"How much tidal stress would be necessary to keep geologically active a world as massive as Mars?"
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Question
Why does the moon change shapes?
The Moon changes shapes because it orbits around the Earth every 28 days, and the fraction that is illuminated by the Sun (and visible on Earth) keeps changing during that time. When the Moon and the Sun are on opposite sides of the Earth, we see a full Moon, i.e., the entire disk is illuminated. When the Moon is on the same side of Earth as the Sun, then we see (or sometimes don't see) a new Moon, that is, when only a tiny sliver is illuminated. When the Moon happens to pass directly between the Earth and the Sun, which happens every now and then, we get an eclipse, in which the Moon blocks all or part of the Sun's light. Answered by Prof. James Kasting
May 1, 2002
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