Now that our rocket has brought us safely back to the earth of to-day, let us consider in more detail the small colony, almost completely isolated in space, which we believe to be the shattered fragments of what was once an ordinary star. It contains a great variety of objects, large, medium-sized, small and very small, which we must discuss in turn.

The Nine Planets

First let us look at its largest members, the nine principal planets. These move round the sun in almost circular paths, rather like circus horses trotting or galloping round the ring-master. They all go round in the same direction, and this must of course be the direction in which the wandering star, which brought them into being, moved round the sun. Because of the way it came into existence, the solar system has only one-way traffic—like Piccadilly Circus. The traffic nearest the centre moves fastest; that further out more slowly, while that at the extreme edge merely crawls—at least by comparison with the fast traffic near the centre. It is true that even the furthest and slowest of the planets covers nearly three miles every second, which is about 200 times the speed of an express train, but this is a mere crawl in astronomy. The planets Mercury and Venus, which constitute the fast traffic near the centre, move, the former ten and the latter seven, times as fast.