Glossary term: Tide

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On Earth, the gravitational force is almost constant, pointing what we call downwards. But over larger distances, gravity's strength and direction vary, and those differences are called tidal forces. Earth and Moon, for instance, orbit a common center of mass due to their mutual gravitational attraction, but objects on the side of Earth nearer the Moon will be accelerated a bit stronger towards the Moon, objects on the opposite side of Earth somewhat less. Earth's ocean water follows those tidal accelerations, forming "tidal bulges" directly under the Moon and on the opposite side, which produces Earth's tides. Closer than the so-called Roche limit, a planet's tidal force can tear apart larger gravitationally-bound objects like moons. Near a black hole, tidal forces grow beyond limit.

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