Among themselves we see the power more plainly. The earth attracts the moon, keeping her in constant close attendance; and the moon attracts the earth, causing a slight movement on her part, and also causing the tides of the sea. Each planet has more or less power to hinder or help forward his nearest brother-planet. For instance, when Jupiter on his orbit draws near the slower Saturn on his orbit, Saturn’s attraction pulls him on, and makes him move faster than usual; but as soon as he gets ahead of Saturn then the same attraction pulls him back, and makes him go more slowly than usual. Jupiter has the same influence over Saturn; and so also have Saturn and Uranus over one another, or Uranus and Neptune. In early days astronomers were often greatly puzzled by these quickened and slackened movements, which could not be explained. Now the “perturbations” of the planets, as they are called, are understood and allowed for in all calculations. Indeed, it is by means of this very attraction that Neptune was discovered, and the planets have actually been weighed. What a wonderful difference we find in this picture of the Solar System, as we now know it to be, from the old-world notion of our earth as the center of the universe! When we think of all the planets, and of the magnificent sun; when we pass onward in imagination through space, and find our sun himself merely one twinkling star amid the myriads of twinkling stars scattered broadcast through the heavens, while planets and comets have sunk to nothing in the far distance,--then indeed we begin to realize the unutterable might of God’s power! Why, our earth and all that it contains may be regarded as but one grain of dust in the wide universe. CHAPTER XII. MORE ABOUT THE SUN. Not among the least of the wondrous things of creation are the tremendous disturbances taking place upon the surface of the sun--that raging, roaring sea of flame. A good many explanations have been from time to time offered as to the dark spots seen to move across the face of the sun. Some one or more of these explanations may be true; but we know little about the matter. A sun-spot does not commonly consist of merely one black patch. There is the dark center, called the _umbra_--plural, _umbræ_. There is the grayish part surrounding the umbra, called the _penumbra_. Also, in the center of the umbra there is sometimes observable an intensely black spot, called the _nucleus_. Sometimes a spot is made up of nucleus and umbra alone, without any penumbra. Sometimes it is made of penumbra alone, without any umbra. Sometimes in one spot there are several umbræ, with the gray penumbra round the whole, and gray bridges dividing the umbræ. The enormous size of these spots has been already described in an earlier chapter. Fifty thousand to one hundred thousand miles across is nothing unusual. In the year 1839 a spot was seen which measured no less than one hundred and eighty-six thousand miles in diameter.