The Moon does not only light the night. Its gravity pulls on Earth’s oceans, raising two watery bulges: one facing the Moon, one on the opposite side, where the planet itself is pulled a little more than the water. As Earth rotates through those bulges, coastlines experience high and low tide, usually twice a day. The Sun pulls as well, though less strongly at the surface because it is so much farther away. When Sun, Moon, and Earth line up at new and full moon, the tides add and become spring tides, higher highs and lower lows. When they form a right angle at quarter moons, neap tides are milder.

Local shape of the seafloor and coastline can delay, amplify, or cancel the pattern. Some bays funnel water into dramatic ranges. Enclosed seas may have almost none. The Mediterranean is relatively quiet. The Bay of Fundy is famous for a huge rise and fall.

Tides also slow Earth’s spin, very slightly, as friction in the moving water and solid Earth dissipates energy. The Moon drifts farther away as a result, a few centimeters a year. Long ago the day was shorter and the Moon looked larger. The calendar of fishermen and the timing of some coral spawning are still written by this three-body dance of rock, water, and light.
