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. We call this the time of the equinoxes; the nights are of the same length as the day in both these positions of the earth with regard to the sun. [Illustration: EARTH AND SUN AT THE EQUINOXES.] In the next figure we have the other condition. Here the earth's axis is inclined at the greatest angle of 23°½, towards, and away from, the sun. If I take a point very near the north pole, that point will not, in summer, be carried by the earth's rotation out of the light, and a part equally near the south pole will not be able to get into it. These are the conditions at and near two other points called the solstices. [Illustration: EARTH AND SUN AT THE SOLSTICES.] On each of these globes I have drawn a line representing the overhead direction from London. If we observe the angle between the direction of the zenith and that to the sun in winter we find it considerable; but if we take the opposite six-monthly condition we get a small angle. In other words, under the first condition the sun at noon will be far from the zenith of London, we shall have winter; and in the other condition the sun will be as near as it can be to the zenith at noon, we shall have summer. These two cases represent the two points in the earth's orbit at which the sun has the greatest declination south and north. With the greatest north declination the sun will come up high, appear to remain at the same height above the horizon at noon for a day or two, as it does at our summer solstice, and then go down again; at the other point, when it has the greatest southern declination, it will go down to the lowest point, as it does in our winter, stop, and come up again--that is, the sun will stand still, so far as its height above the horizon at noon is concerned, and the Latin word solstice exactly expresses that idea. We have, then, two opposite points in the revolution of the earth round the sun at which we have equal altitudes of the sun at noon, two others when the altitude is greatest and least. [Illustration: DIAGRAM SHOWING POSITION OF THE SUN IN RELATION TO THE ZENITH OF LONDON AT THE NORTHERN WINTER SOLSTICE.]