Why Tides Are Linked to the Moon

The most puzzling thing about tides is the second bulge, on the side opposite the Moon. It happens because what matters is not gravity's force itself, but how it differs from one point on Earth to another.

Water on the side closer to the Moon is pulled more strongly than the planet's center and moves ahead. The center is pulled more strongly than the water on the far side, so that water seems to lag behind. As a result, water gathers on both sides, while there is less of it along the sides.

Because the Moon moves along its orbit during the day, the cycle is slightly longer than a day. That is why tides shift by about 50 minutes each day, and tide tables are calculated separately for each port.

The Sun also creates tides, but they are about half as strong as lunar tides, even though it pulls on Earth much more strongly. The reason is distance: the difference in gravitational pull between the near and far sides is smaller for the distant Sun.

The third tile shows that it is not only about the Moon. In the open ocean, the rise is less than 1 meter, while huge values occur in bays with a particular shape, where the wave speeds up and builds, like water in a sloshing basin.

When the Sun and Moon line up, their effects combine, and tides become higher. When they are at right angles, the effects cancel out, and the difference in water levels is smaller. That is why tide height is linked to the phases of the Moon.

Tides also affect land: Earth's crust rises and falls by tens of centimeters twice a day. You do not notice it because everything around you rises at once, but instruments can detect it.

There is also a reverse effect. The tidal wave slightly slows Earth's rotation, and the length of a day increases by about 0.002 seconds per century. The Moon is slowly moving away at the same time, and this is measured using reflectors left on its surface.