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>>>> A dumb question - I recall that an AU, or Astronomical Unit, is the radius of Earth orbit, eg approx 93 million miles. <<<<
The Earth's orbit doesn't *have* a radius, because it's not a circle. The Astronomical Unit is roughly the mean distance from the Earth to the Sun, and it is an arbitrary unit relating purely to our local solar system. For more detail, including the formal definition (which doesn't refer to the Earth's orbit at all, go here:
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