The Tribes and Castes of the Central Provinces of India, Volume 3

by R. V. Russell

2. The apparent path of the sun. The ecliptic or zodiac.

The universe may be conceived for the purpose of understanding the sun’s path among the stars as if it were a huge ball, of which looking from the earth’s surface we see part of the inside with the stars marked on it, as on the inside of a dome. This imaginary inside of a ball is called the celestial sphere, and the ancients believed that it actually existed, and also, in order to account for the varying distances of the stars, supposed that there were several of them, one inside the other, and each with a number of stars fixed to it. The sun and earth may be conceived as smaller solid balls suspended inside this large one. Then looking from the surface of the earth we see the sun outlined against the inner surface of the imaginary celestial sphere. And as the earth travels round the sun in its orbit, the appearance to us is that the sun moves over the surface of the celestial sphere. The following figure will make this clear.[3]

Fig. 1.—The Orbit of the Earth and the Zodiac.

Thus when the earth is at A in its orbit the sun will appear to be at M, and as the earth travels from A to B the sun will appear to move from M to N on the line of the ecliptic. It will be seen that as the earth in a year makes a complete circuit round the sun, the sun will appear to have made a complete circuit among the stars, and have come back to its original position. This apparent movement is annual, and has nothing to do with the sun’s apparent diurnal course over the sky, which is caused by the earth’s daily rotation on its axis. The sun’s annual path among the stars naturally cannot be observed during the day. Professor Newcomb says: “But the fact of the motion will be made very clear if, day after day, we watch some particular fixed star in the west. We shall find that it sets earlier and earlier every day; in other words, it is getting continually nearer and nearer the sun. More exactly, since the real direction of the star is unchanged, the sun seems to be approaching the star.

“If we could see the stars in the daytime all round the sun, the case would be yet clearer. We should see that if the sun and a star were together in the morning, the sun would, during the day, gradually work past the star in an easterly direction. Between the rising and setting it would move nearly its own diameter, relative to the star. Next morning we should see that it had got quite away from the star, being nearly two diameters distant from it. This motion would continue month after month. At the end of the year the sun would have made a complete circuit relative to the star, and we should see the two once more together. This apparent motion of the sun in one year round the celestial sphere was noticed by the ancients, who took much trouble to map it out. They imagined a line passing round the celestial sphere, which the sun always followed in its annual course, and which was called the ecliptic. They noticed that the planets followed nearly the same course as the sun among the stars. A belt extending on each side of the ecliptic, and broad enough to contain all the known planets, as well as the sun, was called the zodiac. It was divided into twelve signs, each marked by a constellation. The sun went through each sign in a month, and through all twelve signs in a year. Thus arose the familiar signs of the zodiac, which bore the same names as the constellations among which they are situated. This is not the case at present, owing to the precession of the equinoxes.” It was by observing the paths of the sun and moon round the celestial sphere along the zodiac that the ancients came to be able to measure the solar and lunar months and years.

3. Inclination of the ecliptic to the equator.

As is well known, the celestial sphere is imagined to be spanned by an imaginary line called the celestial equator, which is in the same plane as the earth’s equator, and as it were, a vast concentric circle. The points in the celestial sphere opposite the north and south terrestrial poles are called the north and south celestial poles, and the celestial equator is midway between these. Owing to the special form of the earth the north celestial pole is visible to us in the northern hemisphere, and marked very nearly by the pole-star, its height above the horizon being equal to the latitude of the place where the observer stands. Owing to the daily rotation of the earth the whole celestial sphere seems to revolve daily on the axis of the north and south celestial poles, carrying the sun, moon and stars with it. To this the apparent daily course of the sun and moon is due. Their course seems to us oblique, as we are north of the equator.

If the earth’s axis were set vertically to the plane of its orbit round the sun, then it would follow that the plane of the equator would pass through the centre of the sun, and that the line drawn by the sun in its apparent revolution against the background of the celestial sphere would be in the same plane. That is, the sun would seem to move round a circle in the heavens in the same plane as the earth’s equator, or round the celestial equator. But the earth’s axis is inclined at 23½° to the plane of its orbit, and therefore the apparent path traced by the sun in the celestial sphere, which is the same path as the earth would really follow to an observer on the surface of the sun, is inclined at 23½° to the celestial equator. This is the ecliptic, and is really the line of the plane of the earth’s orbit extended to cut the celestial sphere.