Astronomical Unit (au)

AstronomicalCurrent unit

Exactly 149,597,870,700 meters, roughly the average distance from the Earth to the Sun. Used for distances within the solar system.

1 astronomical unit = 149,597,870,700 meters (exactly).

About the astronomical unit

Astronomers began calling it the astronomical unit in the mid-nineteenth century, with the name first recorded in 1848, though the quantity itself had been chased since antiquity. Aristarchus put the Sun 18 to 20 times farther away than the Moon, when the true ratio is about 389. Cassini and Richer used the parallax of Mars in 1672, transits of Venus served for generations, and radar echoes from Venus and Mars in the early 1960s finally gave direct distances.

For most of the twentieth century the unit was tied to the Gaussian gravitational constant, which meant its value had to be remeasured as observations improved. In 2012 the IAU removed that dependence by fixing it at exactly 149,597,870,700 meters and recommending the symbol au. The figure used here is that exact value. It is close to the mean Earth-Sun separation, which varies from about 0.983 au at perihelion to about 1.017 au at aphelion. Light needs roughly 499 seconds to cross one au, and Neptune orbits at about 30 au.

Sources: Wikipedia: Astronomical unit; NIST SP 811 Appendix B.8: factors for units outside the SI; Wikipedia: Light-second

Astronomical Unit conversion chart

1 astronomical unit expressed in every other length unit:

More Astronomical units

• light-second (length)
• light-minute (length)
• light-hour (length)
• light-year (length)
• parsec (length)
• megaparsec (length)
• Earth mass (weight & mass)
• solar mass (weight & mass)
• Julian year (time)
• tropical year (time)
• sidereal year (time)
• synodic month (time)
• sidereal day (time)
• aeon (time)
• milliarcsecond (angle)
• zodiac sign (angle)

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