Definition of Astronomical Unit

The astronomical unit, or AU, is the average distance between the Sun and the Earth. It is used as a convenient unit of measurement when calculating distances within the solar system. The AU, as defined in the IAU (1976) system of constants, is equal to 1 AU = 149,597,870.691 kilometres

In terms of other units, 1 AU is equal to;

92 955 807        Miles

499.004 782      Light Seconds

1 / 63 241          Light Years

Historical

In the 15th century it was identified that the phenomenon known as the Transit of Venus could be used to measured the Astronomical Unit.  From that time forward the best Surveyors and Astronomers have traveled the globe to study this event to refine their calculations .  In 1769 a little known mariner by the name James Cook traveled to Tahiti to view the Transit, from there he continued on with his mission to survey the South Pacific, and locate the fabled Terra Australis known as the Great Souther Land.

 

Transit of Venus Australia 2012