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.
Fig. 74.—If the Earth’s Equator Were in a Line With the Sun.
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.
