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| Space on Jan 22, 2014

What is an astronomical unit?

Astronomers use astronomical units – or AU – to describe solar system distances. Definition of AU here. Also, mean distances in AU to prominent solar system objects.

Earth and the sun, as viewed by the Space Shuttle Discovery.   An astronomical unit is the average distance between these two bodies in space.  Image via NASA

Earth and the sun, as viewed by the Space Shuttle Discovery. An astronomical unit is the average distance between these two bodies in space. Image via NASA

Astronomers like to list the distances to objects within our solar system (planets, dwarf planets, asteroids, comets, spacecraft, etc.) in terms of an astronomical unit. How far is that? Follow the links below to learn more about this basic distance unit in our solar system.

Definition of astronomical unit.

Mean distance in AU from sun to each planet.

Mean distance in AU from sun to some dwarf planets.

Mean distance in AU to Kuiper Belt, farthest spacecraft, Oort Cloud.

Amount of distance in a light-year, measured in AU

Definition of astronomical unit. For general reference, we can say that one astronomical unit (AU) represents the mean distance between the Earth and our sun. The AU is approximately 150 million kilometers or 93 million miles. It is approximately 8 light-minutes.

More exactly, one astronomical unit (AU) = 149,597,871 kilometers = 92,955,807 miles.

Earth’s orbit around the sun isn’t a perfect circle. So Earth’s distance from the sun changes throughout the year. Astronomers give the Earth’s changing distance throughout the year relative to the astronomical unit, too. For instance, when the Earth is at perihelion – its nearest point to the sun for the year, in January – it’s about 0.983 AU from the sun. When our planet swings out to aphelion – its farthest point, in July – we’re about 1.017 AU away from the sun.

Distances from the sun of planets in our solar system, expressed in A.U.  Graph via planetsforkids.org

Distances from the sun of planets in our solar system, expressed in A.U. Graph via planetsforkids.org

Mean distance in AU from sun to each planet.

Mercury: 0.387 AU
Venus: 0.723 AU
Earth: 1.000 AU
Mars: 1.524 AU
Jupiter: 5.203 AU
Saturn: 9.529 AU
Uranus: 19.19 AU
Neptune: 30.06 AU

If you want to find out the distances of the solar system planets from the Earth and sun right now, click here or here.

Artist's concept of the dwarf planet Eris, whose distance from the sun varies from 38.255 to 97.661 au.

Artist’s concept of the dwarf planet Eris, whose distance from the sun varies from 38.255 to 97.661 au. Image via HubbleSite

Mean distance in AU from sun to some dwarf planets.

Ceres: 2.767 AU
Pluto: 39.53 AU
Eris: 67.958 AU
Sedna: 518.57 AU

Artist's rendering of the Kuiper Belt and the Oort Cloud, the distant icy realm of the solar system. Image credit: NASA

Artist’s rendering of the Kuiper Belt and the Oort Cloud, the distant icy realm of the solar system. Image credit: NASA

Mean distance in AU to Kuiper Belt, farthest spacecraft, Oort Cloud.

Kuiper Belt: 30 to 55 AU

Farthest spacecraft: Voyager 1: 126.685 AU (as of January 2014)

Oort Cloud: 5,000 to 100,000 AU

Largest circle with yellow arrow indicates one light year from our sun.  Read more about this image at Wikimedia Commons.

Largest circle with yellow arrow indicates one light year from our sun. Smallest yellow sphere is one light-week. Larger yellow sphere is one light-month. Read more about this image at Wikimedia Commons.

Amount of distance in a light-year, measured in AU

One light-year = 63,240 AU

Bottom line: Astronomers like to list the distances to objects within our solar system (planets, dwarf planets, asteroids, comets, spacecraft, etc.) in terms of the astronomical unit, or AU. One astronomical unit is the approximate mean distance between the Earth and sun. It’s about 150 million kilometers or 93 million miles.