Of all the Moon’s influences on Earth, none is more familiar—or more constant—than the tides.

Stand at the shore: water rises, pauses, retreats—twice a day. This rhythm comes from imbalance. The Moon’s gravity pulls more strongly on the near side of Earth than the far side, stretching the oceans into two broad bulges. One faces the Moon; the other forms on the opposite side as Earth is pulled slightly away from its own seas. As the planet turns, coastlines pass through these bulges, and the tides advance and withdraw with metronomic precision.

The Moon is the primary driver. The Sun adds a secondary influence—sometimes reinforcing the Moon’s pull, sometimes countering it, producing the familiar alternation of spring tides and neap tides. When Sun and Moon align at new and full Moon, their forces combine and tidal ranges grow. When they stand at right angles, the oceans settle into more modest swings. The rhythm is steady, global, and relentless, written into the planet’s rotation.

That rhythm was stronger in the past. When the Moon orbited much closer, tidal ranges were far larger, and the constant drag of moving water bled energy from Earth’s spin. The planet slowed. The Moon, conserving that energy, drifted outward. Geological records—from layered tidal sediments to fossil growth bands—show that days were shorter and tides more forceful. Over deep time, the system relaxed into the gentler cycles we experience now.

Today their scale has diminished—the Moon is farther, the pull gentler, the tides less dramatic than they once were. But the exchange continues. Every wave breaking on a shore is the ocean answering a gravitational tug from 239,000 miles away. The Moon doesn’t touch the water. It doesn’t need to.