magnitude equates to a variation in brightness of 5√100 or roughly 2.512 times. Consequently, a first magnitude star is about 2.5 times brighter than a second star,2.52 brighter than a third magnitude star,2.53 brighter than a fourth magnitude star. This is the modern system, which measures the brightness, not the apparent size. Using this logarithmic scale, it is possible for a star to be brighter than “first class”, so Arcturus or Vega are magnitude 0, and Sirius is magnitude −1.46. As mentioned above, the scale appears to work in reverse, the larger the negative value, the brighter
2.
65 Cybele
–
65 Cybele is one of the largest asteroids in the Solar System and is located in the outer asteroid belt. It gives its name to the Cybele family of asteroids that orbit outward from the Sun from the 2,1 orbital resonance with Jupiter, Cybele is a X-type asteroid, meaning that it is dark in color and carbonaceous in composition. It was discovered in 1861 by Ernst Tempel and named after Cybele, Cybele was discovered on March 8,1861, by Ernst Tempel from the Marseilles Observatory. A minor controversy arose from its naming process, Tempel had awarded the honour of naming the asteroid to Carl August von Steinheil in recognition of his achievements in telescope production. Von Steinheil elected to name it Maximiliana after the reigning monarch Maximilian II of Bavaria, at the time, asteroids were conventionally given classical names, and a number of astronomers protested this contemporary appellation. The name Cybele was chosen instead, referring to the Phrygian goddess of the earth, the first Cybelian stellar occultation was observed on October 17,1979 in the Soviet Union. The asteroid appeared to have a shape, with the longest chord being measured as 245 km. During the same 1979 occultation, a hint of a possible 11 km wide satellite was detected, the diameter from IRAS observations in 1983 is 237±4 km. In 1985, Green estimated Cybele to have a diameter of 330 km, in 2004, Müller estimated Cybele using thermophysical modelling to have dimensions of 302×290×232 km. In 2007, David Weintraub suggested that it might be in hydrostatic equilibrium, examination of the asteroids infrared spectrum shows an absorption feature that is similar to the one present in the spectrum of 24 Themis. This can be explained by the presence of water ice, the asteroid may be covered in a layer of fine silicate dust mixed with small amounts of water-ice and organic solids. On August 24,2008, Cybele occulted 2UCAC24389317, a 12. 7-magnitude star in the constellation Ophiuchus which showed a long axis of at least 294 km, on October 11,2009, Cybele occulted a 13. 4-magnitude star in the constellation Aquarius. Asteroids with Satellites, Robert Johnston, johnstonsarchive. net 65 Cybele at the JPL Small-Body Database Discovery · Orbit diagram · Orbital elements · Physical parameters
3.
Celestial object
–
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
4.
Brightness
–
Brightness is an attribute of visual perception in which a source appears to be radiating or reflecting light. In other words, brightness is the perception elicited by the luminance of a visual target and it is not necessarily proportional to luminance. This is a subjective attribute/property of an object being observed and one of the color appearance parameters of color appearance models, brightness refers to an absolute term and should not be confused with Lightness. The adjective bright derives from an Old English beorht with the same meaning via metathesis giving Middle English briht, the word is from a Common Germanic *berhtaz, ultimately from a PIE root with a closely related meaning, *bhereg- white, bright. Brightness was formerly used as a synonym for the term luminance. As defined by the US Federal Glossary of Telecommunication Terms, brightness should now be used only for non-quantitative references to physiological sensations and perceptions of light, a given target luminance can elicit different perceptions of brightness in different contexts, see, for example, Whites illusion. With regard to stars, brightness is quantified as apparent magnitude, brightness is, at least in some respects, the antonym of darkness. The United States Federal Trade Commission has assigned a meaning to brightness when applied to lamps. When appearing on light bulb packages, brightness means Luminous flux, Luminous flux is the total amount of light coming from a source, such as a lighting device. Luminance, the meaning of brightness, is the amount of light per solid angle coming from an area. The table below shows the ways of indicating the amount of light. The term brightness is used in discussions of sound timbres. Luma Luminance Luminosity Media related to brightness at Wikimedia Commons Poyntons Color FAQ
5.
Logarithmic scale
–
A logarithmic scale is a nonlinear scale used when there is a large range of quantities. Common uses include the strength, sound loudness, light intensity. It is based on orders of magnitude, rather than a linear scale. In particular our sense of hearing perceives equal ratios of frequencies as equal differences in pitch, the top left graph is linear in the X and Y axis, and the Y-axis ranges from 0 to 10. A base-10 log scale is used for the Y axis of the left graph. The top right graph uses a scale for just the X axis. A slide rule has logarithmic scales, and nomograms often employ logarithmic scales, the geometric mean of two numbers is midway between the numbers. Before the advent of graphics, logarithmic graph paper was a commonly used scientific tool. If both the vertical and horizontal axes of a plot are scaled logarithmically, the plot is referred to as a log–log plot, if only the ordinate or abscissa is scaled logarithmically, the plot is referred to as a semi-logarithmic plot. Bit Byte Decade John Napier Level Logarithm Logarithmic mean Preferred number Dehaene, Stanislas, Izard, Véronique, Spelke, Elizabeth, Pica, distinct intuitions of the number scale in Western and Amazonian indigene cultures. American Association for the Advancement of Science, why using logarithmic scale to display share prices
6.
Ultraviolet
–
Ultraviolet is an electromagnetic radiation with a wavelength from 10 nm to 400 nm, shorter than that of visible light but longer than X-rays. UV radiation constitutes about 10% of the light output of the Sun. It is also produced by electric arcs and specialized lights, such as lamps, tanning lamps. Consequently, the effects of UV are greater than simple heating effects. Suntan, freckling and sunburn are familiar effects of over-exposure, along with risk of skin cancer. Living things on dry land would be damaged by ultraviolet radiation from the Sun if most of it were not filtered out by the Earths atmosphere. More-energetic, shorter-wavelength extreme UV below 121 nm ionizes air so strongly that it is absorbed before it reaches the ground, Ultraviolet is also responsible for the formation of bone-strengthening vitamin D in most land vertebrates, including humans. The UV spectrum thus has both beneficial and harmful to human health. Ultraviolet rays are invisible to most humans, the lens in a human eye ordinarily filters out UVB frequencies or higher, and humans lack color receptor adaptations for ultraviolet rays. Under some conditions, children and young adults can see ultraviolet down to wavelengths of about 310 nm, near-UV radiation is visible to some insects, mammals, and birds. Small birds have a fourth color receptor for ultraviolet rays, this gives birds true UV vision, reindeer use near-UV radiation to see polar bears, who are poorly visible in regular light because they blend in with the snow. UV also allows mammals to see urine trails, which is helpful for animals to find food in the wild. The males and females of some species look identical to the human eye. Ultraviolet means beyond violet, violet being the color of the highest frequencies of visible light, Ultraviolet has a higher frequency than violet light. He called them oxidizing rays to emphasize chemical reactivity and to them from heat rays. The terms chemical and heat rays were eventually dropped in favour of ultraviolet and infrared radiation, in 1878 the effect of short-wavelength light on sterilizing bacteria was discovered. By 1903 it was known the most effective wavelengths were around 250 nm, in 1960, the effect of ultraviolet radiation on DNA was established. The discovery of the ultraviolet radiation below 200 nm, named vacuum ultraviolet because it is absorbed by air, was made in 1893 by the German physicist Victor Schumann
7.
Visible spectrum
–
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called light or simply light. A typical human eye will respond to wavelengths from about 390 to 700 nm, in terms of frequency, this corresponds to a band in the vicinity of 430–770 THz. The spectrum does not, however, contain all the colors that the human eyes, unsaturated colors such as pink, or purple variations such as magenta, are absent, for example, because they can be made only by a mix of multiple wavelengths. Colors containing only one wavelength are called pure colors or spectral colors. Visible wavelengths pass through the window, the region of the electromagnetic spectrum that allows wavelengths to pass largely unattenuated through the Earths atmosphere. An example of this phenomenon is that clean air scatters blue light more than red wavelengths, the optical window is also referred to as the visible window because it overlaps the human visible response spectrum. The near infrared window lies just out of the vision, as well as the Medium Wavelength IR window. In the 13th century, Roger Bacon theorized that rainbows were produced by a process to the passage of light through glass or crystal. In the 17th century, Isaac Newton discovered that prisms could disassemble and reassemble white light and he was the first to use the word spectrum in this sense in print in 1671 in describing his experiments in optics. The result is red light is bent less sharply than violet as it passes through the prism. Newton divided the spectrum into seven named colors, red, orange, yellow, green, blue, indigo, the human eye is relatively insensitive to indigos frequencies, and some people who have otherwise-good vision cannot distinguish indigo from blue and violet. For this reason, some commentators, including Isaac Asimov, have suggested that indigo should not be regarded as a color in its own right. However, the evidence indicates that what Newton meant by indigo, comparing Newtons observation of prismatic colors to a color image of the visible light spectrum shows that indigo corresponds to what is today called blue, whereas blue corresponds to cyan. In the 18th century, Goethe wrote about optical spectra in his Theory of Colours, Goethe used the word spectrum to designate a ghostly optical afterimage, as did Schopenhauer in On Vision and Colors. Goethe argued that the spectrum was a compound phenomenon. Where Newton narrowed the beam of light to isolate the phenomenon, Goethe observed that a wider aperture produces not a spectrum but rather reddish-yellow, the spectrum appears only when these edges are close enough to overlap. Young was the first to measure the wavelengths of different colors of light, the connection between the visible spectrum and color vision was explored by Thomas Young and Hermann von Helmholtz in the early 19th century
8.
Infrared
–
It extends from the nominal red edge of the visible spectrum at 700 nanometers, to 1000000 nm. Most of the radiation emitted by objects near room temperature is infrared. Like all EMR, IR carries radiant energy, and behaves both like a wave and like its quantum particle, the photon, slightly more than half of the total energy from the Sun was eventually found to arrive on Earth in the form of infrared. The balance between absorbed and emitted infrared radiation has an effect on Earths climate. Infrared radiation is emitted or absorbed by molecules when they change their rotational-vibrational movements and it excites vibrational modes in a molecule through a change in the dipole moment, making it a useful frequency range for study of these energy states for molecules of the proper symmetry. Infrared spectroscopy examines absorption and transmission of photons in the infrared range, Infrared radiation is used in industrial, scientific, and medical applications. Night-vision devices using active near-infrared illumination allow people or animals to be observed without the observer being detected, Infrared thermal-imaging cameras are used to detect heat loss in insulated systems, to observe changing blood flow in the skin, and to detect overheating of electrical apparatuses. Thermal-infrared imaging is used extensively for military and civilian purposes, military applications include target acquisition, surveillance, night vision, homing, and tracking. Humans at normal body temperature radiate chiefly at wavelengths around 10 μm, Infrared radiation extends from the nominal red edge of the visible spectrum at 700 nanometers to 1 mm. This range of wavelengths corresponds to a range of approximately 430 THz down to 300 GHz. Below infrared is the portion of the electromagnetic spectrum. Sunlight, at a temperature of 5,780 kelvins, is composed of near thermal-spectrum radiation that is slightly more than half infrared. At zenith, sunlight provides an irradiance of just over 1 kilowatt per square meter at sea level, of this energy,527 watts is infrared radiation,445 watts is visible light, and 32 watts is ultraviolet radiation. Nearly all the radiation in sunlight is near infrared, shorter than 4 micrometers. On the surface of Earth, at far lower temperatures than the surface of the Sun, almost all thermal radiation consists of infrared in mid-infrared region, much longer than in sunlight. Of these natural thermal radiation processes only lightning and natural fires are hot enough to produce much visible energy, thermal infrared radiation also has a maximum emission wavelength, which is inversely proportional to the absolute temperature of object, in accordance with Wiens displacement law. Therefore, the band is often subdivided into smaller sections. Due to the nature of the blackbody radiation curves, typical hot objects, such as exhaust pipes, the three regions are used for observation of different temperature ranges, and hence different environments in space
9.
Passband
–
A passband is the range of frequencies or wavelengths that can pass through a filter. For example, a radio receiver contains a filter to select the frequency of the desired radio signal out of all the radio waves picked up by its antenna. The passband of a receiver is the range of frequencies it can receive, a bandpass-filtered signal, is known as a bandpass signal, in contrast to a baseband signal. In telecommunications, optics, and acoustics, a passband is the portion of the spectrum that is transmitted by some filtering device. In other words, it is a band of frequencies which passes through some filter or set of filters, the accompanying figure shows a schematic of a waveform being filtered by a bandpass filter consisting of a highpass and a lowpass filter. Radio receivers generally include a tunable band-pass filter with a passband that is enough to accommodate the bandwidth of the radio signal transmitted by a single station. There are two categories of digital communication transmission methods, baseband and passband. In baseband transmission, line coding is utilized, resulting in a train or pulse amplitude modulated signal. This is typically used over non-filtered wires such as fiber optical cables and short-range copper links, for example, V.29, V.35, IEEE802.3, SONET/SDH. In passband transmission, digital methods are employed so that only a limited frequency range is used in some bandpass filtered channel. Passband transmission is utilized in wireless communication and in bandpass filtered channels such as POTS lines. It also allows for frequency-division multiplexing, the digital bit stream is converted first into an equivalent baseband signal, and then to a RF signal. On the receiver side a demodulator is used to detect the signal, a combined equipment for modulation and demodulation is called a modem. In general, there is a relationship between the width of a filters passband and the time required for the filter to respond to new inputs. This is a consequence of the mathematics of Fourier analysis, the limiting frequencies of a passband are defined as those at which the relative intensity or power decreases to a specified fraction of the maximum intensity or power. This decrease in power is often specified to be the half-power points, the difference between the limiting frequencies is called the bandwidth, and is expressed in hertz. The related term bandpass is an adjective that describes a type of filter or filtering process, it is confused with passband. The two words are both words that follow the English rules of formation, the primary meaning is the latter part of the compound
10.
Vega
–
It is relatively close at only 25 light-years from the Sun, and, together with Arcturus and Sirius, one of the most luminous stars in the Suns neighborhood. Vega has been studied by astronomers, leading it to be termed “arguably the next most important star in the sky after the Sun. ”Vega was the northern pole star around 12,000 BC. Vega was the first star other than the Sun to be photographed and it was one of the first stars whose distance was estimated through parallax measurements. Vega has served as the baseline for calibrating the photometric brightness scale, Vega has an unusually low abundance of the elements with a higher atomic number than that of helium. Vega is also a star that varies slightly in brightness. It is rotating rapidly with a velocity of 274 km/s at the equator and this is causing the equator to bulge outward because of centrifugal effects, and, as a result, there is a variation of temperature across the stars photosphere that reaches a maximum at the poles. From Earth, Vega is being observed from the direction of one of these poles, based on an observed excess emission of infrared radiation, Vega appears to have a circumstellar disk of dust. This dust is likely to be the result of collisions between objects in a debris disk, which is analogous to the Kuiper belt in the Solar System. Stars that display an infrared excess because of dust emission are termed Vega-like stars, α Lyrae is the stars Bayer designation. The traditional name Vega comes from a transliteration of the Arabic word wāqi‘ meaning falling or landing, via the phrase an-nasr al-wāqi‘. 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 Vega for this star. It is now so entered in the IAU Catalog of Star Names, astrophotography, the photography of celestial objects, began in 1840 when John William Draper took an image of the Moon using the daguerreotype process. On July 17,1850, Vega became the first star to be photographed, when it was imaged by William Bond and John Adams Whipple at the Harvard College Observatory, also with a daguerreotype. Henry Draper took the first photograph of a spectrum in August 1872 when he took an image of Vega. Similar lines had already identified in the spectrum of the Sun. In 1879, William Huggins used photographs of the spectra of Vega and these were later identified as lines from the Hydrogen Balmer series. Since 1943, the spectrum of this star has served as one of the anchor points by which other stars are classified. The distance to Vega can be determined by measuring its parallax shift against the stars as the Earth orbits the Sun
11.
Sirius
–
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
12.
Hellenistic Greece
–
During the Hellenistic period the importance of Greece proper within the Greek-speaking world declined sharply. The great centers of Hellenistic culture were Alexandria and Antioch, capitals of Ptolemaic Egypt, cities such as Pergamon, Ephesus, Rhodes and Seleucia were also important, and increasing urbanization of the Eastern Mediterranean was characteristic of the time. The quests of Alexander had a number of consequences for the Greek city-states and it greatly widened the horizons of the Greeks, making the endless conflicts between the cities which had marked the 5th and 4th centuries BC seem petty and unimportant. It led to a steady emigration, particularly of the young and ambitious, the Greeks valued their local independence too much to consider actual unification, but they made several attempts to form federations through which they could hope to reassert their independence. Following Alexanders death a struggle for power broke out among his generals, which resulted in the break-up of his empire, Macedon fell to Cassander, son of Alexanders leading general Antipater, who after several years of warfare made himself master of most of the rest of Greece. He founded a new Macedonian capital at Thessaloniki and was generally a constructive ruler, Cassanders power was challenged by Antigonus, ruler of Anatolia, who promised the Greek cities that he would restore their freedom if they supported him. This led to successful revolts against Cassanders local rulers, in 307 BC, Antigonuss son Demetrius captured Athens and restored its democratic system, which had been suppressed by Alexander. But in 301 BC a coalition of Cassander and the other Hellenistic kings defeated Antigonus at the Battle of Ipsus, after Cassanders death in 298 BC, however, Demetrius seized the Macedonian throne and gained control of most of Greece. He was defeated by a coalition of Greek rulers in 285 BC. Lysimachus was in turn defeated and killed in 280 BC, the Macedonian throne then passed to Demetriuss son Antigonus II, who also defeated an invasion of the Greek lands by the Gauls, who at this time were living in the Balkans. The battle against the Gauls united the Antigonids of Macedon and the Seleucids of Antioch, an alliance which was directed against the wealthiest Hellenistic power. Antigonus II ruled until his death in 239 BC, and his family retained the Macedonian throne until it was abolished by the Romans in 146 BC. Their control over the Greek city states was intermittent, however, since other rulers, particularly the Ptolemies, Sparta also remained independent, but generally refused to join any league. In 267 BC, Ptolemy II persuaded the Greek cities to revolt against Antigonus, in became the Chremonidian War. The cities were defeated and Athens lost her independence and her democratic institutions, the Aetolian League was restricted to the Peloponnese, but on being allowed to gain control of Thebes in 245 BC became a Macedonian ally. This marked the end of Athens as a actor, although it remained the largest, wealthiest and most cultivated city in Greece. In 255 BC, Antigonus defeated the Egyptian fleet at Cos and brought the Aegean islands, except Rhodes, in spite of their decreased political power and autonomy, the Greek city state or polis continued to be the basic form of political and social organization in Greece. Classical city states such as Athens and Ephesus grew and even thrived in this period, the Aetolians and the Achaeans developed strong federal states or leagues, which were governed by councils of city representatives and assemblies of league citizens
13.
Human
–
Modern humans are the only extant members of Hominina tribe, a branch of the tribe Hominini belonging to the family of great apes. Several of these hominins used fire, occupied much of Eurasia and they began to exhibit evidence of behavioral modernity around 50,000 years ago. In several waves of migration, anatomically modern humans ventured out of Africa, the spread of humans and their large and increasing population has had a profound impact on large areas of the environment and millions of native species worldwide. Humans are uniquely adept at utilizing systems of communication for self-expression and the exchange of ideas. Humans create complex structures composed of many cooperating and competing groups, from families. Social interactions between humans have established a wide variety of values, social norms, and rituals. These human societies subsequently expanded in size, establishing various forms of government, religion, today the global human population is estimated by the United Nations to be near 7.5 billion. In common usage, the word generally refers to the only extant species of the genus Homo—anatomically and behaviorally modern Homo sapiens. In scientific terms, the meanings of hominid and hominin have changed during the recent decades with advances in the discovery, there is also a distinction between anatomically modern humans and Archaic Homo sapiens, the earliest fossil members of the species. The English adjective human is a Middle English loanword from Old French humain, ultimately from Latin hūmānus, the words use as a noun dates to the 16th century. The native English term man can refer to the species generally, the species binomial Homo sapiens was coined by Carl Linnaeus in his 18th century work Systema Naturae. The generic name Homo is a learned 18th century derivation from Latin homō man, the species-name sapiens means wise or sapient. Note that the Latin word homo refers to humans of either gender, the genus Homo evolved and diverged from other hominins in Africa, after the human clade split from the chimpanzee lineage of the hominids branch of the primates. The closest living relatives of humans are chimpanzees and gorillas, with the sequencing of both the human and chimpanzee genome, current estimates of similarity between human and chimpanzee DNA sequences range between 95% and 99%. The gibbons and orangutans were the first groups to split from the leading to the humans. The splitting date between human and chimpanzee lineages is placed around 4–8 million years ago during the late Miocene epoch, during this split, chromosome 2 was formed from two other chromosomes, leaving humans with only 23 pairs of chromosomes, compared to 24 for the other apes. There is little evidence for the divergence of the gorilla, chimpanzee. Each of these species has been argued to be an ancestor of later hominins
14.
Visual perception
–
Visual perception is the ability to interpret the surrounding environment using light in the visible spectrum reflected by the objects in the environment. The resulting perception is known as visual perception, eyesight, sight. The visual system in animals allows individuals to assimilate information from their surroundings, the act of seeing starts when the cornea and then the lens of the eye focuses light from its surroundings onto a light-sensitive membrane in the back of the eye, called the retina. The retina is actually part of the brain that is isolated to serve as a transducer for the conversion of light into neuronal signals. These signals are processed via complex feedforward and feedback processes by different parts of the brain, signals from the retina can also travel directly from the retina to the superior colliculus. The perception of objects and the totality of the scene is accomplished by the visual association cortex. The visual association cortex combines all sensory information perceived by the cortex which contains thousands of modules that are part of modular neural networks. The neurons in the cortex send axons to the extrastriate cortex. The major problem in perception is that what people see is not simply a translation of retinal stimuli. Thus people interested in perception have long struggled to explain what visual processing does to create what is actually seen, there were two major ancient Greek schools, providing a primitive explanation of how vision is carried out in the body. The first was the theory which maintained that vision occurs when rays emanate from the eyes and are intercepted by visual objects. If an object was seen directly it was by means of coming out of the eyes. This theory was championed by scholars like Euclid and Ptolemy and their followers, the second school advocated the so-called intro-mission approach which sees vision as coming from something entering the eyes representative of the object. Plato makes this assertion in his dialogue Timaeus, as does Aristotle, alhazen carried out many investigations and experiments on visual perception, extended the work of Ptolemy on binocular vision, and commented on the anatomical works of Galen. Leonardo da Vinci is believed to be the first to recognize the special qualities of the eye. He wrote The function of the human eye, was described by a large number of authors in a certain way. But I found it to be completely different and his main experimental finding was that there is only a distinct and clear vision at the line of sight—the optical line that ends at the fovea. Although he did not use these words literally he actually is the father of the distinction between foveal and peripheral vision
15.
Telescope
–
A telescope is an optical instrument that aids in the observation of remote objects by collecting electromagnetic radiation. The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s and they found use in both terrestrial applications and astronomy. Within a few decades, the telescope was invented, which used mirrors to collect. In the 20th century many new types of telescopes were invented, including radio telescopes in the 1930s, the word telescope now refers to a wide range of instruments capable of detecting different regions of the electromagnetic spectrum, and in some cases other types of detectors. The word telescope was coined in 1611 by the Greek mathematician Giovanni Demisiani for one of Galileo Galileis instruments presented at a banquet at the Accademia dei Lincei, in the Starry Messenger, Galileo had used the term perspicillum. The earliest recorded working telescopes were the telescopes that appeared in the Netherlands in 1608. Their development is credited to three individuals, Hans Lippershey and Zacharias Janssen, who were spectacle makers in Middelburg, Galileo heard about the Dutch telescope in June 1609, built his own within a month, and improved upon the design in the following year. Also in 1609, Thomas Harriot became the first person known to point a telescope skyward in order to make observations of a celestial object. The idea that the objective, or light-gathering element, could be a mirror instead of a lens was being investigated soon after the invention of the refracting telescope. The potential advantages of using parabolic mirrors—reduction of spherical aberration and no chromatic aberration—led to many proposed designs, in 1668, Isaac Newton built the first practical reflecting telescope, of a design which now bears his name, the Newtonian reflector. The invention of the lens in 1733 partially corrected color aberrations present in the simple lens and enabled the construction of shorter. The largest reflecting telescopes currently have objectives larger than 10 m, the 20th century also saw the development of telescopes that worked in a wide range of wavelengths from radio to gamma-rays. The first purpose built radio telescope went into operation in 1937, since then, a tremendous variety of complex astronomical instruments have been developed. The name telescope covers a range of instruments. Most detect electromagnetic radiation, but there are differences in how astronomers must go about collecting light in different frequency bands. The near-infrared can be collected much like light, however in the far-infrared and submillimetre range. For example, the James Clerk Maxwell Telescope observes from wavelengths from 3 μm to 2000 μm, on the other hand, the Spitzer Space Telescope, observing from about 3 μm to 180 μm uses a mirror. Also using reflecting optics, the Hubble Space Telescope with Wide Field Camera 3 can observe in the range from about 0.2 μm to 1.7 μm
16.
Ptolemy
–
Claudius Ptolemy was a Greek writer, known as a mathematician, astronomer, geographer, astrologer, and poet of a single epigram in the Greek Anthology. He lived in the city of Alexandria in the Roman province of Egypt, wrote in Koine Greek, beyond that, few reliable details of his life are known. His birthplace has been given as Ptolemais Hermiou in the Thebaid in a statement by the 14th-century astronomer Theodore Meliteniotes. This is a very late attestation, however, and there is no reason to suppose that he ever lived elsewhere than Alexandria. Ptolemy wrote several treatises, three of which were of importance to later Byzantine, Islamic and European science. The first is the astronomical treatise now known as the Almagest, although it was entitled the Mathematical Treatise. The second is the Geography, which is a discussion of the geographic knowledge of the Greco-Roman world. The third is the treatise in which he attempted to adapt horoscopic astrology to the Aristotelian natural philosophy of his day. This is sometimes known as the Apotelesmatika but more known as the Tetrabiblos from the Greek meaning Four Books or by the Latin Quadripartitum. The name Claudius is a Roman nomen, the fact that Ptolemy bore it indicates he lived under the Roman rule of Egypt with the privileges and political rights of Roman citizenship. It would have suited custom if the first of Ptolemys family to become a citizen took the nomen from a Roman called Claudius who was responsible for granting citizenship, if, as was common, this was the emperor, citizenship would have been granted between AD41 and 68. The astronomer would also have had a praenomen, which remains unknown and it occurs once in Greek mythology, and is of Homeric form. All the kings after him, until Egypt became a Roman province in 30 BC, were also Ptolemies, abu Mashar recorded a belief that a different member of this royal line composed the book on astrology and attributed it to Ptolemy. The correct answer is not known”, Ptolemy wrote in Greek and can be shown to have utilized Babylonian astronomical data. He was a Roman citizen, but most scholars conclude that Ptolemy was ethnically Greek and he was often known in later Arabic sources as the Upper Egyptian, suggesting he may have had origins in southern Egypt. Later Arabic astronomers, geographers and physicists referred to him by his name in Arabic, Ptolemys Almagest is the only surviving comprehensive ancient treatise on astronomy. Ptolemy presented his models in convenient tables, which could be used to compute the future or past position of the planets. The Almagest also contains a catalogue, which is a version of a catalogue created by Hipparchus
17.
Almagest
–
The Almagest is the critical source of information on ancient Greek astronomy. It has also been valuable to students of mathematics because it documents the ancient Greek mathematician Hipparchuss work, Hipparchus wrote about trigonometry, but because his works appear to have been lost, mathematicians use Ptolemys book as their source for Hipparchuss work and ancient Greek trigonometry in general. The treatise was later titled Hē Megalē Syntaxis, and the form of this lies behind the Arabic name al-majisṭī. Ptolemy set up a public inscription at Canopus, Egypt, in 147 or 148, the late N. T. Hamilton found that the version of Ptolemys models set out in the Canopic Inscription was earlier than the version in the Almagest. Hence it cannot have been completed before about 150, a century after Ptolemy began observing. The Syntaxis Mathematica consists of thirteen sections, called books, an example illustrating how the Syntaxis was organized is given below. It is a 152-page Latin edition printed in 1515 at Venice by Petrus Lichtenstein, then follows an explanation of chords with table of chords, observations of the obliquity of the ecliptic, and an introduction to spherical trigonometry. There is also a study of the angles made by the ecliptic with the vertical, Book III covers the length of the year, and the motion of the Sun. Ptolemy explains Hipparchus discovery of the precession of the equinoxes and begins explaining the theory of epicycles. Books IV and V cover the motion of the Moon, lunar parallax, the motion of the apogee. Book VI covers solar and lunar eclipses, books VII and VIII cover the motions of the fixed stars, including precession of the equinoxes. They also contain a catalogue of 1022 stars, described by their positions in the constellations. The brightest stars were marked first magnitude, while the faintest visible to the eye were sixth magnitude. Each numerical magnitude was twice the brightness of the following one and this system is believed to have originated with Hipparchus. The stellar positions too are of Hipparchan origin, despite Ptolemys claim to the contrary, Book IX addresses general issues associated with creating models for the five naked eye planets, and the motion of Mercury. Book X covers the motions of Venus and Mars, Book XI covers the motions of Jupiter and Saturn. Book XII covers stations and retrograde motion, which occurs when planets appear to pause, Ptolemy understood these terms to apply to Mercury and Venus as well as the outer planets. Book XIII covers motion in latitude, that is, the deviation of planets from the ecliptic, the cosmology of the Syntaxis includes five main points, each of which is the subject of a chapter in Book I
18.
Hipparchus
–
Hipparchus of Nicaea was a Greek astronomer, geographer, and mathematician. He is considered the founder of trigonometry but is most famous for his discovery of precession of the equinoxes. Hipparchus was born in Nicaea, Bithynia, and probably died on the island of Rhodes and he is known to have been a working astronomer at least from 162 to 127 BC. Hipparchus is considered the greatest ancient astronomical observer and, by some and he was the first whose quantitative and accurate models for the motion of the Sun and Moon survive. For this he made use of the observations and perhaps the mathematical techniques accumulated over centuries by the Babylonians. He developed trigonometry and constructed trigonometric tables, and he solved problems of spherical trigonometry. With his solar and lunar theories and his trigonometry, he may have been the first to develop a method to predict solar eclipses. Relatively little of Hipparchuss direct work survives into modern times, although he wrote at least fourteen books, only his commentary on the popular astronomical poem by Aratus was preserved by later copyists. There is a tradition that Hipparchus was born in Nicaea, in the ancient district of Bithynia. His birth date was calculated by Delambre based on clues in his work, Hipparchus must have lived some time after 127 BC because he analyzed and published his observations from that year. Hipparchus obtained information from Alexandria as well as Babylon, but it is not known when or if he visited these places and he is believed to have died on the island of Rhodes, where he seems to have spent most of his later life. It is not known what Hipparchuss economic means were nor how he supported his scientific activities and his appearance is likewise unknown, there are no contemporary portraits. In the 2nd and 3rd centuries coins were made in his honour in Bithynia that bear his name and show him with a globe, this supports the tradition that he was born there. As an astronomer of antiquity his influence, supported by ideas from Aristotle, held sway for nearly 2000 years, Hipparchuss only preserved work is Τῶν Ἀράτου καὶ Εὐδόξου φαινομένων ἐξήγησις. This is a critical commentary in the form of two books on a popular poem by Aratus based on the work by Eudoxus. Hipparchus also made a list of his works, which apparently mentioned about fourteen books. His famous star catalog was incorporated into the one by Ptolemy, the first trigonometric table was apparently compiled by Hipparchus, who is now consequently known as the father of trigonometry. There are a variety of mis-steps in the more ambitious 2005 paper, According to one book review, both of these claims have been rejected by other scholars
19.
N. R. Pogson
–
Norman Robert Pogson, CIE was an English astronomer who worked in India at the Madras observatory. Norman was born in Nottingham, the son of George Owen Pogson, a manufacturer, lace dealer and commission agent, with enough income to support an extended family. His early education was largely informal and he left school at 16, intending to teach mathematics. At the age of eighteen, he calculated with the help of John Russell Hind of the Royal Astronomical Society and he was introduced to astronomy through George Bishops Observatory at South Villa Regents Park from 1846. He took an interest in comets and studied Iris, a planet that had been recently discovered. He was engaged as an assistant at the Radcliffe Observatory in 1852, after working as an assistant at the South Villa Observatory in 1851, he moved to the Radcliffe Observatory in Oxford in 1852. He received the Lalande medal upon his discovery of the minor planet Isis and his Oxford period was spent studying variable stars and other routine research. In 1854 he helped Sir George Airy conduct an experiment to determine the density of the earth, Pogson was appointed as director at the Hartwell Observatory belonging to John Lee in 1859. He published around fourteen papers from 1859 to 1860 in the Monthly Notices of the Royal Astronomical Society, mostly on variable stars, Sir Charles Wood appointed him as government astronomer for Madras in October 1860. Reaching India in 1861 and working at the Madras Observatory he worked tirelessly, in the next seven years he found five minor planets and seven variable stars. He continued worked on Taylors Madras Catalogue of 11,015 stars which had published in 1835 based on work begun in 1831 by T. G. Taylor. Pogson continued work on this to add 51,101 observations and after his death in 1891 the catalogue was revised by Arthur Downing and published in 1901. Despite Pogsons isolation he had at the time of his death discovered 134 stars,106 variable stars,21 possible variable stars and 7 possible supernovae, Pogson also made special expeditions, observing a total solar eclipse on 18 August 1868 at Masulipatnam and making spectrometric studies. He observed and commented on the line associated with Helium. Pogsons suggestion in 1856 was to make this a standard, thus and this fifth root of 100 is known as Pogsons Ratio. The magnitude relation is given as follows, m1 - m2 = -2.5 log10 where m is the magnitude and L is the luminosity. In 1868 and 1871, Pogson joined the Indian solar eclipse expeditions and he received a telegram from Ernst Friedrich Wilhelm Klinkerfues on November 30th,1872 which read Biela touched Earth on 27th. Search near Theta Centauri, a message so esoteric that it caught the fancy of the newspapers of the time, one of Pogsons assistants was Chintamani Raghunatha Chary
20.
Polaris
–
Polaris, designated Alpha Ursae Minoris, commonly the North Star or Pole Star, is the brightest star in the constellation of Ursa Minor. It is very close to the celestial pole, making it the current northern pole star. The revised Hipparcos parallax gives a distance to Polaris of about 433 light-years, Polaris is a multiple star, comprising the main star in orbit with a smaller companion, the pair in orbit with Polaris B. There were once thought to be two more distant components—Polaris C and Polaris D—but these have been not to be physically associated with the Polaris system. Polaris Aa is a 4.5 solar mass F7 yellow supergiant of spectral type Ib and this is the first classical Cepheid to have a mass determined from its orbit. William Herschel discovered the star in 1780 while using a hand-built reflecting telescope, in 1929 it was discovered, by examining the spectrum of Polaris A, that it was a very close binary, with the secondary being a dwarf, which had been theorized in earlier observations. In January 2006, NASA released images, from the Hubble telescope, that showed the three members of the Polaris ternary system. The nearest dwarf star is in an orbit of only 18.5 au from Polaris A, about the distance between the Sun and Uranus), which explains why its light is swamped by its close and much brighter companion. Polaris A, the supergiant primary component, is a low-amplitude Population I classical Cepheid variable, cepheids constitute an important standard candle for determining distance, so Polaris, as the closest such star, is heavily studied. The variability of Polaris had been suspected since 1852, this variation was confirmed by Ejnar Hertzsprung in 1911, the range of brightness of Polaris during its pulsations is given as 1. 86–2.13, but the amplitude has changed since discovery. Prior to 1963, the amplitude was over 0.1 magnitude and was gradually decreasing. After 1966 it very rapidly decreased until it was less than 0.05 magnitude, since then and it has been reported that the amplitude is now increasing again, a reversal not seen in any other Cepheid. The period, roughly 4 days, has changed over time. It has steadily increased by around 4.5 seconds per year except for a hiatus in 1963–1965 and this was originally thought to be due to secular redward evolution across the Cepheid instability strip, but it may be due to interference between the primary and the first-overtone pulsation modes. Authors disagree on whether Polaris is a fundamental or first-overtone pulsator, the temperature of Polaris varies by only a small amount during its pulsations, but the amount of this variation is variable and unpredictable. The erratic changes of temperature and the amplitude of changes during each cycle, from less than 50 K to at least 170 K. Research reported in Science suggests that Polaris is 2.5 times brighter today than when Ptolemy observed it, because of its importance in celestial navigation, Polaris is known by numerous names. It became known as Polaris during the Renaissance, its derived from the Latin polaris of/near the pole
21.
Black body
–
A black body is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. A white body is one with a surface reflects all incident rays completely and uniformly in all directions. A black body in thermal equilibrium emits electromagnetic radiation called black-body radiation, the radiation is emitted according to Plancks law, meaning that it has a spectrum that is determined by the temperature alone, not by the bodys shape or composition. A black body in equilibrium has two notable properties, It is an ideal emitter, at every frequency, it emits as much energy as – or more energy than – any other body at the same temperature. It is an emitter, the energy is radiated isotropically. An approximate realization of a surface is a hole in the wall of a large enclosure. Any light entering the hole is reflected indefinitely or absorbed inside and is unlikely to re-emerge, the radiation confined in such an enclosure may or may not be in thermal equilibrium, depending upon the nature of the walls and the other contents of the enclosure. Real materials emit energy at a fraction—called the emissivity—of black-body energy levels, by definition, a black body in thermal equilibrium has an emissivity of ε =1.0. A source with lower emissivity independent of frequency often is referred to as a gray body, construction of black bodies with emissivity as close to one as possible remains a topic of current interest. I shall call such bodies perfectly black, or, more briefly, a more modern definition drops the reference to infinitely small thicknesses, An ideal body is now defined, called a blackbody. A blackbody allows all incident radiation to pass into it and internally absorbs all the incident radiation and this is true for radiation of all wavelengths and for all angles of incidence. Hence the blackbody is a perfect absorber for all incident radiation and this section describes some concepts developed in connection with black bodies. A widely used model of a surface is a small hole in a cavity with walls that are opaque to radiation. Radiation incident on the hole will pass into the cavity, and is unlikely to be re-emitted if the cavity is large. The hole is not quite a perfect black surface — in particular, if the wavelength of the incident radiation is longer than the diameter of the hole, part will be reflected. Similarly, even in perfect equilibrium, the radiation inside a finite-sized cavity will not have an ideal Planck spectrum for wavelengths comparable to or larger than the size of the cavity. Suppose the cavity is held at a fixed temperature T and the radiation trapped inside the enclosure is at equilibrium with the enclosure. The hole in the enclosure will allow some radiation to escape, if the hole is small, radiation passing in and out of the hole has negligible effect upon the equilibrium of the radiation inside the cavity
22.
Infrared astronomy
–
Infrared astronomy is the branch of astronomy and astrophysics that studies astronomical objects visible in infrared radiation. The wavelength of infrared light ranges from 0.75 to 300 micrometers, Infrared falls in between visible radiation, which ranges from 380 to 750 nanometers, and submillimeter waves. Infrared astronomy began in the 1830s, a few decades after the discovery of infrared light by William Herschel in 1800. Early progress was limited, and it was not until the early 20th century that conclusive detections of astronomical objects other than the Sun, both fields also use solid state detectors, though the specific type of solid state detectors used are different. Infrared light is absorbed at many wavelengths by water vapor in the Earths atmosphere, so most infrared telescopes are at elevations in dry places. There are also infrared observatories in space, including the Spitzer Space Telescope, the discovery of infrared radiation is attributed to William Herschel, who performed an experiment where he placed a thermometer in sunlight of different colors after it passed through a prism. He noticed that the temperature increase induced by sunlight was highest outside the visible spectrum and he dubbed this radiation calorific rays, and went on to show that it could be reflected, transmitted, and absorbed just like visible light. Efforts were made starting in the 1830s and continuing through the 19th century to detect infrared radiation from astronomical sources. Radiation from the Moon was first detected in 1873 by William Parsons, ernest Fox Nichols used a modified Crookes radiometer in an attempt to detect infrared radiation from Arcturus and Vega, but Nichols deemed the results inconclusive. The field was mostly neglected by traditional astronomers though until the 1960s, Infrared radiation with wavelengths just longer than visible light, known as near-infrared, behaves in a very similar way to visible light, and can be detected using similar solid state devices. For this reason, the infrared region of the spectrum is commonly incorporated as part of the optical spectrum. Many optical telescopes, such as those at Keck Observatory, operate effectively in the infrared as well as at visible wavelengths. The far-infrared extends to submillimeter wavelengths, which are observed by such as the James Clerk Maxwell Telescope at Mauna Kea Observatory. Like all other forms of radiation, infrared is utilized by astronomers to study the universe. Indeed, infrared measurements taken by the 2MASS and WISE astronomical surveys have been effective at unveiling previously undiscovered star clusters. Examples of such embedded star clusters are FSR1424, FSR1432, Camargo 394, Camargo 399, Majaess 30, and Majaess 99. Infrared telescopes, which includes most major optical telescopes as well as a few dedicated infrared telescopes, need to be chilled with liquid nitrogen, the reason for this is that objects with temperatures of a few hundred Kelvin emit most of their thermal energy at infrared wavelengths. If infrared detectors were not kept cooled, the radiation from the detector itself would contribute noise that would dwarf the radiation from any celestial source and this is particularly important in the mid-infrared and far-infrared regions of the spectrum
23.
Circumstellar disc
–
A circumstellar disc is a torus, pancake or ring-shaped accumulation of matter composed of gas, dust, planetesimals, asteroids or collision fragments in orbit around a star. Around the youngest stars, they are the reservoirs of material out of which planets may form, around mature stars, they indicate that planetesimal formation has taken place and around white dwarfs, they indicate that planetary material survived the whole of stellar evolution. Such a disc can manifest itself in various ways, the infalling material possesses some amount of angular momentum, which results in the formation of a gaseous protoplanetary disc around the young, rotating star. The former is a circumstellar disc of dense gas and dust that continues to feed the central star. It may contain a few percent of the mass of the central star, the main accretion phase lasts a few million years, with accretion rates typically between 10−7 and 10−9 solar masses per year. The disc gradually cools in what is known as the T Tauri star stage, within this disc, the formation of small dust grains made of rocks and ices can occur, and these can coagulate into planetesimals. If the disc is sufficiently massive, the runaway accretions begin, the formation of planetary systems is thought to be a natural result of star formation. A sun-like star usually takes around 100 million years to form, asteroid belt is a reservoir of small bodies in the Solar System located between the orbit of Mars and Jupiter. It is a source of interplanetary dust, edgeworth-Kuiper belt, beyond the orbit of Neptune Scattered disc, beyond the orbit of Neptune Hills cloud, only the inner Oort cloud has a toroid-like shape. The outer Oort cloud is more spherical in shape, making it a circumstellar envelope, the infall of gas onto a binary system allows the formation of circumstellar and circumbinary discs. The formation of such a disc will occur for any system in which infalling gas which contains some degree of angular momentum. A general progression of disc formation is observed with increasing levels of angular momentum and this type of disc will form through accretion if any angular momentum is present in the infalling gas. Circumsecondary disc is one which orbits around the star of the binary star system. This type of disc will form when a high enough level of angular momentum is present within the infalling gas. The amount of angular momentum required is dependent on the mass ratio. Circumbinary disc is one which orbits about both the primary and secondary stars, such a disc will form at a later time than the circumprimary and circumsecondary discs, with an inner radius much larger than the orbital radius of the binary system. An example of a disc may be seen around the star system GG Tauri. Once a circumstellar disc has formed, spiral density waves are created within the material via a differential torque due to the binarys gravity
24.
Cosmic dust
–
Cosmic dust, or Extraterrestrial dust, is dust which exists in outer space, as well as all over planet Earth. Most cosmic dust particles are between a few molecules to 0.1 µm in size, a smaller fraction of all dust in space consists of larger refractory minerals that condensed as matter left the stars. It is called stardust and is included in a section below. The dust density falling to Earth is approximately 10−6/m3 with each grain having a mass between 10−16kg and 10−4 kg, cosmic dust can be further distinguished by its astronomical location, intergalactic dust, interstellar dust, interplanetary dust and circumplanetary dust. In the Solar System, interplanetary dust causes the zodiacal light, sources of Solar System dust include comet dust, asteroidal dust, dust from the Kuiper belt, and interstellar dust passing through the Solar System. The terminology has no application for describing materials found on the planet Earth except for dust that has demonstrably fallen to Earth. By one estimate, as much as 40,000 tons of cosmic dust reaches the Earths surface every year, in October 2011, scientists reported that cosmic dust contains complex organic matter that could be created naturally, and rapidly, by stars. In August 2014, scientists announced the collection of interstellar dust particles from the Stardust spacecraft since returning to Earth in 2006. In March 2017, scientists reported that extraterrestrial dust particles have been identified all over planet Earth. According to one of the researchers, “Once I knew what to look for, I found them everywhere. ”Cosmic dust was once solely an annoyance to astronomers, when infrared astronomy began, those dust particles were observed to be significant and vital components of astrophysical processes. Their analysis can reveal information about phenomena like the formation of the Solar System, for example, cosmic dust can drive the mass loss when a star is nearing the end of its life, play a part in the early stages of star formation, and form planets. In the Solar System, dust plays a role in the zodiacal light, Saturns B Ring spokes, the outer diffuse planetary rings at Jupiter, Saturn, Uranus and Neptune. These disparate research areas can be linked by the following theme, the astronomers accumulate observational ‘snapshots’ of dust at different stages of its life and, over time, form a more complete movie of the Universes complicated recycling steps. Parameters such as the initial motion, material properties, intervening plasma. Slightly changing any of these parameters can give significantly different dust dynamical behavior, therefore, one can learn about where that object came from, and what is the intervening medium. Cosmic dust can be detected by indirect methods utilizing the properties of cosmic dust. Cosmic dust can also be detected using a variety of collection methods. Estimates of the influx of extraterrestrial material entering the Earths atmosphere range between 5 and 300 tonnes
25.
Planck's law
–
Plancks law describes the spectral density of electromagnetic radiation emitted by a black body in thermal equilibrium at a given temperature T. The law is named after Max Planck, who proposed it in 1900 and it is a pioneering result of modern physics and quantum theory. The spectral radiance of a body, Bν, describes the amount of energy it gives off as radiation of different frequencies. It is measured in terms of the power emitted per unit area of the body, the spectral radiance can also be measured per unit wavelength λ instead of per unit frequency. In this case, it is given by B λ =2 h c 2 λ51 e h c λ k B T −1. The law may also be expressed in terms, such as the number of photons emitted at a certain wavelength. The SI units of Bν are W·sr−1·m−2·Hz−1, while those of Bλ are W·sr−1·m−3, in the limit of low frequencies, Plancks law tends to the Rayleigh–Jeans law, while in the limit of high frequencies it tends to the Wien approximation. Every physical body spontaneously and continuously emits electromagnetic radiation, near thermodynamic equilibrium, the emitted radiation is nearly described by Plancks law. Because of its dependence on temperature, Planck radiation is said to be thermal radiation, the higher the temperature of a body the more radiation it emits at every wavelength. Planck radiation has an intensity at a specific wavelength that depends on the temperature. For example, at room temperature, a body emits thermal radiation that is mostly infrared, at higher temperatures the amount of infrared radiation increases and can be felt as heat, and the body glows visibly red. At even higher temperatures, a body is bright yellow or blue-white and emits significant amounts of short wavelength radiation, including ultraviolet. The surface of the sun emits large amounts of infrared and ultraviolet radiation, its emission is peaked in the visible spectrum. Planck radiation is the greatest amount of radiation that any body at thermal equilibrium can emit from its surface, the passage of radiation across an interface between media can be characterized by the emissivity of the interface, usually denoted by the symbol ε. It is in general dependent on chemical composition and physical structure, on temperature, on the wavelength, on the angle of passage, the emissivity of a natural interface is always between ε =0 and 1. A body that interfaces with another medium which both has ε =1 and absorbs all the incident upon it, is said to be a black body. At equilibrium, the radiation inside this enclosure follows Plancks law, just as the Maxwell–Boltzmann distribution is the unique maximum entropy energy distribution for a gas of material particles at thermal equilibrium, so is Plancks distribution for a gas of photons. If the photon gas is not Planckian, the law of thermodynamics guarantees that interactions will cause the photon energy distribution to change
26.
Celestial sphere
–
In astronomy and navigation, the celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with Earth. All objects in the sky can be thought of as projected upon the inside surface of the celestial sphere. The celestial sphere is a tool for spherical astronomy, allowing observers to plot positions of objects in the sky when their distances are unknown or unimportant. Because astronomical objects are at such distances, casual observation of the sky offers no information on the actual distances. All objects seem equally far away, as if fixed to the inside of a sphere of large but unknown radius, which rotates from east to west overhead while underfoot, the celestial sphere can be considered to be infinite in radius. This means any point within it, including that occupied by the observer, all parallel planes will seem to intersect the sphere in a coincident great circle. On an infinite-radius celestial sphere, all observers see the things in the same direction. For some objects, this is over-simplified, objects which are relatively near to the observer will seem to change position against the distant celestial sphere if the observer moves far enough, say, from one side of the Earth to the other. This effect, known as parallax, can be represented as an offset from a mean position. The celestial sphere can be considered to be centered at the Earths center, the Suns center, or any convenient location. Individual observers can work out their own small offsets from the mean positions, in many cases in astronomy, the offsets are insignificant. The celestial sphere can thus be thought of as a kind of astronomical shorthand, for many rough uses, this position, as seen from the Earths center, is adequate. This greatly abbreviates the amount of detail necessary in such almanacs and these concepts are important for understanding celestial coordinate systems – frameworks for measuring the positions of objects in the sky. Certain reference lines and planes on Earth, when projected onto the celestial sphere and these include the Earths equator, axis, and the Earths orbit. At their intersections with the sphere, these form the celestial equator, the north and south celestial poles. As the celestial sphere is considered infinite in radius, all observers see the celestial equator, celestial poles, from these bases, directions toward objects in the sky can be quantified by constructing celestial coordinate systems. Similar to terrestrial longitude and latitude, the coordinate system specifies positions relative to the celestial equator and celestial poles. The ecliptic coordinate system specifies positions relative to the Earths orbit, besides the equatorial and ecliptic systems, some other celestial coordinate systems, such as the galactic coordinate system, are more appropriate for particular purposes
27.
Tarantula Nebula
–
The Tarantula Nebula is an H II region in the Large Magellanic Cloud. The Tarantula Nebula was observed by Nicolas Louis de Lacaille during an expedition to the Cape of Good Hope between 1751 and 1753 and he catalogued it as the second of the Nebulae of the First Class, Nebulosities not accompanied by any star visible in the telescope of two feet. It was described as a diffuse nebula 20 across, instead of being given a stellar magnitude, it was noted to be nebulous. The name Tarantula Nebula arose the mid 20th century from the appearance in deep photographic exposures. 30 Doradus has often been treated as the designation of a star, or of the star cluster NGC2070. The Tarantula Nebula has an apparent magnitude of 8, considering its distance of about 49 kpc, this is an extremely luminous non-stellar object. Its luminosity is so great if it were as close to Earth as the Orion Nebula. In fact it is the most active starburst region known in the Local Group of galaxies and it is also one of the largest such regions in the Local Group with an estimated diameter of 200 pc. The nebula resides on the edge of the LMC where ram pressure stripping. 30 Doradus has at its centre the star cluster NGC2070 which includes the compact concentration of stars known as R136 that produces most of the energy that makes the nebula visible. The estimated mass of the cluster is 450,000 solar masses, in addition to NGC2070, the Tarantula Nebula contains a number of other star clusters including the much older Hodge 301. The most massive stars of Hodge 301 have already exploded in supernovae, the closest supernova observed since the invention of the telescope, Supernova 1987A, occurred in the outskirts of the Tarantula Nebula. There is a prominent supernova remnant enclosing the open cluster NGC2060, Tarantula Nebula and Its Huge Stars
28.
European Southern Observatory
–
The European Southern Observatory is a 16-nation intergovernmental research organization for ground-based astronomy. Created in 1962, ESO has provided astronomers with state-of-the-art research facilities, the organisation employs about 730 staff members and receives annual member state contributions of approximately €131 million. Its observatories are located in northern Chile, ESO has built and operated some of the largest and most technologically advanced telescopes. The Atacama Large Millimeter Array observes the universe in the millimetre and submillimetre wavelength ranges and it was completed in March 2013 in an international collaboration by Europe, North America, East Asia and Chile. Currently under construction is the European Extremely Large Telescope and it will use a 39. 3-metre-diameter segmented mirror, and become the worlds largest optical reflecting telescope when operational in 2024. ESOs observing facilities have made discoveries and produced several astronomical catalogues. Its findings include the discovery of the most distant gamma-ray burst, in 2004, the VLT allowed astronomers to obtain the first picture of an extrasolar planet orbiting a brown dwarf 173 light-years away. The idea that European astronomers should establish a large observatory was broached by Walter Baade. It was pursued by Oort, who gathered a group of astronomers in Leiden to consider it on June 21 that year, immediately thereafter, the subject was further discussed at the Groningen conference in the Netherlands. On January 26,1954, an ESO declaration was signed by astronomers from six European countries expressing the wish that a joint European observatory be established in the southern hemisphere. At the time, all reflector telescopes with an aperture of 2 metres or more were located in the northern hemisphere. The decision to build the observatory in the southern hemisphere resulted from the necessity of observing the southern sky, although it was initially planned to set up telescopes in South Africa, tests from 1955 to 1963 demonstrated that a site in the Andes was preferable. On November 15,1963 Chile was chosen as the site for ESOs observatory, the decision was preceded by the ESO Convention, signed 5 October 1962 by Belgium, Germany, France, the Netherlands and Sweden. Otto Heckmann was nominated as the organisations first director general on 1 November 1962, a preliminary proposal for a convention of astronomy organisations in these five countries was drafted in 1954. Although some amendments were made in the document, the convention proceeded slowly until 1960 when it was discussed during that years committee meeting. The new draft was examined in detail, and a member of CERN highlighted the need for a convention between governments. The convention and government involvement became pressing due to rising costs of site-testing expeditions. The final 1962 version was adopted from the CERN convention
29.
Nebula
–
A nebula is an interstellar cloud of dust, hydrogen, helium and other ionized gases. Originally, nebula was a name for any diffuse astronomical object, the Andromeda Galaxy, for instance, was once referred to as the Andromeda Nebula before the true nature of galaxies was confirmed in the early 20th century by Vesto Slipher, Edwin Hubble and others. Most nebulae are of vast size, even millions of years in diameter. The Orion Nebula, the brightest nebula in the sky that occupies a region twice the diameter of the full Moon, can be viewed with the naked eye but was missed by early astronomers. Many nebulae are visible due to their fluorescence caused by the hot stars, while others are so diffuse they can only be detected with long exposures. Some nebulae are variably illuminated by T Tauri variable stars, Nebulae are often star-forming regions, such as in the Pillars of Creation in the Eagle Nebula. In these regions the formations of gas, dust, and other materials together to form denser regions, which attract further matter. The remaining material is believed to form planets and other planetary system objects. Around 150 AD, Claudius Ptolemaeus recorded, in books VII–VIII of his Almagest and he also noted a region of nebulosity between the constellations Ursa Major and Leo that was not associated with any star. The first true nebula, as distinct from a cluster, was mentioned by the Persian astronomer Abd al-Rahman al-Sufi. He noted a little cloud where the Andromeda Galaxy is located and he also cataloged the Omicron Velorum star cluster as a nebulous star and other nebulous objects, such as Brocchis Cluster. The supernova that created the Crab Nebula, the SN1054, was observed by Arabic, in 1610, Nicolas-Claude Fabri de Peiresc discovered the Orion Nebula using a telescope. This nebula was observed by Johann Baptist Cysat in 1618. However, the first detailed study of the Orion Nebula was not performed until 1659 by Christiaan Huygens, in 1715, Edmund Halley published a list of six nebulae. This number steadily increased during the century, with Jean-Philippe de Cheseaux compiling a list of 20 in 1746, from 1751 to 1753, Nicolas Louis de Lacaille cataloged 42 nebulae from the Cape of Good Hope, most of which were previously unknown. Charles Messier then compiled a catalog of 103 nebulae by 1781, his interest was detecting comets, the number of nebulae was then greatly expanded by the efforts of William Herschel and his sister Caroline Herschel. Their Catalogue of One Thousand New Nebulae and Clusters of Stars was published in 1786, a second catalog of a thousand was published in 1789 and the third and final catalog of 510 appeared in 1802. During much of their work, William Herschel believed that these nebulae were merely unresolved clusters of stars, in 1790, however, he discovered a star surrounded by nebulosity and concluded that this was a true nebulosity, rather than a more distant cluster
30.
Common logarithm
–
In mathematics, the common logarithm is the logarithm with base 10. It is indicated by log10, or sometimes Log with a capital L, on calculators it is usually log, but mathematicians usually mean natural logarithm rather than common logarithm when they write log. To mitigate this ambiguity the ISO80000 specification recommends that log10 should be written lg, before the early 1970s, handheld electronic calculators were not available and mechanical calculators capable of multiplication were bulky, expensive and not widely available. Instead, tables of logarithms were used in science, engineering. Use of logarithms avoided laborious and error prone paper and pencil multiplications and divisions, because logarithms were so useful, tables of base-10 logarithms were given in appendices of many text books. Mathematical and navigation handbooks included tables of the logarithms of trigonometric functions as well, see log table for the history of such tables. The fractional part is known as the mantissa, thus log tables need only show the fractional part. Tables of common logarithms typically listed the mantissa, to 4 or 5 decimal places or more, of number in a range. Such a range would cover all possible values of the mantissa, the integer part, called the characteristic, can be computed by simply counting how many places the decimal point must be moved so that it is just to the right of the first significant digit. For example, the logarithm of 120 is given by, log 10 120 = log 10 =2 + log 10 1.2 ≈2 +0.07918. The last number —the fractional part or the mantissa of the logarithm of 120—can be found in the table shown. The location of the point in 120 tells us that the integer part of the common logarithm of 120. Numbers greater than 0 and less than 1 have negative logarithms, when reading a number in bar notation out loud, the symbol n ¯ is read as bar n, so that 2 ¯.07918 is read as bar 2 point 07918. The following table shows how the same mantissa can be used for a range of numbers differing by powers of ten and this holds for any positive real number x because, log 10 = log 10 + log 10 = log 10 + i. Since i is always an integer the mantissa comes from log 10 which is constant for given x and this allows a table of logarithms to include only one entry for each mantissa. In the example of 5×10i,0.698970 will be listed once indexed by 5, or 0.5, common logarithms are sometimes also called Briggsian logarithms after Henry Briggs, a 17th-century British mathematician. In 1616 and 1617 Briggs visited John Napier, the inventor of what are now called natural logarithms at Edinburgh in order to suggest a change to Napiers logarithms. During these conferences the alteration proposed by Briggs was agreed upon, because base 10 logarithms were most useful for computations, engineers generally simply wrote log when they meant log10
31.
Flux
–
Flux is either of two separate simple and ubiquitous concepts throughout physics and applied mathematics. Within a discipline, the term is used consistently. Both concepts have mathematical rigor, enabling comparison of the underlying math when the terminology is unclear, for transport phenomena, flux is a vector quantity, describing the magnitude and direction of the flow of a substance or property. In electromagnetism, flux is a quantity, defined as the surface integral of the component of a vector field perpendicular to the surface at each point. As will be clear, the easiest way to relate the two concepts is that the surface integral of a flux according to the first definition is a flux according to the second definition. The word flux comes from Latin, fluxus means flow, as fluxion, this term was introduced into differential calculus by Isaac Newton. One could argue, based on the work of James Clerk Maxwell, the specific quote from Maxwell is, In the case of fluxes, we have to take the integral, over a surface, of the flux through every element of the surface. The result of operation is called the surface integral of the flux. It represents the quantity which passes through the surface, according to the first definition, flux may be a single vector, or flux may be a vector field / function of position. In the latter case flux can readily be integrated over a surface, by contrast, according to the second definition, flux is the integral over a surface, it makes no sense to integrate a second-definition flux for one would be integrating over a surface twice. Thus, Maxwells quote only makes sense if flux is being used according to the first definition and this is ironic because Maxwell was one of the major developers of what we now call electric flux and magnetic flux according to the second definition. This implies that Maxwell conceived as these fields as flows/fluxes of some sort, given a flux according to the second definition, the corresponding flux density, if that term is used, refers to its derivative along the surface that was integrated. By the Fundamental theorem of calculus, the flux density is a flux according to the first definition. Given a current such as electric current—charge per time, current density would also be a flux according to the first definition—charge per time per area. Concrete fluxes in the rest of this article will be used in accordance to their acceptance in the literature. In transport phenomena, flux is defined as the rate of flow of a property per unit area, the area is of the surface the property is flowing through or across. Here are 3 definitions in increasing order of complexity, each is a special case of the following. In all cases the frequent symbol j, is used for flux, q for the quantity that flows, t for time
32.
Sun
–
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
33.
Moon
–
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
34.
Full moon
–
A full moon is the lunar phase that occurs when the Moon is completely illuminated as seen from Earth. This occurs when Earth is located directly between the Sun and the Moon and this means that the hemisphere of the Moon that is facing Earth is almost fully illuminated by the Sun and appears round. On some occasions at the time of full moon there is also a lunar eclipse so the face appears reddish due to the rayleigh scattering of blue light in Earths atmosphere. Lunar eclipses can occur only at full moon, where the Moons orbit allows it to pass through Earths shadow, Lunar eclipses do not occur every month because the Moon usually passes above or below Earths shadow. Lunar eclipses can only when the full moon occurs near the two nodes of the orbit, either the ascending or descending node. This causes eclipses to occur about every 6 months. The time interval between similar lunar phases—the synodic month—averages about 29.53 days, therefore, in those lunar calendars in which each month begins on the new moon, the full moon falls on either the 14th or 15th of the lunar month. Because calendar months have a number of days, lunar months may be either 29 or 30 days long. A full moon is thought of as an event of a full nights duration. This is somewhat misleading because the Moon seen from Earth is continuously becoming larger and/or smaller and its absolute maximum size occurs at the moment expansion has stopped. For any given location, about half of these absolute maximum full moons will be visible, as the other half occur during the day. Many almanacs list full moons not just by date, but by their time as well. Typical monthly calendars that include phases of the Moon may be offset by one day if intended for use in a different time zone, Full moons are generally a poor time to conduct astronomical observations, because the bright reflected sunlight from the Moon overwhelms the dimmer light from stars. On 12 December 2008, the moon occurred closer to the Earth than it had been at any time for the previous 15 years. On 19 March 2011, another full moon occurred, closer to the Earth than at any time for the previous ~18 years. On 14 November 2016, a full moon occurred closer to the Earth than at any time for the previous 68 years. The true time of a full moon may differ from this approximation by up to about 14.5 hours as a result of the non-circularity of the moons orbit, see New moon for an explanation of the formula and its parameters. The age and apparent size of the full moon vary in a cycle of just under 14 synodic months, Full moons are traditionally associated with temporal insomnia, insanity and various magical phenomena such as lycanthropy
35.
Double star
–
In observational astronomy, a double star or visual double is a pair of stars that appear close to each other in the sky as seen from Earth when viewed through an optical telescope. Multiple stars are studied in this way, although the dynamics of multiple stellar systems are more complex than those of binary stars. There are three types of paired stars, Optical Doubles are unrelated stars that appear close together through chance alignment with Earth, visual Binaries are gravitationally-bound stars that are separately visible with a telescope. Non-Visual Binaries are stars whose binary status was deduced through more esoteric means, such as occultation, spectroscopy, improvements in telescopes can shift previously non-visual binaries into visual binaries, as happened with Polaris A in 2006. It is only the inability to observe two separate stars that distinguish non-visual and visual binaries. Mizar, in Ursa Major, was observed to be double by Benedetto Castelli, since that time, the search has been carried out thoroughly and the entire sky has been examined for double stars down to a limiting apparent magnitude of about 9.0. At least 1 in 18 stars brighter than 9.0 magnitude in the half of the sky are known to be double stars visible with a 36-inch telescope. The unrelated categories of optical doubles and true binaries are lumped together for historical and practical reasons, when Mizar was found to be a binary, it was quite difficult to determine whether a double star was a binary system or only an optical double. Improved telescopes, spectroscopy, and photography are the tools used to make the distinction. After it was determined to be a binary, Mizars components were found to be spectroscopic binaries themselves. Observation of visual double stars by visual measurement will yield the separation, or angular distance, the position angle specifies the direction in which the stars are separated and is defined as the bearing from the brighter component to the fainter, where north is 0°. In the measures of a binary, the position angle will change progressively. Plotting the measures in the plane will produce an ellipse and this is the apparent orbit, the projection of the orbit of the two stars onto the celestial sphere, the true orbit can be computed from it. Although it is expected that the majority of catalogued visual doubles are visual binaries, visual double stars may be distinguished from binary stars by observing their relative motion. Some bright visual double stars have a Bayer designation, in this case, the components may be denoted by superscripts. An example of this is α Crucis, whose components are α1 Crucis, since α1 Crucis is a spectroscopic binary, this is actually a multiple star. Superscripts are also used to more distant, physically unrelated, pairs of stars with the same Bayer designation, such as α1,2 Capricorni, ξ1,2 Centauri. These optical pairs are resolvable by the naked eye, apart from these pairs, the components of a double star are generally denoted by the letters A and B appended to the designation, of whatever sort, of the double star
. Bibcode:2014CQGra..31t5001U. doi:10.1088/0264-9381/31/20/205001.
