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Moon and Regulus

July 1, 2014

Ocean tides are strong and relentless. Over the millennia, they help reshape the coastlines. And over eons, they slow Earth’s rotation on its axis, making the day a little longer.

The tides are powered by the gravitational pull of the Moon and Sun, with lunar tides much stronger than solar tides. The Moon’s gravity pulls a little more strongly on the side of Earth that faces it than on the side that’s away from it. That creates “bulges” in the oceans that travel around the planet. When a bulge hits land, the ocean level rises, creating high tide. And as a bulge moves away, the ocean level drops, creating low tide.

Earth’s gravity creates tides on the Moon as well. Since the Moon doesn’t have oceans, the tides are in the solid surface — the rock actually bulges outward by a few inches. These tides have slowed the Moon’s rotation rate so that it matches the length of its orbit around Earth. As a result, the same lunar hemisphere always faces our way.

That means that if you were standing on that side of the Moon, Earth would remain at almost exactly the same spot in your sky, day after day, night after night. You’d see Earth go through phases, just as the Moon does, but our bright blue planet would always hover at the same spot in the sky.

And the crescent Moon is hovering low in the west as night falls this evening. Regulus, the bright star that marks the heart of Leo, the lion, is close to the Moon’s upper right.


Script by Damond Benningfield, Copyright 2014

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