Astronomical Unit

The astronomical unit 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

A variety of unit symbols and abbreviations have been in use for the astronomical unit. In a 1976 resolution, the International Astronomical Union (IAU) used the symbol A for the astronomical unit

The new definition also recognizes as a consequence that the astronomical unit is now to play a role of reduced importance, limited in its use to that of a convenience in some applications.

• 1 astronomical unit = 149597870700 metres (exactly)
• ≈ 92.955807 million miles
• ≈ 499.004 light-seconds
• ≈ 4.8481368 millionths (4.8481368×10−6) of a parsec
• ≈ 15.812507 millionths (15.812507×10−6) of a light-year

This definition makes the speed of light, defined as exactly 299792458 m/s, equal to exactly 299792458 × 86400 ÷ 149597870700 or about 173.144632674240 AU/d, some 60 parts per trillion less than the 2009 estimate.