Tidal force

Tidal force is a kind of force made by gravity that stretches objects unevenly. This is because the gravitational field changes across the middle of a body (the diameter). Gravity pulls more strongly on the part of an object that is closer to another object, like the Moon or the Sun, than on the part that is farther away. This difference in pull stretches the object.

Comet Shoemaker-Levy 9 after breaking up under the influence of Jupiter's tidal forces.

On Earth, tidal forces mainly come from the Moon and the Sun. They make the oceans move and cause tides — the regular rise and fall of sea levels. Tidal force also stretches the whole Earth a tiny bit, not just the oceans.

Tidal forces can do other things too. They can lock the rotation of moons so they always show the same side to their planet. This is called tidal locking. If an object gets too close to a big planet or a black hole, tidal forces can even pull it apart. This is called spaghettification.

Tidal forces are strongest when objects are big and close together. They get weaker very quickly when the objects are farther apart.