AU

Astronomical Unit (A.U.)

1 AU = 149,597,870.691 kilometers Definition: An Astronomical Unit is an approximation of the mean distance between the Earth and the Sun, used to quantify distance within a solar system. Since an AU is based on radius of a circular orbit, one AU is actually slightly less than the average distance between the Earth and the Sun (approximately 150 million km or 93 million miles).

Historical Background: Tycho Brahe estimated the distance between the Sun and the Earth at 8 million kilometers (5 million miles). Later, Johannes Kepler estimated the AU was at 24 million kilometers (15 million miles). In 1672, Giovanni Cassini made a much better estimate by using Mars. By observing Mars from Paris and having a colleague, Jean Richer, also observe Mars at the same time in French Guiana in South America, Cassini determined the parallax of Mars. From that Cassini was able to calculate the distance from Earth to Mars, and then the distance from Earth to the Sun. Cassini calculated the AU to be at 140 million kilometers (87 million miles), which is lower, but very close to the modern day number.

Source: National Aeronautics and Space Administration (NASA).