Earth's Spin Is Slowing Down
As the moon's gravity causes the tides to rise and fall, the motion drains energy from Earth's rotation, slowing it down. And if one rotation equals one day, then the days are slowly but surely getting longer. In 100 years, one day will be about two milliseconds longer than it is now. In billions of years, when the Earth spins at the same rate as the moon orbits it, one day will be equal to about 27 current days.
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Key Facts In This Video
Tidal forces cause high tides twice a day: once when the moon is overhead, and once when it's on the other side of the Earth. (0:20)
Eventually, the Earth will spin slowly enough that the same side will always face the moon. (1:06)
If the moon was too close to Earth, tidal forces from the Earth would cause the moon to disintegrate. (1:21)