Page:A Voyage in Space (1913).djvu/39

Rh and when very far away it loiters terribly. That is really because the gravity or attraction of the Sun is feeble at a distance and strong when close; but Kepler did not quite realize about this attraction, he contented himself with noting that the planet moved more slowly or more quickly. And finally he found a third law, which is rather tiresomely

mathematical at first sight, but was the most important of all for taking the next step, and therefore we should pay it some attention.

It states that if a planet be placed at a distance from the Sun 4 times as great as that of our Earth, it will take 8 times as long as we do to go round the Sun completely, that is to say, 8 years; and if it is 9 times as far away, then 27 years; if 100 times as far away, then 1000 years. Can you see how these