Astronomical unit

Astronomical unit
The red line indicates the Earth–Sun distance, which on average is about 1 astronomical unit.
Standard: Astronomical system of units
(Accepted for use with the SI)
Quantity: length
Symbol: au or ua
Expressed in: 1 au or ua =
metric (SI) units Script error: No such module "val".
imperial & US units Script error: No such module "val".
astronomical units Script error: No such module "val".
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File:Aphelion (PSF).svg
1. Planet at aphelion 2. Planet at perihelion 3. Sun. Drawing not to scale

The astronomical unit (AU) is a unit of length derived from the Earth's orbit. It is the average distance the Earth gets from the Sun on the long axis of the ellipse. Its definition is: the length of the semi-major axis of the Earth's elliptical orbit around the Sun. "Semi-major" means half the long axis.

The AU is about 150 million kilometers or 93 million miles. Astronomers usually measure distances within the Solar System in astronomical units. Mars is about 1.4 AU from the Sun, Jupiter lies at roughly 5.2 AU, and Neptune is roughly 30 AU from the Sun. Light travels an AU in about 8.317 minutes.

More accuracy

From 1976 to 2012 the AU was defined as “the radius of an unperturbed circular Newtonian orbit about the Sun of a particle having infinitesimal mass, moving with a mean motion of 0.01720209895 radians per day (known as the Gaussian constant)”. [1][2][3] In 2012, the IAU redefined it to be simply Script error: No such module "val"..[4]

In the IERS numerical standards, the speed of light in a vacuum is defined as c0 = Script error: No such module "val"., in accordance with the SI units. The time to cover an AU is τA = Script error: No such module "val"., resulting in the astronomical unit in metres as c0τA = Script error: No such module "val"..[5] It is very roughly the distance from the Earth to the Sun.


  1. Resolution No. 10 of the XVIth General Assembly of the International Astronomical Union, Grenoble, 1976
  2. H. Hussmann, F. Sohl, J. Oberst (2009), "§ Astronomical units", in Joachim E Trümper (ed.), Astronomy, astrophysics, and cosmology. Volume VI/4B Solar System, Springer, p. 4, ISBN 3-540-88054-2CS1 maint: Multiple names: authors list (link)
  3. Gareth V Williams (1997), "Astronomical unit", in James H. Shirley, Rhodes Whitmore Fairbridge (ed.), Encyclopedia of planetary sciences, Springer, p. 48, ISBN 0-412-06951-2
  4. Geoff Brumfiel (14-Sep-2012). "The astronomical unit gets fixed: Earth–Sun distance changes from slippery equation to single number". Retrieved 14-Sep-2012. Check date values in: |accessdate=, |date= (help)
  5. Gérard Petit and Brian Luzum, eds. (2010). "Table 1.1: IERS numerical standards" (PDF). IERS technical note no. 36: General definitions and numerical standards. International Earth Rotation and Reference Systems Service.CS1 maint: Extra text: authors list (link) For complete document see Template:Cite book

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