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Astronomical Unit (AU) ○◂｜Definition｜1st｜20260419155516-00-⌔

Astronomical unit - Wikipedia

Astronomical unit

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The astronomical unit (symbol: au1234 or AU) is a unit of length defined as 149 597 870 700 m.5 Historically, the astronomical unit was conceived as the average Earth-Sun distance (the average of Earth’s aphelion and perihelion), before its modern redefinition in 2012.

The astronomical unit is used primarily for measuring distances within the Solar System or around other stars. It is also a fundamental component in the definition of another unit of astronomical length, the parsec.6 One au is approximately equivalent to 499 light-seconds.

Printed 2026-06-28.

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Footnotes

  1. On the re-definition of the astronomical unit of length (PDF). XXVIII General Assembly of International Astronomical Union. Beijing, China: International Astronomical Union. 31 August 2012. Resolution B2. Archived from the original (PDF) on 5 March 2025… recommends… 5. that the unique symbol “au” be used for the astronomical unit. ↩

  2. “Monthly Notices of the Royal Astronomical Society: Instructions for Authors”. Oxford Journals. Archived from the original on 22 October 2012. Retrieved 20 March 2015. The units of length/distance are Å, nm, μm, mm, cm, m, km, au, light-year, pc. ↩

  3. “Manuscript Preparation: AJ & ApJ Author Instructions”. American Astronomical Society. Archived from the original on 21 February 2016. Retrieved 29 October 2016. Use standard abbreviations for… natural units (e.g., au, pc, cm). ↩

  4. The International System of Units (PDF), V4.01 (9th ed.), International Bureau of Weights and Measures, June 2026, p. 145, ISBN 978-92-822-2272-0 ↩

  5. On the re-definition of the astronomical unit of length (PDF). XXVIII General Assembly of International Astronomical Union. Beijing: International Astronomical Union. 31 August 2012. Resolution B2. Archived from the original (PDF) on 5 March 2025… recommends[adopted] that the astronomical unit be re-defined to be a conventional unit of length equal to exactly 149597870700 metres, in agreement with the value adopted in IAU 2009 Resolution B2 ↩

  6. Luque, B.; Ballesteros, F.J. (2019). “Title: To the Sun and beyond”. Nature Physics. 15 (12): 1302. Bibcode:2019NatPh..15.1302L. doi:10.1038/s41567-019-0685-3. ↩

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