1 AU = 149,597,870.691 kilometers
Definition:
An Astronomical Unit is approximately the mean distance between
the Earth and the Sun. It is a derived constant and used to indicate
distances within the solar system.
Its formal definition is the radius of
an unperturbed circular orbit a massless body would revolve about the sun
in 2*(pi)/k days (i.e., 365.2568983.... days), where k is defined as the
Gaussian constant exactly equal to 0.01720209895.
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.
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