Glossary term: Astronomical Unit


An astronomical unit (AU) is a descriptive and convenient unit used by astronomers to measure distances. One AU is equal to exactly 149,597,870.7 kilometers (km) or about 150 million km. It is based on the average distance from Earth to the Sun. It is a unit used by astronomers to measure distances in the Solar System. The farthest planet in our system is Neptune located at about 30 AU. In contrast, the closest star, Proxima Centauri, lies at about 268,770 AU or 4.25 light years (which represents the time taken by light emitted by the Sun to get there). The AU is also used to measure orbital dimensions in multiple-star systems and exoplanet systems.

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The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".