Tides rise and fall mainly because the Moon's gravity pulls on Earth's oceans as the planet rotates.

The longer version

Gravity weakens with distance, so the Moon tugs harder on the near side of Earth than on its centre, and harder on the centre than on the far side. That difference stretches the planet's water into two bulges, one facing the Moon and one opposite it.

Rotation carries coastlines through both bulges each day, so high water usually arrives twice. The Sun contributes a smaller pull that adds to the effect at alignment, producing spring tides, and partly offsets it at other times. Basin shape and seabed depth then reshape local timing and height. Local wind and weather can push the water higher or lower than predicted.

Where this entry sits

Oceans and Sea Life looks at the salt water that covers most of the planet, from crashing waves and tides to the darkest deep-sea trenches. Expect notes on coral, fish, whales, sharks, jellyfish, shells, and the strange creatures that live far below the surface. The full list sits on the Oceans and Sea Life page, where every entry is listed in order.