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An astronomical unit (AU) is a distance unit representing the distance between the Earth and the Sun, or more specifically the mean of the Earth's orbit's aphelion and perihelion. The International Astronomical Union (IAU) has defined a standard AU length of 149597870700 meters, which is considered a reasonable exact value (i.e., "nominal" value) approximating this distance, and tying it to SI standard units. The AU is commonly used for distances within the solar system and other star systems, e.g., for orbital radii. An AU is:
Some distances in AU:
| 0.000085 AU | Earth diameter |
| 0.0026 AU | distance from Earth to Moon |
| 0.0093 AU | diameter of the Sun |
| 5.2 AU | radius of Jupiter's orbit |
| 30 AU | radius of Neptune's orbit |
| 276200 AU | distance to Alpha Centauri |
| 1.6 billion AU | distance to galactic center |
| 1.6 trillion AU | distance to Andromeda |