Ocean tides are caused mainly by the Moon pulling on Earth's water.

The longer version

Tidal bulges form because gravity weakens with distance, so the Moon pulls harder on the near side of Earth than on its center, and harder on the center than on the far side. That difference stretches the planet slightly along the line to the Moon, and the fluid ocean responds far more than rock.

As Earth rotates beneath these bulges, coastal locations pass through high and low water roughly twice a day. The solid ground rises and falls by a small amount as well. The Sun contributes a smaller tide, since it is far more distant despite being enormously massive. When Sun, Moon and Earth line up, the two effects add together and produce especially high spring tides.

Where this entry sits

The night sky looks calm, yet almost everything up there is moving, burning, or slowly changing. These facts cover planets, stars, the Moon, gravity, black holes, comets, and the story of how people began exploring beyond Earth. The full list sits on the Space and Sky page, where every entry is listed in order.