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Encyclopedia > Astronomical unit
1 astronomical unit =
SI units
149.598×106 km 149.598×109 m
Astronomical units
4.8481×10-6 pc 15.8125×10−6 ly}}
US customary / Imperial units
92.9558×106 mi 490.807×109 ft

The astronomical unit (AU or au or a.u. or sometimes ua) is a unit of length approximately equal to the distance from the Earth to the Sun. The currently accepted value of the AU is 149,597,870,691 ± 30 metres (nearly 150 million kilometres or 93 million miles). “SI” redirects here. ... “km” redirects here. ... This article is about the unit of length. ... A parsec is the distance from the Earth to an astronomical object which has a parallax angle of one arcsecond. ... A light-year, symbol ly, is the distance light travels in one year: exactly 9. ... U.S. customary units, also known in the United States as English units[1] (but see English unit) or standard units, are units of measurement that are currently used in the USA, in some cases alongside units from SI (the International System of Units — the modern metric system). ... The Imperial units are an irregularly standardized system of units that have been used in the United Kingdom and its former colonies, including the Commonwealth countries. ... “Miles” redirects here. ... A foot (plural: feet or foot;[1] symbol or abbreviation: ft or, sometimes, ′ – a prime) is a unit of length, in a number of different systems, including English units, Imperial units, and United States customary units. ... For other uses of this word, see Length (disambiguation). ... This article is about Earth as a planet. ... Sol redirects here. ... The plus-minus sign (±) is a mathematical symbol commonly used to indicate the precision of an approximation, or as a convenient shorthand for a quantity which has two possible values opposite in sign. ...


The symbol ua is recommended by the Bureau International des Poids et Mesures[1] but in anglophone countries the reverse - au - is more common. The International Astronomical Union recommends au[2] and international standard ISO 31-1 uses AU. In general, capital letters are only used for the symbols of units which are named after individual scientists, while au or a.u. can also mean atomic unit or even arbitrary unit; however, the use of AU to refer to the astronomical unit is widespread. The Bureau International des Poids et Mesures (International Bureau of Weights and Measures, or BIPM) is a standards organization, one of the three organizations established to maintain the SI system under the terms of the Metre Convention. ... Look up Anglophone in Wiktionary, the free dictionary. ... IAU redirects here. ... Standards are produced by many organizations, some for internal usage only, others for use by a groups of people, groups of companies, or a subsection of an industry. ... ISO 31-1 is the part of international standard ISO 31 that defines names and symbols for quantities and units related to space and time. ... The atomic mass unit (amu), unified atomic mass unit (u), or dalton (Da), is a small unit of mass used to express atomic masses and molecular masses. ...


Originally, the AU was defined as the length of the semi-major axis of the Earth's elliptical orbit around the Sun. In 1976, the International Astronomical Union revised the definition of the AU for greater precision, defining it as the distance from the centre of the Sun at which a particle of negligible mass, in an unperturbed circular orbit, would have an orbital period of 365.2568983 days (one Gaussian year). This definition gives a value that is slightly less than the mean Earth-Sun distance. An alternative way of stating the definition is that an AU is the distance at which the heliocentric gravitational constant (the product GM) is equal to (0.017 202 093 95)² AU³/d². For other uses of this word, see Length (disambiguation). ... The semi-major axis of an ellipse In geometry, the term semi-major axis (also semimajor axis) is used to describe the dimensions of ellipses and hyperbolae. ... Sol redirects here. ... In metric theories of gravitation, particularly general relativity, a test particle is an idealized model of a small object whose mass is so small that it does not appreciably disturb the ambient graviational field. ... For other uses, see Mass (disambiguation). ... The orbital period is the time it takes a planet (or another object) to make one full orbit. ... A Gaussian year is defined as 365. ... According to the law of universal gravitation, the attractive force between two bodies is proportional to the product of their masses and inversely proportional to the square of the distance between them. ...

Contents

History

Aristarchus of Samos estimated the distance to the Sun to be about 20 times the distance to the moon, whereas the true ratio is about 390. His estimate was based on the angle between the half moon and the sun, which he estimated as 87°. For other uses of this name, including the grammarian Aristarchus of Samothrace, see Aristarchus Statue of Aristarchus at Aristotle University in Thessalonica, Greece Aristarchus (Greek: Ἀρίσταρχος; 310 BC - ca. ... This article is about Earths moon. ...


According to Eusebius of Caesarea in the Praeparatio Evangelica, Eratosthenes found the distance to the sun to be "σταδιων μυριαδας τετρακοσιας και οκτωκισμυριας" (literally "of stadia myriads 400 and 80000"). This has been translated either as 4,080,000 stadia (1903 translation by Edwin Hamilton Gifford), or as 804,000,000 stadia (edition of Édouard des Places, dated 1974-1991). Using the Greek stadium of 185 to 190 metres, the former translation comes to a far-too-low 755,000 km, whereas the second translation comes to 148.7 to 152.8 million km (accurate within 2%). Eusebius of Caesarea Eusebius of Caesarea (c. ... This article is about the Greek scholar of the third century BC. For the ancient Athenian statesman of the fifth century BC, see Eratosthenes (statesman). ... For other uses, see Myriad (disambiguation). ... Stadiametric rangefinding, or the stadia method is a technique of measuring distances with a telescopic instrument. ...


At the time the AU was introduced, its actual value was very poorly known, but planetary distances in terms of AU could be determined from heliocentric geometry and Kepler's laws of planetary motion. The value of the AU was first estimated by Jean Richer and Giovanni Domenico Cassini in 1672. By measuring the parallax of Mars from two locations on the Earth, they arrived at a figure of about 140 million kilometres. Illustration of Keplers three laws with two planetary orbits. ... Jean Richer (born 1630; died 1696 in Paris) was a French astronomer and assistant (élève astronome) of Giovanni Domenico Cassini. ... Giovanni Domenico (Jean-Dominique) Cassini Portrait Giovanni Domenico Cassini (June 8, 1625–September 14, 1712) was an Italian astronomer, engineer, and astrologer. ... For other uses, see Parallax (disambiguation). ... Mars is the fourth planet from the Sun in the solar system, named after the Roman god of war (the counterpart of the Greek Ares), on account of its blood red color as viewed in the night sky. ...


A somewhat more accurate estimate can be obtained by observing the transit of Venus. This method was devised by James Gregory and published in his Optica Promata. It was strongly advocated by Edmond Halley and was applied to the transits of Venus observed in 1761 and 1769, and then again in 1874 and 1882. This article is about the astronomical phenomenon. ... James Gregory For other people with the same name, see James Gregory. ... Edmond Halley FRS (IPA: ) (November 8, 1656 – January 14, 1742) was an English astronomer, geophysicist, mathematician, meteorologist, and physicist. ...


Another method involved determining the constant of aberration, and Simon Newcomb gave great weight to this method when deriving his widely accepted value of 8.80″ for the solar parallax (close to the modern value of 8.794148″). The aberration of light (also referred to as astronomical aberration or stellar aberration) is an astronomical phenomenon which produces an apparent motion of celestial objects. ... Simon Newcomb. ... Motion Parallax (Greek: παραλλαγή (parallagé) = alteration) is the change of angular position of two stationary points relative to each other as seen by an observer, due to the motion of an observer. ...


The discovery of the near-Earth asteroid 433 Eros and its passage near the Earth in 1900–1901 allowed a considerable improvement in parallax measurement. More recently very precise measurements have been carried out by radar and by telemetry from space probes. Computer model of the Apollo Asteroid 6489 Golevka Near-Earth asteroids (NEAs) are asteroids whose orbits are close to Earths orbit. ... The asteroid 433 Eros (eer-os) was named after the Greek god of love Eros. ... For other uses, see Radar (disambiguation). ... Telemetry is a technology that allows the remote measurement and reporting of information of interest to the system designer or operator. ... Italic textBold text Technicians work on the Ulysses space probe. ...


While the value of the astronomical unit is now known to great precision, the value of the mass of the Sun is not, because of uncertainty in the value of the gravitational constant. Because the gravitational constant is known to only five or six significant digits while the positions of the planets are known to 11 or 12 digits, calculations in celestial mechanics are typically performed in solar masses and astronomical units rather than in kilograms and kilometres. This approach makes all results dependent on the gravitational constant. A conversion to SI units would separate the results from the gravitational constant, at the cost of introducing additional uncertainty by assigning a specific value to that unknown constant. According to the law of universal gravitation, the attractive force between two bodies is proportional to the product of their masses and inversely proportional to the square of the distance between them. ... Look up si, Si, SI in Wiktionary, the free dictionary. ...


Examples

The distances are approximate mean distances. It has to be taken into consideration that the distances between celestial bodies change in time due to their orbits and other factors. Astronomical objects are significant physical entities, associations or structures which current science has confirmed to exist in space. ... This article is about the concept of time. ... Two bodies with a slight difference in mass orbiting around a common barycenter. ...

  • The Earth is 1.00 ± 0.02 AU from the Sun.
  • The Moon is 0.0026 ± 0.0001 AU from the Earth.
  • Mars is 1.52 ± 0.14 AU from the Sun.
  • Jupiter is 5.20 ± 0.05 AU from the Sun.
  • Pluto is 39.5 ± 9.8 AU from the Sun.
  • The Kuiper Belt begins at roughly 35 AU
  • Beginning of Scattered disk at 45 AU (10 AU overlap with Kuiper Belt)
  • Ending of Kuiper Belt at 50-55 AU
  • 90377 Sedna's orbit ranges between 76 and 942 AU from the Sun; Sedna is currently (as of 2006) about 90 AU from the Sun.
  • 94 AU: Termination shock between Solar winds/Interstellar winds/Interstellar medium.
  • 100 AU: Heliosheath
  • 105 AU: As of February 2008, Voyager 1 is the furthest of any human-made objects from the Sun.
  • 100-150 AU: Ending of Scattered Disk
  • 500-3000 AU: Beginning of Hills cloud/"Inner Oort Cloud"
  • 20,000 AU: Ending of Hills Cloud/"Inner Oort Cloud", beginning of "Outer Oort Cloud"
  • 50,000 AU: possible closest estimate of the "Outer Oort Cloud" limits (1.0 ly)
  • 100,000 AU: possible farthest estimate of the "Outer Oort Cloud" limits (1.6 ly).
  • 125,000 AU: maximum extent of influence of the Sun's gravitational field (Hill/Roche sphere). beyond this is true interstellar space. This distance is roughly 1.8-2.0 light-years.
  • Proxima Centauri (the nearest star to Earth, excluding our own Sun) is ~268 000 AU away from the Sun.
  • The mean diameter of Betelgeuse is 2.57 AU.
  • The distance from the Sun to the centre of the Milky Way is approximately 1.7×109 AU.

Some conversion factors: This article is about Earth as a planet. ... Sol redirects here. ... This article is about Earths moon. ... Mars is the fourth planet from the Sun in the solar system, named after the Roman god of war (the counterpart of the Greek Ares), on account of its blood red color as viewed in the night sky. ... For other uses, see Jupiter (disambiguation). ... For other uses, see Pluto (disambiguation). ... The Kuiper belt, derived from data from the Minor Planet Center. ... The scattered disc (or scattered disk) is a distant region of our solar system, thinly populated by icy planetoids known as scattered disk objects (SDOs), a subset of the broader family of trans-Neptunian objects (TNOs). ... you are abunch of bull | bgcolour=#FFFFC0 | name=90377 Sedna | image= | caption= Sedna is located in the center of the green circle | discovery=yes | discoverer=M. Brown, C. Trujillo, D. Rabinowitz | discovered=November 14, 2003 | mp_name=90377 Sedna | alt_names= | mp_category=Trans-Neptunian object | epoch=September 26, 1990 (JD 2448160. ... 2006 is a common year starting on Sunday of the Gregorian calendar. ... The heliosphere is a bubble in space produced by the solar wind. ... The plasma in the solar wind meeting the heliopause The solar wind is a stream of charged particles (i. ... A solar wind is a stream of particles (mostly high-energy protons ~ 500 keV) which are ejected from the upper atmosphere of a star (in the case of a star other than the Earths Sun, it may be called a stellar wind instead). ... The interstellar medium (or ISM) is the name astronomers give to the tenuous gas and dust that pervade interstellar space. ... The locations of Voyagers 1 and 2 as of 2005 The heliosheath is the zone between the termination shock and the heliopause at the outer border of the solar system. ... For the album by The Verve, see Voyager 1 (album). ... The Hills cloud is a hypothetical inner region of the Oort cloud with an outer boundary of 2-3×104 AU, and a less well defined inner boundary at 50 to 3000 AU, proposed in 1981 by J. G. Hill. ... Artists rendering of the Oort cloud and the Kuiper Belt. ... A light-year, symbol ly, is the distance light travels in one year: exactly 9. ... A gravitational field is a model used within physics to explain how gravity exists in the universe. ... A Hill sphere approximates the gravitational sphere of influence of one astronomical body in the face of perturbations from another heavier body around which it orbits. ... Interstellar Space was one of the last albums recorded before the death of John Coltrane in 1967. ... Proxima Centauri (Latin proximus, -a, -um: meaning next to or nearest to)[4] is a red dwarf star that is likely a part of the Alpha Centauri star system and is the nearest star to the Sun at a distance of 4. ... This article is about the astronomical object. ... This article is about Earth as a planet. ... This article is about the star. ... For other uses, see Milky Way (disambiguation). ...

A light-year, symbol ly, is the distance light travels in one year: exactly 9. ... A light second is a unit of length. ... A light second is a unit of length. ... A gigametre (American spelling: gigameter) (symbol: Gm) is a unit of length equal to 109 metres. ... A light minute (also written light-minute) is a unit of length. ... A parsec is the distance from the Earth to an astronomical object which has a parallax angle of one arcsecond. ... A terametre (American spelling: terameter) (symbol: Tm) is a unit of length equal to 1012 metres. ... A light hour (also written light-hour) is a unit of length. ... A light day (also written light-day) is a unit of length. ... A parsec is the distance from the Earth to an astronomical object which has a parallax angle of one arcsecond. ... A light week (also written light-week) is a unit of length. ... A light month (also written light-month) is a unit of length. ... A light-year, symbol ly, is the distance light travels in one year: exactly 9. ... A parsec is the distance from the Earth to an astronomical object which has a parallax angle of one arcsecond. ...

See also

Categories: | ...

References

  • E. Myles Standish. "Report of the IAU WGAS Sub-group on Numerical Standards". In Highlights of Astronomy, I. Appenzeller, ed. Dordrecht: Kluwer Academic Publishers, 1995. (Complete report available online: PostScript. Tables from the report also available: Astrodynamic Constants and Parameters)
  • D. D. McCarthy ed., IERS Conventions (1996), IERS Technical Note 21, Observatoire de Paris, July 1996
  1. ^ Bureau International des Poids et Mesures (2006). The International System of Units (SI) (8th Edn). ISBN 92-822-2213-6. p. 126.,
  2. ^ IAU Website: Units

The Bureau International des Poids et Mesures (International Bureau of Weights and Measures, or BIPM) is a standards organization, one of the three organizations established to maintain the SI system under the terms of the Metre Convention. ...

External links

  • Chasing Venus, Observing the Transits of Venus Smithsonian Institution Libraries
  • Units outside the SI (at the NIST web site)
  • Recommendations concerning Units (at the IAU web site)
  • Solar Mass Loss, the Astronomical Unit, and the Scale of the Solar System (a discussion of the relation between the AU and other quantities)
  • AU may need to be redefined (measuring distances in AU will become increasingly imprecise as the Sun radiates away its energy) New Scientist, 6 February 2008
As a non-regulatory agency of the United States Department of Commerce’s Technology Administration, the National Institute of Standards (NIST) develops and promotes measurement, standards, and technology to enhance productivity, facilitate trade, and improve the quality of life. ... IAU redirects here. ...

  Results from FactBites:
 
Astronomical unit - Wikipedia, the free encyclopedia (939 words)
For greater precision, the International Astronomical Union in 1976 defined the AU as the distance from the centre of the Sun at which a particle of negligible mass, in an unperturbed circular orbit, would have an orbital period of 365.256 898 3 days (a Gaussian year).
While the value of the astronomical unit is now known to great precision, the value of the mass of the Sun is not, because of uncertainty in the value of the gravitational constant.
A conversion to SI units would separate the results from the gravitational constant, at the cost of introducing additional uncertainty by assigning a specific value to that unknown constant.
Astronomical distance - Wikipedia, the free encyclopedia (447 words)
The distance to an astronomical body is often, therefore, referred to by the time it takes light to reach the body.
Another often used unit, on the same order of magnitude as the light year is the parsec.
In the early 1900s an astronomer named Henrietta Leavitt examined the relationship between the absolute magnitude and period of the variable stars known as Cepheid variables.
  More results at FactBites »

 
 

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