Page:A Voyage in Space (1913).djvu/169

Rh looking at, we have some choice of opinion. Most astronomers would say the spots are cloud effects high above the real surface of Jupiter, which is hidden from view.

Another thing I would like you to realize about Jupiter is his vast size. Here is a model of Jupiter on the scale of 10,000 miles to the inch, and the model is about 9 inches across, so that Jupiter himself must be 90,000 miles from side to side; and now look at our Earth less than an inch across, and our little Moon less than a quarter of an inch. How small they are compared with Jupiter! Why, to go round Jupiter is a longer voyage than from the Earth to the Moon!

Another big planet is Saturn, which I hope you recognize by his ring. In this model we have had to imitate the ring in solid cardboard, but if the real ring were solid like this, it would soon fall on to Saturn, as a great mathematician (Clerk Maxwell) showed by calculation. He went on to show that the ring must be made up of millions of little tiny satellites all separate from one another and revolving round Saturn at different rates. They are, however, so close together that we cannot see them separate even with our best telescopes; consequently we do not see that one travels faster than another, as we can see one of Jupiter's spots catch up and pass another; at any rate we cannot see this in the ordinary way; but by a wonderful property of light, the spectroscope enables us to see it in another way of which we will say more in another lecture. We are able to measure the speed with which a shining body is coming towards us or going away from us;