Why are there tides?

Pulling to and fro

Why does the tide go in and out again? It is all to do with the gravitational force of the Moon and Sun, on the Earth. There is a gravitational force between any two objects, anywhere on Earth or in Space. Usually we cannot feel it because unless an object is really, really big, the force is very small. However, we can feel the force of the Earth acting on us. It is why we fall back towards the ground when we jump up in the air, instead of floating away.

Feeling the moon

We do not feel the force of the Moon acting on us, but the water on the surface of the Earth in the oceans ‘feels the force’. Because the moon is massive and quite close to the Earth, the water on the surface of the Earth actually gets pulled towards the moon and bunches up on the side facing the moon. On the opposite side of the Earth, the water also bulges out.

This bulge happens because there is another force, a centrifugal force, throwing  the water outwards as the earth rotates around the barycentre (a point between the Earth and the moon

Sun and moon join forces

The Earth is also rotating around its axis every 24 hours. Imagine you are standing on the shore facing the moon at 12:00 midday. The tide will be fully in. Six hours later the Earth will have moved ¼ of the way around, and the tide will be fully out. At 12:00 midnight the moon will be on the opposite side of the world to you and the tide will be fully in again. In practice the tidal cycle is a bit more than 12 hours (about 12.5 hours), as the moon will not be where you left it as you revolve back. It will have moved on a little bit as the Moon revolves around the Earth every 4 weeks.

The sun also pulls on the oceans but with less force than the Moon. Although the Sun is very much bigger than the Moon it is also very much further away, so its gravitational pull on the oceans is much less. 

When the Sun and Moon are in line, their gravitational forces pull in the same direction and add together. This causes a “spring” tide (the high tides are particularly high and the low tides are particularly low). When they are positioned at right angles (90°) from each other, their gravitational pulls work in opposite directions, causing a “neap” tide (when there is the least difference between the high tide line and the low tide line)

 

Compressing

There are two spring tides and two neap tides every 4 weeks. The size of the tide gradually decreases over the course of one week to become neap tides, then gradually increses again.

 

 

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