The big draw

Michael Shirley, of Ardgowan School, asks: Does the Moon have any gravitational pull on the Earth?

Richard Dowden, a physicist at the University of Otago, responds:

Yes, it does. Every mass, solid or liquid or gas, has a gravitation pull on every other mass. For any two masses, the pull is proportional to the product of the masses. So the gravitational pull of the Moon on the Earth is exactly the same as the gravitational pull of the Earth on the Moon.

The gravitational pull of the Earth on the Moon is what keeps the Moon in orbit around the Earth. If you tie a heavy thing to a string, you can whirl it around you. If the string breaks, the thing will fly off at a tangent. If you could "switch off" the gravitational pull of the Earth on the Moon, the Moon would fly off at a tangent!

So what does the gravitational pull of the Moon on the Earth do? If you whirl a heavy thing on a string around you, you may notice that your body moves around a bit too.

Have you seen a pair of skaters on television where the man whirls his partner around in a horizontal circle? Did you notice that he is moving in a circle too? So you could say they are circling around one another. There is a fixed point about which the man and his partner are circling. Since they have about the same weight (or mass, rather), the fixed point is about where they join hands.

In the same way, the Earth and the Moon are circling around one another.

Since the Earth is so much heavier than the Moon, the fixed point about which they are both circling is close to the centre of the Earth. If it wasn't for the gravitational pull of the Moon on the Earth, the fixed point would be exactly at the centre of the Earth.

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