An astronomical object or celestial object is a occurring physical entity, association, or structures that exists in the observable universe. In astronomy, the terms object and body are used interchangeably. However, an astronomical body or celestial body is a single bound, contiguous entity, while an astronomical or celestial object is a complex, less cohesively bound structure, which may consist of multiple bodies or other objects with substructures. Examples of astronomical objects include planetary systems, star clusters and galaxies, while asteroids, moons and stars are astronomical bodies. A comet may be identified as both body and object: It is a body when referring to the frozen nucleus of ice and dust, an object when describing the entire comet with its diffuse coma and tail; 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 into groups and clusters within larger superclusters, that are strung along great filaments between nearly empty voids, forming a web that spans the observable universe.
The universe has a variety of morphologies, with irregular and disk-like shapes, depending on their formation and evolutionary histories, including interaction with other galaxies, which may lead to a merger. Disc galaxies encompass lenticular and spiral galaxies with features, such as spiral arms and a distinct halo. At the core, most galaxies have a supermassive black hole, which may result in an active galactic nucleus. Galaxies can have satellites in the form of dwarf galaxies and globular clusters; 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 resulting fundamental components are the stars, which are assembled in clusters from the various condensing nebulae; the great variety of stellar forms are determined entirely by the mass and evolutionary state of these stars. Stars may be found in multi-star systems. A planetary system and various minor objects such as asteroids and debris, can form in a hierarchical process of accretion from the protoplanetary disks that surrounds newly formed stars.
The various distinctive types of stars are shown by the Hertzsprung–Russell diagram —a plot of absolute stellar luminosity versus surface temperature. Each star follows an evolutionary track across this diagram. If this track takes the star through a region containing an intrinsic variable type its physical properties can cause it to become a variable star. An example of this is the instability strip, a region of the H-R diagram that includes Delta Scuti, RR Lyrae and Cepheid variables. Depending on the initial mass of the star and the presence or absence of a companion, a star may spend the last part of its life as a compact object; the table below lists the general categories of bodies and objects by their structure. List of light sources List of Solar System objects List of Solar System objects by size Lists of astronomical objects SkyChart, Sky & Telescope at the Library of Congress Web Archives Monthly skymaps for every location on Earth
The horizon or skyline is the apparent line that separates earth from sky, the line that divides all visible directions into two categories: those that intersect the Earth's surface, those that do not. The true horizon is a theoretical line, which can only be observed when it lies on the sea surface. At many locations, this line is obscured by land, buildings, etc. and the resulting intersection of earth and sky is called the visible horizon. When looking at a sea from a shore, the part of the sea closest to the horizon is called the offing; the true horizon is horizontal. It surrounds the observer and it is assumed to be a circle, drawn on the surface of a spherical model of the Earth, its center is below sea level. Its distance from the observer varies from day to day due to atmospheric refraction, affected by weather conditions; the higher the observer's eyes are from sea level, the farther away is the horizon from the observer. For instance, in standard atmospheric conditions, for an observer with eye level above sea level by 1.70 metres, the horizon is at a distance of about 5 kilometres.
When observed from high standpoints, such as a space station, the horizon is much farther away and it encompasses a much larger area of Earth's surface. In this case, it becomes evident that the horizon more resembles an ellipse than a perfect circle when the observer is above the equator, that the Earth's surface can be better modeled as an ellipsoid than as a sphere; the word horizon derives from the Greek "ὁρίζων κύκλος" horizōn kyklos, "separating circle", where "ὁρίζων" is from the verb ὁρίζω horizō, "to divide", "to separate", which in turn derives from "ὅρος", "boundary, landmark". The distance to the visible horizon has long been vital to survival and successful navigation at sea, because it determined an observer's maximum range of vision and thus of communication, with all the obvious consequences for safety and the transmission of information that this range implied; this importance lessened with the development of the radio and the telegraph, but today, when flying an aircraft under visual flight rules, a technique called attitude flying is used to control the aircraft, where the pilot uses the visual relationship between the aircraft's nose and the horizon to control the aircraft.
A pilot can retain his or her spatial orientation by referring to the horizon. In many contexts perspective drawing, the curvature of the Earth is disregarded and the horizon is considered the theoretical line to which points on any horizontal plane converge as their distance from the observer increases. For observers near sea level the difference between this geometrical horizon and the true horizon is imperceptible to the naked eye. In astronomy the horizon is the horizontal plane through the eyes of the observer, it is the fundamental plane of the horizontal coordinate system, the locus of points that have an altitude of zero degrees. While similar in ways to the geometrical horizon, in this context a horizon may be considered to be a plane in space, rather than a line on a picture plane. One sees further along the Earth's curved surface than a simple geometric calculation allows for because of refraction error. If the ground, or water, surface is colder than the air above it, a cold, dense layer of air forms close to the surface, causing light to be refracted downward as it travels, therefore, to some extent, to go around the curvature of the Earth.
The reverse happens if the ground is hotter than the air above it, as happens in deserts, producing mirages. As an approximate compensation for refraction, surveyors measuring distances longer than 100 meters subtract 14% from the calculated curvature error and ensure lines of sight are at least 1.5 meters from the ground, to reduce random errors created by refraction. However, ignoring the effect of atmospheric refraction, distance to the true horizon from an observer close to the Earth's surface is about d ≈ 3.57 h, where d is in kilometres and h is height above sea level in metres. The constant 3.57 has units of km/m½. When d is measured in miles and h in feet, the distance is d ≈ 1.5 h ≈ 1.22 h. where the constant 1.22 has units of mi/ft½. In this equation Earth's surface is assumed to be spherical, with radius equal to about 6,371 kilometres. Assuming no atmospheric refraction and a spherical Earth with radius R=6,371 kilometres: For an observer standing on the ground with h = 1.70 metres, the horizon is at a distance of 4.7 kilometres.
For an observer standing on the ground with h = 2 metres, the horizon is at a distance of 5 kilometres. For an observer standing on a hill or tower 100 feet above sea level, the horizon is at a distance of 12.2 miles. For an observer standing on a hill or tower 100 metres above sea level, the horizon is at a distance of 36 kilometres. For an observer standing on the roof of the Burj Khalifa, 828 metres from ground, about 834 metres above sea level, the horizon is at a distance of 103 kilometres. For an observe
Longitude by chronometer
Longitude by chronometer is a method, in navigation, of determining longitude using a marine chronometer, developed by John Harrison during the first half of the eighteenth century. It is an astronomical method of calculating the longitude at which a position line, drawn from a sight by sextant of any celestial body, crosses the observer's assumed latitude. In order to calculate the position line, the time of the sight must be known so that the celestial position i.e. the Greenwich Hour Angle and Declination, of the observed celestial body is known. All that can be derived from a single sight is a single position line, which can be achieved at any time during daylight when both the sea horizon and the sun are visible. To achieve a fix, more than one celestial body and the sea horizon must be visible; this is only possible at dawn and dusk. The angle between the sea horizon and the celestial body is measured with a sextant and the time noted; the Sextant reading is known as the'Sextant Altitude'.
This is corrected by use of tables to a'True Altitude'. The actual declination and hour angle of the celestial body are found from astronomical tables for the time of the measurement and together with the'True Altitude' are put into a formula with the assumed latitude; this formula calculates the'True Hour Angle', compared to the assumed longitude providing a correction to the assumed longitude. This correction is applied to the assumed position so that a position line can be drawn through the assumed latitude at the corrected longitude at 90° to the azimuth on the celestial body; the observer's position is somewhere along the position line, not at the found longitude at the assumed latitude. If two or more sights or measurements are taken within a few minutes of each other a'fix' can be obtained and the observer's position determined as the point where the position lines cross; the azimuth of the celestial body is determined by use of astronomical tables and for which the time must be known.
From this, it can be seen that a navigator will need to know the time accurately so that the position of the observed celestial body is known just as accurately. The position of the sun is given in degrees and minutes north or south of the equational or celestial equator and east or west of Greenwich, established by the English as the Prime Meridian; the desperate need for an accurate chronometer was met in the mid 18th century when an Englishman, John Harrison, produced a series of chronometers that culminated in his celebrated model H-4 that satisfied the requirements for a shipboard standard time-keeper. Many nations, such as France, have proposed their own reference longitudes as a standard, although the world’s navigators have come to accept the reference longitudes tabulated by the British; the reference longitude adopted by the British became known as the Prime Meridian and is now accepted by most nations as the starting point for all longitude measurements. The Prime Meridian of zero degrees longitude runs along the meridian passing through the Royal Observatory at Greenwich, England.
Longitude is measured west from the Prime Meridian. To determine "longitude by chronometer," a navigator requires a chronometer set to the local time at the Prime Meridian. Local time at the Prime Meridian has been called Greenwich Mean Time, but now, due to international sensitivities, has been renamed as Coordinated Universal Time, is known colloquially as "zulu time". Noon sights obtain the observer's Latitude, it is impossible to determine longitude with an accuracy better than 10nmi by means of a noon sight. A noon sight is called a Meridian Altitude. While it is easy to determine the observer's latitude at noon without knowing the exact time, longitude cannot be measured at noon. At noon the sun's change of altitude is slow, so determining the exact time that the sun is at its zenith by direct observation is impossible, therefore it is impossible to obtain an accurate longitude at the moment of Zenith. However, it is possible to determine the time of zenith for longitude with a useful accuracy by performing a mean time of observation when the sun is on its ascent and descent prior to and following its moment of Zenith.
By taking a sextant reading within 15 to 30 minutes prior to local noon and noting the time leaving the sextant set to the same angle and subsequently observing the moment in time at which the sun passes through the sight tube on its descent from Zenith between a half-hour and hour the two times can be averaged to obtain a longitude sufficiently accurate for navigation. The Earth does not make a perfect circular orbit around the Sun. Due to the elliptical nature of the Earth’s orbit around the Sun, the speed of the Sun’s apparent orbit around the Earth varies throughout the year and that causes it to appear to speed up and slow down slightly. Noon at the Prime Meridian is if exactly at 1200 UTC, but rather it occurs some minutes and seconds before or after that time each day; this slight daily variation has been calculated and is listed for each day of the year in the Nautical Almanac under the title of Equation of time. This variation must be added to or subtracted from the UTC of local apparent noon to improve the accuracy of the calculation.
With that, other factors, including the difficulty of determining the exact moment of local apparent noon due to the flattening of the Sun’s arc across the sky at its highest point, diminish the accuracy of determining longitude by chronometer as a method of cele
A nautical almanac is a publication describing the positions of a selection of celestial bodies for the purpose of enabling navigators to use celestial navigation to determine the position of their ship while at sea. The Almanac specifies for each whole hour of the year the position on the Earth's surface at which the sun, moon and first point of Aries is directly overhead; the positions of 57 selected stars are specified relative to the first point of Aries. In Great Britain, The Nautical Almanac has been published annually by HM Nautical Almanac Office since the first edition was published in 1767. In the United States, a nautical almanac has been published annually by the US Naval Observatory since 1852, it was titled American Ephemeris and Nautical Almanac. Since 1958, the USNO and HMNAO have jointly published a unified nautical almanac, the Astronomical Almanac for use by the navies of both countries. Almanac data is now available online from the US Naval Observatory. Commercial almanacs were produced that combined other information.
A good example would be Brown's — which commenced in 1877 – and is still produced annually, its early twentieth century subtitle being "Harbour and Dock Guide and Advertiser and Daily Tide Tables". This combination of trade advertising, information "by permission... of the Hydrographic Department of the Admiralty" provided a useful compendium of information. More recent editions have kept up with the changes in technology – the 1924 edition for instance had extensive advertisements for coaling stations. Meanwhile, the Reeds Nautical Almanac, published by Adlard Coles Nautical, has been in print since 1932, in 1944 was used by landing craft involved in the Normandy landings; the "Air Almanac" of the United States and Great Britain tabulates celestial coordinates for 10-minute intervals for the use in aerial navigation. The Sokkia Corporation's annual "Celestial Observation Handbook and Ephemeris" tabulated daily celestial coordinates for the Sun and nine stars. To find the position of a ship or aircraft by celestial navigation, the navigator measures with a sextant the apparent height of a celestial body above the horizon, notes the time from a marine chronometer.
That height is compared with the height predicted for a trial position. American Practical Navigator Nautical Almanac, Board of Longitude Collection Her Majesty's Nautical Almanac Office Online Nautical Almanac A free nautical Almanac in PDF format Navigation Spreadsheets: Almanac data History of the Nautical Almanac
The South Pole known as the Geographic South Pole or Terrestrial South Pole, is one of the two points where Earth's axis of rotation intersects its surface. It is the southernmost point on the surface of Earth and lies on the opposite side of Earth from the North Pole. Situated on the continent of Antarctica, it is the site of the United States Amundsen–Scott South Pole Station, established in 1956 and has been permanently staffed since that year; the Geographic South Pole is distinct from the South Magnetic Pole, the position of, defined based on Earth's magnetic field. The South Pole is at the center of the Southern Hemisphere. For most purposes, the Geographic South Pole is defined as the southern point of the two points where Earth's axis of rotation intersects its surface. However, Earth's axis of rotation is subject to small "wobbles", so this definition is not adequate for precise work; the geographic coordinates of the South Pole are given as 90°S, since its longitude is geometrically undefined and irrelevant.
When a longitude is desired, it may be given as 0°. At the South Pole, all directions face north. For this reason, directions at the Pole are given relative to "grid north", which points northwards along the prime meridian. Along tight latitude circles, clockwise is east, counterclockwise is west, opposite to the North Pole; the Geographic South Pole is located on the continent of Antarctica. It sits atop a featureless, barren and icy plateau at an altitude of 2,835 metres above sea level, is located about 1,300 km from the nearest open sea at Bay of Whales; the ice is estimated to be about 2,700 metres thick at the Pole, so the land surface under the ice sheet is near sea level. The polar ice sheet is moving at a rate of 10 metres per year in a direction between 37° and 40° west of grid north, down towards the Weddell Sea. Therefore, the position of the station and other artificial features relative to the geographic pole shift over time; the Geographic South Pole is marked by a stake in the ice alongside a small sign.
The sign records the respective dates that Roald Amundsen and Robert F. Scott reached the Pole, followed by a short quotation from each man, gives the elevation as "9,301 FT.". A new marker stake is fabricated each year by staff at the site; the Ceremonial South Pole is an area set aside for photo opportunities at the South Pole Station. It is located some meters from the Geographic South Pole, consists of a metallic sphere on a short bamboo pole, surrounded by the flags of the original Antarctic Treaty signatory states. Amundsen's Tent: The tent was erected by the Norwegian expedition led by Roald Amundsen on its arrival on 14 December 1911, it is buried beneath the snow and ice in the vicinity of the Pole. It has been designated a Historic Site or Monument, following a proposal by Norway to the Antarctic Treaty Consultative Meeting; the precise location of the tent is unknown, but based on calculations of the rate of movement of the ice and the accumulation of snow, it is believed, as of 2010, to lie between 1.8 and 2.5 km from the Pole at a depth of 17 m below the present surface.
Argentine Flagpole: A flagpole erected at the South Geographical Pole in December 1965 by the First Argentine Overland Polar Expedition has been designated a Historic Site or Monument following a proposal by Argentina to the Antarctic Treaty Consultative Meeting. In 1820, several expeditions claimed to have been the first to have sighted Antarctica, with the first being the Russian expedition led by Fabian Gottlieb von Bellingshausen and Mikhail Lazarev; the first landing was just over a year when American Captain John Davis, a sealer, set foot on the ice. The basic geography of the Antarctic coastline was not understood until the mid-to-late 19th century. American naval officer Charles Wilkes claimed that Antarctica was a new continent, basing the claim on his exploration in 1839–40, while James Clark Ross, in his expedition of 1839–43, hoped that he might be able to sail all the way to the South Pole. British explorer Robert Falcon Scott on the Discovery Expedition of 1901–04 was the first to attempt to find a route from the Antarctic coastline to the South Pole.
Scott, accompanied by Ernest Shackleton and Edward Wilson, set out with the aim of travelling as far south as possible, on 31 December 1902, reached 82°16′ S. Shackleton returned to Antarctica as leader of the British Antarctic Expedition in a bid to reach the Pole. On 9 January 1909, with three companions, he reached 88°23' S – 112 miles from the Pole – before being forced to turn back; the first men to reach the Geographic South Pole were the Norwegian Roald Amundsen and his party on December 14, 1911. Amundsen named his camp Polheim and the entire plateau surrounding the Pole King Haakon VII Vidde in honour of King Haakon VII of Norway. Robert Falcon Scott returned to Antarctica with his second expedition, the Terra Nova Expedition unaware of Amundsen's secretive expedition. Scott and four other men reached the South Pole on January 17, 1912, thirty-four days after Amundsen. On the return trip and his four companions all died of starvation and extreme cold. In 1914 Ernest Shackleton's Imperial Trans-Antarctic Expedition set out with the goal of crossing Antarctica via the South Pole, but his ship, the Endurance, was frozen in pack ice and sank 1
A geographical pole is either of the two points on a rotating body where its axis of rotation intersects its surface. As with Earth's North and South Poles, they are called that body's "north pole" and "south pole", one lying 90 degrees in one direction from the body's equator and the other lying 90 degrees in the opposite direction from the equator; every planet has geographical poles. If, like the Earth, a body generates a magnetic field, it will possess magnetic poles. Perturbations in a body's rotation mean that geographical poles wander on its surface; the Earth's North and South Poles, for example, move by a few metres over periods of a few years. As cartography requires exact and unchanging coordinates, the averaged locations of geographical poles are taken as fixed cartographic poles and become the points where the body's great circles of longitude intersect. Antipodes Equatorial bulge Polar regions of Earth Poles of astronomical bodies Polar wander
The north and south celestial poles are the two imaginary points in the sky where the Earth's axis of rotation, indefinitely extended, intersects the celestial sphere. The north and south celestial poles appear permanently directly overhead to an observer at the Earth's North Pole and South Pole, respectively; as the Earth spins on its axis, the two celestial poles remain fixed in the sky, all other points appear to rotate around them, completing one circuit per day. The celestial poles are the poles of the celestial equatorial coordinate system, meaning they have declinations of +90 degrees and −90 degrees; the celestial poles do not remain permanently fixed against the background of the stars. Because of a phenomenon known as the precession of the equinoxes, the poles trace out circles on the celestial sphere, with a period of about 25,700 years; the Earth's axis is subject to other complex motions which cause the celestial poles to shift over cycles of varying lengths. Over long periods the positions of the stars themselves change, because of the stars' proper motions.
An analogous concept applies to other planets: a planet's celestial poles are the points in the sky where the projection of the planet's axis of rotation intersects the celestial sphere. These points vary. Celestial bodies other than Earth have defined celestial poles; the north celestial pole is within a degree of the bright star Polaris. This makes Polaris useful for navigation in the northern hemisphere: not only is it always above the north point of the horizon, but its altitude angle is always equal to the observer's geographic latitude. Polaris can, of course, only be seen from locations in the northern hemisphere. Polaris is near the celestial pole for only a small fraction of the 25,700-year precession cycle, it will remain a good approximation for about 1,000 years, by which time the pole will have moved to be closer to Alrai. In about 5,500 years, the pole will have moved near the position of the star Alderamin, in 12,000 years, Vega will become our north star, but it will be about six degrees from the true north celestial pole.
To find Polaris, face north and locate the Big Dipper and Little Dipper asterisms. Looking at the "cup" part of the Big Dipper, imagine that the two stars at the outside edge of the cup form a line pointing upward out of the cup; this line points directly at the star at the tip of the Little Dipper's handle. That star is the North Star; the south celestial pole is visible only from the Southern Hemisphere. It lies in the Octant. Sigma Octantis is identified as the south pole star, over a degree away from the pole, but with a magnitude of 5.5 it is visible on a clear night. The south celestial pole can be located from the Southern Cross and its two "pointer" stars α Centauri and β Centauri. Draw an imaginary line from γ Crucis to α Crucis—the two stars at the extreme ends of the long axis of the cross—and follow this line through the sky. Either go four-and-a-half times the distance of the long axis in the direction the narrow end of the cross points, or join the two pointer stars with a line, divide this line in half at right angles draw another imaginary line through the sky until it meets the line from the Southern Cross.
This point is 6 degrees from the south celestial pole. Few bright stars of importance lie between Crux and the pole itself, although the constellation Musca is easily recognised beneath Crux; the second method uses Achernar. Make a large equilateral triangle using these stars for two of the corners; the third imaginary corner will be the south celestial pole. If Canopus has not yet risen, the second-magnitude Alpha Pavonis can be used to form the triangle with Achernar and the pole; the third method is best for moonless and clear nights, as it uses two faint "clouds" in the Southern Sky. These are marked in astronomy books as Small Magellanic Clouds; these "clouds" are dwarf galaxies near the Milky Way. Make an equilateral triangle, the third point of, the south celestial pole. A line from Sirius, the brightest star in the sky, through Canopus, the second-brightest, continued for the same distance lands within a couple of degrees of the pole. In other words, Canopus is halfway between the pole. Celestial sphere Celestial equator Circumpolar star Orbital pole Polaris Pole star visual representation of finding Polaris using the Big Dipper