Tides and the Moon

Why there are two bulges

The Moon's gravity pulls on all of Earth, but not equally. The side facing the Moon is pulled a little harder than Earth's center, and the far side a little less. This difference in pull, called the tidal force, stretches Earth slightly along the Earth–Moon line.

Ocean water responds most visibly. It heaps up on the side facing the Moon, where water is pulled ahead of Earth, and also on the opposite side, where water is left behind as Earth is pulled away from it. The result is two bulges, one on each side of the planet.

Two high tides a day

As Earth spins, a coastline passes through both bulges and the two low areas between them. That gives most places two high tides and two low tides each day.

The cycle takes about 24 hours 50 minutes, not 24 hours, because the Moon moves along its orbit while Earth turns. Earth must spin a little extra to catch up. That is why high tide arrives roughly 50 minutes later each day.

Real tides are messier than this simple picture. Continents block the bulges, ocean basins slosh like water in a bathtub, and local shape matters a lot. Some places get only one high tide a day, and the Bay of Fundy in Canada can see differences of about 50 feet between high and low water. Enclosed seas such as the Mediterranean have small tides, often under a foot.

The Sun's role: spring and neap tides

The Sun is far more massive than the Moon, but it is also about 390 times farther away. Because tidal force drops very fast with distance, the Sun's tidal effect is a bit less than half of the Moon's.

When the Sun, Moon and Earth line up, at new Moon and full Moon, their tides add together. High tides are higher and low tides lower. These are spring tides, a name that comes from water springing up, not from the season.

When the Sun and Moon pull at right angles, at first and last quarter, they partly cancel. The tidal range is smaller, giving neap tides. Each pattern comes around about twice a month. Tides are also a little stronger when the Moon is near perigee, its closest point to Earth.

Tidal locking

Tidal forces work on solid bodies too. Long ago, Earth raised bulges in the young Moon's rock. Earth's gravity tugged on those bulges and gradually slowed the Moon's spin until it matched its orbit. Today the Moon rotates once every 27.3 days, the same time it takes to orbit, so it always shows us the same face.

This is called tidal locking, and it is common. Most large moons in the Solar System, including Jupiter's Galilean moons, are tidally locked to their planets. Pluto and its moon Charon go further: each keeps the same face toward the other.

The Moon is slowly moving away

Earth spins faster than the Moon orbits, so friction drags Earth's tidal bulges slightly ahead of the Moon. Those bulges pull the Moon forward along its orbit, giving it energy and lifting it into a wider orbit. At the same time the Moon pulls back on the bulges and slowly brakes Earth's spin.

Laser beams bounced off reflectors left by Apollo astronauts and Soviet rovers show that the Moon moves away by about 1.5 inches a year, roughly the rate your fingernails grow. Earth's day lengthens in step, by about 2 thousandths of a second per century. Hundreds of millions of years ago, a day lasted only about 21 to 22 hours.

Watching the tides

If you live near an ocean coast, check the local tide table, which lists predicted times and heights of high and low water. Compare a week around full or new Moon with a week around a quarter Moon, and you will see the spring and neap difference.

Keep safety in mind. Spring tides can cover beaches and cut off paths quickly, so check times before walking out on sand flats or rocks. On the Mediterranean or Black Sea coasts of Türkiye, tides are so small that wind and air pressure usually change the water level more than the Moon does.