Friday, March 02, 2007

A Total Eclipse of the Sun -- as seen from the moon

From our Earth-based view, tomorrow's total eclipse of the moon is caused when the full moon slips into Earth's shadow. But from the moon, it appears that the sun passes behind the "new"-phase Earth. As I mentioned yesterday, from the moon you would see every sunset and sunrise on the Earth at one time. Today's Astronomy Picture of the Day shows a simulation of what an astronaut on the moon might see if she were to look back at the Earth during a total eclipse.

Why do I say that, during the eclipse, it appears that the sun is passing behind the Earth? You probably remember how the moon always keeps the same face toward the Earth. This phenomenon means that, at a given point on the moon, the Earth appears to stay at the same place in the sky. The sun and the stars will move through the sky once each month, but the Earth would appear to magically hang in the sky, changing phases once every moon day (the moon spins once every lunar cycle). So, as the Earth waned from a thin crescent to the new phase, the sun, moving through the sky as the moon turned, would pass behind the Earth, plunging the blisteringly hot midday moon into a few hours of brutal, but beautiful, cold.

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