The word tide is a bit confusing; most people think of it as the ebb and flow of the ocean at the beach, caused by the moon as it orbits Earth. But astronomers think of it as a force... that causes the water to wash in and out.
Gravity is what holds the moon in orbit around Earth. But gravity has a critical characteristic: it gets weaker with distance. This change in the force of gravity is what astronomers call the tidal force.
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