Читать книгу The dawn of astronomy. A study of the temple-worship and mythology of the ancient Egyptians онлайн

53 страница из 97

The angle of the inclination of the earth's plane of rotation to the plane of its revolution round the sun, which, as I have said, is at the present time something like 23½°, is called the obliquity of the ecliptic. This obliquity is subject to a slight change, to which I shall refer in a subsequent chapter.

In order to give a concrete idea of the most important points in the yearly path of the earth round the sun, let us imagine four globes arranged on a circle representing the earth at different points of its orbit, with another globe in the centre representing the sun, marking the two practically opposite points of the earth's orbit, in which the axis is not inclined to or from the sun but is at right angles to the line joining the earth in these two positions, and the two opposite and intermediate points at which the north pole of the axis is most inclined towards and away from the sun.

A diagram will show what will happen under these conditions. If we take first the points at which the axis, instead of being inclined towards the sun, is inclined at right angles to it, it is perfectly obvious that we shall get a condition of things in which the movement of the earth on its axis will cause the dark side of the earth and also the light side represented by the side nearest to the sun, both being of equal areas, to extend from pole to pole; so that any place on the earth rotating under those conditions will be brought for half a period of rotation into the sunlight, and be carried for half a period of the rotation out of the sunlight; the day, therefore, will be of the same length as the night, and the days and nights will therefore be equal all over the world.