{{Unit of length|m=149597870691}}
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 [[Plus-minus sign|±]] 30 metres (nearly 150 million kilometres or 93 million miles).

The symbol ''ua'' is recommended by the [[Bureau International des Poids et Mesures]]<ref>{{SIbrochure|page=126}},</ref> but in the United States and other [[anglophone]] countries the reverse - ''au'' - is more common. The [[International Astronomical Union]] recommends ''au''<ref>http://www.iau.org/IAU/Activities/nomenclature/units.html</ref> 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.

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 [[test particle|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.

==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°.

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 [[myriad]]s 400 and 80000"). This has been translated either as 4,080,000 [[Stadiametric rangefinding|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%).

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 (planet)|Mars]] from two locations on the Earth, they arrived at a figure of about 140 million kilometres.

A somewhat more accurate estimate can be obtained by observing the [[transit of Venus]]. This method was devised by [[James Gregory (astronomer and mathematician)|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]].

Another method involved determining the constant of [[aberration of light|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 discovery of the [[near-Earth asteroid]] [[433 Eros]] and its passage near the Earth in [[1900]]&ndash;[[1901]] allowed a considerable improvement in parallax measurement. More recently very precise measurements have been carried out by [[radar]] and by [[telemetry]] from [[space probe]]s.

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.

== Examples ==
The distances are approximate mean distances. It has to be taken into consideration that the distances between [[astronomical object|celestial bodies]] change in [[time]] due to their [[orbit]]s and other factors.

* The [[Earth]] is 1.00 ± 0.02 AU from the [[Sun]].
* The [[Moon]] is 0.0026 ± 0.0001 AU from the Earth.
* [[Mars (planet)|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: [[Heliosphere#Termination_shock|Termination shock]] between [[Solar Winds]]/Inter[[stellar wind]]s/[[Interstellar Medium]]
* As of August 2006, [[Voyager 1]] is 100 AU from the Sun, the furthest of any human-made object.
* 100 AU: [[Heliosheath]]
* 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 sphere|Hill/Roche sphere]]). beyond this is true [[interstellar space]]
* [[Proxima Centauri]] (the second-nearest [[star]] to [[Earth]]) 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:
* 1 AU = 149 597 870.691 ± 0.030 km ≈ 92 955 807 miles ≈ 8.317 [[light-year|light minutes]] ≈ 499 [[light-second]]s
* 1 [[light-second]] ≈ 0.002 AU
* 1 [[gigameter]] ≈ 0.007 AU
* 1 [[light-minute]] ≈ 0.120 AU
* 1 [[microparsec]] ≈ 0.206 AU
* 1 [[terameter]] ≈ 6.685 AU
* 1 [[light-hour]] ≈ 7.214 AU
* 1 [[light-day]] ≈ 173.263 AU
* 1 [[milliparsec]] ≈ 206.265 AU
* 1 [[light-week]] ≈ 1212.84 AU
* 1 [[light-month]] ≈ 5197.9 AU
* 1 [[light-year]] ≈ 63,241 AU
* 1 [[parsec]] ≈ 206,265 AU

==See also==
* [[Orders of magnitude (length)]]

== 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: [http://ssd.jpl.nasa.gov/iau-comm4/iausgnsrpt.ps PostScript]. Tables from the report also available: [http://ssd.jpl.nasa.gov/astro_constants.html Astrodynamic Constants and Parameters])''
* D. D. McCarthy ed., IERS Conventions (1996), IERS Technical Note 21, Observatoire de Paris, July 1996
{{reflist}}

== External links ==
*''[http://www.sil.si.edu/exhibitions/chasing-venus/intro.htm Chasing Venus, Observing the Transits of Venus]'' Smithsonian Institution Libraries
* [http://physics.nist.gov/cuu/Units/outside.html Units outside the SI] ''(at the [[NIST]] web site)''
* [http://www.iau.org/Units.234.0.html Recommendations concerning Units] ''(at the [[International Astronomical Union|IAU]] web site)''
* [http://home.comcast.net/~pdnoerd/SMassLoss.html Solar Mass Loss, the Astronomical Unit, and the Scale of the Solar System] ''(a discussion of the relation between the AU and other quantities)''

[[Category:Units of length in astronomy]]
[[Category:Celestial mechanics]]

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