Skipper Info

Why do we have two tides a day?

We think of the moon orbiting around the earth, but in fact they revolve together once a month around a point in between - their centre of gravity.

The earth is kept in a circular orbit around this point by two forces; its gravitational attraction to the moon and its centrifugal force around the centre of gravity. Because these forces are always equal and opposite, the earth (and the moon) just keep going around and around.

But for the water it's a bit different because it is free to move about on the surface of the earth. And for the water on the side closest to the moon, the gravitational attraction of the moon is a little bit bigger than the centrifugal force, so the water feels a force upward, towards the moon. It tries to flow in that direction and you get a bulge.

The opposite is true on the far side. The centrifugal force is bigger than the moon's attraction, so the water feels a force outwards. Result: another bulge.

So there's a bulge on each side of the earth. pointing towards and away from the moon. As the earth spins once a day, these bulges of water sweeps past you, making two tides a day. In fact, the water is always playing catch-up, so the bulge is always a little ahead of the moon's direction. As the water is always flowing to try to catch up with the turning earth, local conditions, such as the shape of the shoreline, make small local differences to the tides.

Acknowledgment

Article in Financial Times Magazine - September 20 2003

Written by Professor Manuel Grande, Astrophysics Group, Warwick University, and principal investigator of DCIXS, the UK contribution to SMART-1, the first European mission to the moon.