23rd June 2000 at 01:00
Just how big?

Ask children to select, from a range of spheres, three that they think would best represent the Sun, Earth and Moon. Offer everything from a beach ball to a peppercorn. Look for approximate answers. The largest possible ball - the melon or football - should be the Sun - the tiny peppercorn Earth will fit inside it a million times - and the Moon has a mass one-eightieth that ofthe Earth.

How can the Moon and Sun both look the same size?

It's all a question of scale. The Sun's face is 400 times the size of the Moon's - but it is also 400 times as far away. This means that the Moon neatly fits the Sunduring a solar eclipse.

Just how far is that?

Using the melon or football scale, the peppercorn Earth should be orbiting smewhere round the school boundaries, with the minuscule Moon orbiting it quite closely.

How do they move?

The Earth would be travelling anti-clockwise round the school and the Moon anti-clockwise round the Earth, 13-and-a-half times for every orbit of the Sun.

Is that all?

Not quite. All three are also rotating on their axes.

How can I understand that?

The best way is by getting involved in the action. You need three of you. One stands still as the Sun. Another orbits the Sun - anti-clockwise - and a third orbits the Earth. Got that? Let's complicate that a bit.

All three of you should be rotating. The Sun should also be moving in a straight line while it is being orbited. Try that. It takes a bit of practice!

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