Astronomy
The Influence of the Moon on Earth's Tides and Rotation
Quick fact
The Moon is drifting away from Earth at about 3.8 centimeters per year, and it has slowed Earth's rotation so much that a day was only about 22 hours long during the age of the dinosaurs.
Why this is interesting
Each time you visit the beach, the Moon is quietly tugging on the ocean and even slowing down your planet's spin. In fact, the day you spend 24 hours long is slightly longer than it was a century ago—and the Moon is to blame.
Read the full explanation
Understanding The Influence of the Moon on Earth's Tides and Rotation
Imagine stretching a rubber band between two balls—the pull is stronger on the side facing the other ball. Similarly, the Moon's gravity pulls on Earth, but it doesn't pull uniformly: the side of Earth facing the Moon feels a stronger pull than the center, and the center feels a stronger pull than the far side. This difference in pull stretches Earth, creating two bulges of water—one facing the Moon and one opposite it. This is why in most places you see two high tides and two low tides each day. As Earth rotates, different parts of the planet pass through these bulges, creating the rhythm of the tide. The Moon's gravity holds these bulges in place while Earth turns underneath them, so the bulges seem to move around the planet.
A deeper explanation
The mechanism behind the tides is the gradient of the Moon's gravitational force across Earth's diameter. The gravitational pull is stronger on the near side, weaker on the far side, creating a differential force that pulls water outward on the near side and permits a bulge on the far side because the solid Earth is pulled away from the water. This tidal force also acts on Earth's solid crust, but water moves more visibly. Because Earth rotates faster (once a day) than the Moon orbits (once a month), the tidal bulges are dragged ahead of the Earth-Moon line. This creates tidal friction: as Earth spins, it tries to drag the bulges with it, but the Moon's gravity pulls back. This interaction transfers rotational energy from Earth to the Moon's orbit. As a result, Earth's rotation slows down (the day lengthens) and the Moon's orbit expands, moving it farther away. Over billions of years, this energy transfer has significantly altered Earth's day length and continues to shape both bodies.