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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.

Artist's concept of our Earth and sun. Image via NASA.

Artist’s concept of Earth and sun. One astronomical unit = the average distance between them. Image via NASA.

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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 from sun to each planet, in AU.

Mean distance from sun to some dwarf planets, in AU.

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

Amount of distance in a light-year, 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. An AU is approximately 93 million miles (150 million km). It’s approximately 8 light-minutes.

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

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 (semi-major axis) from sun to each planet, in AU.

Mercury: 0.387 AU
Venus: 0.723 AU
Earth: 1.000 AU
Mars: 1.524 AU
Jupiter: 5.203 AU
Saturn: 9.582 AU
Uranus: 19.201 AU
Neptune: 30.047 AU

Source: Planetary Fact Sheet

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 from sun to some dwarf planets, in AU.

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 via NASA

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

Kuiper Belt: 30 to 55 AU

Farthest spacecraft: Voyager 1: 137.053 AU (as of October 2016)

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, 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 93 million miles (150 million km), or 8 light-minutes.

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Bruce McClure

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