astronomical unit (AU)

An astronomical unit (AU) is the mean distance between the center of the Earth and the center of the sun. The actual distance varies depending on the time of year, because the Earth's orbit around the sun is not a perfect circle. The mean, or average, separation is approximately 1.4959787 x 10 ¹¹ meters, which is 1.4959787 x 10 ⁸ kilometers or 9.2955807 x 10 ⁷ miles.

Distances within the solar system are commonly measured in AU. Mercury, the innermost planet, orbits at a mean distance of about 0.4 AU from the sun, while Neptune, one of the most distant planets known, has an average orbital radius of about 30 AU. Interstellar and intergalactic distances are expressed in light-years or parsec s.

Interestingly, the Earth is closest to the Sun during the month of January, and farthest during July. The variation is about ±1 percent of the mean distance. Seasonal weather results from the tilt of the Earth's axis, and is not significantly affected by the distance of the planet from the sun.

Also see the Table of Physical Units and Constants.