Astronomical unit

Astronomical Unit

An astronomical unit (symbol: au, or AU) is a unit of length, roughly the distance from Earth to the Sun. However, that distance varies as Earth orbits the Sun, from a maximum (aphelion) to a minimum (perihelion) and back again once a year. Originally conceived as the average of Earth's aphelion and perihelion, since 2012 it has been defined as exactly 149,597,870.7 kilometers.

History

The astronomical unit was originally defined as the radius of an unperturbed circular Newtonian orbit about the Sun of a particle having infinitesimal mass, moving with a mean motion of 0.01720209895 radians per day (known as the Gaussian gravitational constant).

Usage and significance

The astronomical unit provides a convenient way to express and relate measurements of distances in the solar system, and to carry out astronomical calculations. For example, the Laws of Kepler describe the motion of the planets in terms of astronomical units and years.

See also

• Parsec
• Light-year
• Orders of magnitude (length)