The heavy pendulum, which after being started swung for hours, seemed to move away from the line in the direction of the hands of a watch. This showed that the floor of the Panthéon was really skewing around under the swinging pendulum. This is easily explained as due to the rotation of the earth because the northern edge of the floor was nearer the axis of the earth than the southern edge and therefore was carried more slowly eastward. (2) A much simpler way to show the turning of the Earth on its axis, though this experiment only proves that either the Earth or the whole sky turns, is to make a photograph of the North Star and the stars in its neighborhood, as explained in Chapter XII. During the time the sensitive plate is being exposed the camera will be carried round by the Earth turning on its axis and the fixed stars will leave bright trails on the plate in _arcs of circles_. =The Earth Turning on Its Axis Makes Day and Night.=—The Earth, in turning round on its axis once every 24 hours, receives the light of the Sun on half of its surface at a time, making the day, while the other half is in the shadow, which makes the night. [Illustration: FIG. 73.—THE EARTH MOVES AWAY FROM THE SWINGING PENDULUM.] If the equator of the earth was in a plane with the Sun, as shown in Pig. 74, it is easy to see that the days and nights all over the world would be of the same length, that is, each would be 12 hours long. Instead, the Earth is tilted a little, as shown in Fig. 75—to be exact, its axis is tilted 23½ degrees out of the _perpendicular_ and this makes the day and the night at the equator each 12 hours long and the day and the night at the north and south poles each six months long. The circle round the Sky which is in a plane with the Sun is called the _ecliptic_. The Sun seems to follow a path round the Earth in this plane, and this is called the _path of the Sun_. =To Show That the Earth Travels Round the Sun.=—When we look at the Sun and the stars it is hard to believe that they are standing still and that it is the Earth which is whirling round on its own axis and also round the Sun. [Illustration: FIG. 74.—IF THE EARTH’S EQUATOR WERE IN A LINE WITH THE SUN.] [Illustration: FIG. 75.—THE EARTH TILTED ON ITS AXIS.] We have given a couple of experiments to show that the Earth turns on its own axis, and here is one to make it clear how the Earth travels in a great circle, or rather _ellipse_, round the Sun and gives us our year. Further, the Earth’s axis being tilted away from the axis of the orbit, its movement round the Sun causes the north pole to be turned toward the Sun half of the time, and then the south pole to be turned toward the Sun an equal length of time, which gives each of them a day and a night that is six months long. [Illustration: FIG. 76.—LIGHT IN ROOM TO REPRESENT SUN.]