1 AU = 149,597,870.700 kilometers (or 149,597,870,700 meters)
An Astronomical Unit is the mean distance between
the Earth and the Sun. In 2012, the International Astronomical Union defined the distance to be 149,597,870,700 meters.
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
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.