getting it from fierce blows from meteors; but it is known to be just as real a way, and it can be calculated how much contraction is required to give out the heat we now experience. You might scarcely credit it, but the amount is so small that we could not have noticed it in the 200 years for which we have been measuring the Sun's size accurately. Until we had really good telescopes, (that is to say until the beginning of the eighteenth century), no measures could be made of the Sun's size sufficiently accurate to be worth considering. Hence we practically began measuring the Sun 200 years ago, and all the heat he has sent out during these 200 years could have been produced by his whole body shrinking ten miles inwards. Now, ten miles seems a long distance when we have to walk it, but it is quite imperceptible on the Sun, owing to his great distance. An inch is easy to see when close to us; but put an inch 135 miles away and how big would it look? Just as big as ten miles on the Sun. The Sun might have shrunk since astronomers watched him closely, not merely ten miles but even 100 miles without our being able to detect it.
Can this shrinking go on for ever? It is a hard question to answer. If it goes on steadily there must come a time when the Sun is as compact and solid as our Earth. At present we find that he is not solid, and not nearly so dense as the Earth. But even our solid Earth is still shrinking, as we know from the occurrence of earthquakes and the existence of mountains and valleys. When an orange dries and shrinks, its skin goes into folds: