As the various planets move about the sun, in paths which we may call concentric circles, or rings set one inside the other, it is evident that there may be occasions when the different planets will come between the sun and some planet with an orbit beyond their own. For instance, Mercury is a planet travelling in a circle around the sun, but a circle smaller than that in which the earth travels. Mercury may then sometimes pass across the face of the sun at a time when the earth is on the same side of the sun, and in that case we can observe Mercury moving like a little black ball across the glowing front of the King of the Day. To see such an occurrence we must use smoked or darkened glasses, else Mercury would simply be lost in the unshaded splendor of the sunlight. Why is not such a transit of Mercury seen every year? Every year Mercury must cross the disk of the sun, and the reason we do not see a transit yearly is that the orbits of Mercury and the earth differ in plane, and long periods will elapse between the occasions when the sun, Mercury, and the earth come into line thus: [Illustration] As Venus overtakes the earth in its journey around the sun once every nineteen months, shall we have the benefit of a transit at each time of passing? No; because Venus is usually either above or below the line of the earth’s orbit. But when at long intervals such an event as a transit of Venus does occur, no astronomical incident can be more valuable, because it gives us the best means of measuring the distance between the earth and the sun. A grand opportunity for studying the sun is afforded by an eclipse. An eclipse of the sun is occasioned by the moon’s passing between the earth and the sun. Children are frequently warned that it is discourteous to pass between a person and the fire or light; our child, the moon, happily for us, often passes between us and our fire, the sun, and on the fortunate occasions when this can be observed out come the glasses for sun-study. As the moon revolves about the earth, she sometimes stands between the sun and the earth. When the moon passes between us and the sun on a clear day, between sunrise and sunset, the day grows dark, a twilight comes over the world, and continues until the moon has crossed the disk of the sun.