From a still Earth to a moving one

Stand outside and it seems obvious. The ground is still. The Sun, the Moon and the stars wheel overhead. For most of history almost everyone believed just that: the Earth sits still in the middle, and the sky turns round it. They were not fools. They did know the Earth was a ball. One reason is in this photograph. When the Earth’s shadow crosses the Moon in an eclipse, its edge is always curved. Around 240 BC a Greek called Eratosthenes even measured how big the ball is, using shadows, and came close.

One thing spoiled the picture. Five of the lights in the sky wander among the stars, and now and then one of them slows, goes backwards for a few weeks, and then goes forwards again. The Greeks called them planets, which means wanderers. In the second century AD an astronomer called Ptolemy explained the loops like this. Each planet rides on a small circle, and the small circle rides on a big one round the Earth. It was clumsy, but it worked well enough to be used for more than a thousand years.

In 1543 a Polish churchman, Nicolaus Copernicus, published a simpler idea. Put the Sun in the middle. Let the Earth be a planet that spins once a day and goes round the Sun once a year. Then the loops need no small circles at all. The Earth, on the inside track, overtakes Mars, and for a few weeks Mars seems to slip backwards, just as a slower car does when you pass it. Few people believed him. Nobody can feel the Earth move.

Then came the telescope. On 7 January 1610, in Padua, Galileo Galilei pointed one at Jupiter and saw small stars beside it. Night after night they changed places. Within a week he understood: they were moons, going round Jupiter. So not everything went round the Earth. This is a page from the book he printed two months later. Each circle is Jupiter on a different night, and the little stars are its moons.

One puzzle was left. Even with the Sun in the middle, circles did not quite fit where Mars was seen to be. A German, Johannes Kepler, struggled with Mars for years, using the best measurements ever made by eye. In 1609 he found the answer. The path is not a circle. It is an ellipse, a slightly squashed circle, with the Sun to one side of the centre. And a planet moves faster when it is nearer the Sun.

Why an ellipse? In 1687 Isaac Newton showed that one rule explains everything. All things pull on each other, and the pull weakens with distance. The pull that brings an apple down is the same pull that holds the Moon in its path. The Moon is sixty times further from the centre of the Earth than the apple is, so it is pulled 3,600 times more weakly. The sky and the ground obey the same law. Nobody had known that before.