One red bit of water

One bit of water turning in place

Let’s slice the sea like a cake and look from the side. The wave passes from left to right. But the red dot, one bit of water, does not follow the wave. It goes around in a circle right where it is. When a crest comes, it moves forward along the top; when a trough comes, it moves back along the bottom.

Why?As a wave passes, one bit of water does two things at once. It rises and falls, and at the same time it is pushed forward and then comes back. Put those two movements together and you get a circle.

Learn moreEach corner of the grid on screen is one bit of water. When every point turns around the same size circle, each one a little later than its neighbor, the line joining the points becomes the shape of the wave. A wave drawn this way is called a Gerstner wave. For a wave 3 m high, the circle is also 3 m across (drawn 4× larger on screen).

Time for one bit of water to go around once

8 s

Wave height

3 m → circle 3 m across

Crest to crest

about 100 m

One turn

8 seconds

calculated from the textbook equations for water waves (linear wave theory)