Lecture Eleven

We need only compare the behaviour of Saturn, who stands far out in the Universe, with that of the Earth, to notice a certain difference. Astronomy recognises this difference by saying that Saturn goes round the Sun in 30 years, the Earth in one year. We will not now stop to discuss whether these assertions are correct or whether they show a superficial view. We will only point to the fact that the observation which can be gained by following Saturn in cosmic space and comparing the rapidity of his progress with that of the Earth, brings us to the conclusion that according to the astronomical system of Copernicus and Kepler, Saturn needs 30 years and the Earth only one year, in which to go round the Sun. Looking at Jupiter, we assign to him a revolution lasting 12 years. Much shorter is that of Mars. And when we come to the other planets, Venus and Mercury, we find that they have even shorter periods of rotation than the Earth. All these conclusions are obviously well thought out, worked out on the basis of observations made in one way or another.