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

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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.

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.

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.