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  • The earth completes a full rotation once per day, and hence the moon passes overhead pretty

  • much once daily. So then why are there two high and low tides every day?

  • Simple: the moon's gravitational attraction is stronger the closer you are to it. So the

  • close side of the earth gets pulled away from the middle, which in turn gets pulled away

  • from the far side. As the earth rotates, these so-called "tidal forces" cause water on the

  • surface to slosh around as "high tides" twice per day - when the moon is directly over your

  • head and directly below your feet.

  • So why is gravity weaker at a greater distance? Well, imagine you're spraying a stream of

  • water at a sheep [flamingo]. If the sheep's more distant, less of the water hits it than

  • when it's close, so there's less force pushing it away. Gravity behaves the same way, except

  • it's attractive.

  • And there are tidal forces from the sun as well: they're weaker than the lunar ones,

  • but when the sun and moon line up [on the same or opposite sides of the earth] the tides

  • pack an extra punch.

  • It's not just water that sloshes, though - tidal forces cause the earth's crust to bulge as

  • well - we just don't notice because we get lifted along with it. And tidal forces from

  • the earth cause the moon to bulge, too! All this bulging and sloshing dissipates some

  • of the earth's rotational energy, causing it to gradually slow its rotation. Eventually,

  • the earth will spin slowly enough that the same side of the earth will always face the

  • moon - just as the same side of the moon already always faces the earth! This means that in

  • the distant future, a day and a lunar month will each have the same length: about 50 of

  • our current earth days.

  • What's more, if the moon were to get too close to us, tidal forces from the earth would be

  • stronger than the gravity holding the moon together, so rocks on the moon's surface would

  • literally start flying off into space, and the moon would disintegrate. This would also

  • happen if the earth were to get too close to the sun - and if you decided to fly into

  • a black hole, it's what would actually do you in: your spaceship would get ripped apart

  • from tip to tail by tidal forces. Now that's bad tidings.

The earth completes a full rotation once per day, and hence the moon passes overhead pretty

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タイズ (The Tides)

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    Why Why に公開 2021 年 01 月 14 日
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