In the center of a large room set a light so that it will be about as high as your eyes. Let this light represent the Sun; you must play now that you are the Earth, and think of the pictures on the wall as being the stars fixed in the sky away off in space. Now walk in a circle around the light toward the right and facing the light all the time. As you move around the light you will see that it _seems_ to move in the opposite direction and that it _seems_ to move past the pictures on the wall. The experiment is shown in Fig. 76. This, then, is exactly what happens when we look at the Sun and the stars. The Earth moves round the Sun in a circle, nearly, and since the Sun is so much closer to us than any of the other fixed stars the Sun apparently moves by the stars, because the Earth changes its position relative to it and the stars. =The Earth Turning Round the Sun Makes the Seasons.=—We have seen how the days and nights would be equal all over the world if the equator of the Earth was in a plane with the Sun, but since the Earth is tilted the days and nights are unequal except twice a year, and this is when the places where the _ecliptic_ and the _equator_ cross each other are facing the Sun. [Illustration: FIG. 77—TOP SPRING ON PLATE.] Again, if the Earth’s equator was always in a plane with the Sun the day would be just as long as the night all through the year and there would be no seasons. But the Earth having its axis tilted, and which is always set in the same direction, together with the Earth speeding in a circle around the Sun, causes some curious things to happen and the seasons are one of them. To make clearer the reason the axis of the Earth always stays in one position take a top and give it a good spin. The top, of course, turns round its axis very fast, and this is like the Earth turning round its axis every 24 hours. Now, if you place the blunt end of a pencil on the upper axis of the spinning top, as shown in Fig. 77, and try to tilt it in some other direction than that it took when it began to spin, you will find it rather a hard thing to do. In other words, once a body is rapidly turning on its own axis it very strongly tends to keep its axis pointing in the same direction. This rule also applies to the Earth, for having been tilted at an angle when it was thrown off by the Sun in the making, no other forces have ever been able to change the position of its axis to any great extent, though, the Earth spins easily on its axis and also revolves round the Sun. To understand how the seasons are made you must first have clearly in mind the positions of the tilted Earth in different parts of its orbit round the Sun. The things needed for this experiment are a nice round apple, a candle, a piece of string, a safety-pin and a knitting needle. [Illustration: FIG. 78.—CIRCLE AROUND CANDLE MARKED WITH SEASONS.]