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Re: Astronomical Unit (AU)



I have seen one astronomical unit (AU) defined as:

1) the mean distance between the earth and the sun
2) the semi-major axis of the earth's orbit around the sun

Both of these are more or less correct, where the mean distance is taken
to be the time average rather than the average focal distance taken around
the ellipse. A complication arises because the Earth has a large satellite
which prevents its following a simple elliptical orbit about the Sun. The
usual fix is to take Earth's mean orbit about the Sun, neglecting a small,
nearly periodic variation in the orbit.

A useful number to carry around with you is that the astronomical unit is
499. light seconds (the next two digits are zeros). Given the speed of
light is 299 792 458 m/s one can calculate an accurate distance in km. No,
it's not eight minutes - that's a lousy number. Stamp it out!

Leigh