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Shadow of the Moon

The tidal effects of the Earth on the Moon have long since stopped the spin of the Moon with respect to the Earth, so the Moon now keeps the same face toward us. But the Moon still spins once a month with respect to the Sun. And we can watch that from the Earth. In a little over two weeks the Moon waxes from new to full and then, in a little over two weeks, it wanes from full to new. If you lived on the Moon, the Sun would come up only once a month and stay up for two weeks at a stretch, and the rocks get hotter than boiling water. And, although on the Earth when the Sun sets over the mountains, you can still find your campsite, on the Moon, when the Sun sets over the mountains, all you could see would be stars.


Now the plane of the Moon’s orbit around the Earth is not in the plane of the Earth’s equator, so, as seen from the Earth, the Moon goes north and south once a month. And sometimes it crosses through the shadow of the Earth and we have what is known in the trade as an eclipse of the Moon. But the Earth is much bigger than the Moon, so the total shadow of the Earth (the umbra) more than covers the whole face of the Moon. And if you were an astronaut on the Moon at that time you’d see sunset light all around the edge of the Earth. That is why, as seen from the Earth, there is that lovely reddish glow on the face of the eclipsed Moon.

But occasionally, when the Moon crosses, it goes between us and the Sun, so that then we see only its shady side, and the darkness of the sky around the unlit face of the Moon we see the Sun’s corona. And great excitement ensues.


But the Moon is much smaller than the Earth, so sometimes, when the Moon is a little too far away, in its elliptical orbit, the tip of the Moon’s total shadow (the umbra) doesn’t quite reach the Earth’s surface, and we see the disk of the Moon not quite covering the disk of the Sun. We see a little rim of the Sun’s disk around the Moon. And it is know in the trade, not as a total eclipse of the sun, but as an annular eclipse. But if the Moon, when it crosses between us and the Sun, is a little closer to Earth, then the tip of the Moon’s total shadow moves across the face of the Earth as a shadow in the sunlight a few miles wide. And this August, the path of the Moon’s shadow crosses Europe, where, if the weather is clear, many people will have the opportunity to bask in the shadow of the Moon.


But wait! The tidal effect of the Moon on the Earth are slowing the daily spin of the Earth and our days are getting longer. And the Exterior Decorator has rules. You can’t get rid of angular momentum. You can push it around or cancel it with an equal and opposite angular momentum, but you can’t just make it disappear. So the angular momentum of the Earth’s spin is being transfered to the Moon’s orbit and the Moon is going farther away. So after a time (and I won’t say how long) we won’t see any more total eclipses of the Sun because the Moon will be too far away to let the tip of its shadow move across the face of the Earth. So, COME BASK IN THE SHADOW OF THE MOON WHILE YET YOU CAN!