What is an astronomical unit?
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. 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.
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
Ceres: 2.767 AU
Pluto: 39.53 AU
Eris: 67.958 AU
Sedna: 518.57 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
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.