The Tides

Tides on Earth A tidal force is the difference in gravity from one side of a body to the other that is exerted by a 2nd object. The Moon exerts a tidal force on the Earth that causes the oceans facing the Moon to bulge out toward it, and the oceans on the opposite side of the Earth to bulge out away from the Moon. These varying ocean levels are called the tides.

Tides on Earth The Sun also causes tides, but they are smaller than the Lunar tides. When the Moon and Sun are aligned (during New and Full Moons), they pull the oceans in the same direction. This is when the high tide is highest, and low tide is lowest. During the quarter moons, the Sun partially cancels the Lunar tides, so the tides are less extreme.

Tides on Earth

Tidal Friction Because of friction between the Earth’s crust and the oceans, the rotation of Earth pulls the ocean’s tidal bulge out of alignment with the Moon. This has 2 effects: 1) The gravitational pull of the Moon on the tidal bulge slows Earth’s rotation (by a few milliseconds per century). This will continue until Earth keeps the same face towards the Moon. 2) The gravitation pull of the tidal bulge on the Moon slings the the Moon ahead in its orbit, causing its orbit to expand (by a few cm per year).

Tidal Friction

Tides on the Moon Just as the Moon exerts tidal forces on Earth, Earth also exerts tidal forces on the Moon. Since Earth is much more massive than the Moon, its tidal forces are much stronger. So Earth’s tidal forces have already slowed the Moon’s rotation to the point that it always keeps the same face towards Earth. As a result, there’s a far side of the Moon that we never see from Earth. We didn’t know what it looked like until satellites orbited the Moon and photographed the far side.

The Moon’s Rotation