257 II THE APPARENT PATH OF THE SUN telligible it is desirable first to recapitulate some elementary facts of astronomy. apparent path of The ecliptic or The universe may be conceived for the purpose of 2. The 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 the sun. 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 zodiac. 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.1 *SCORPIO SAGITTARIUS CAPRICORNUS' * * * 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 VOL. III 1 Newcomb's Astronomy for Everybody, p. 33. S 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 II INCLINATION OF ECLIPTIC TO THE EQUATOR 259 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. ecliptic As is well known, the celestial sphere is imagined to be 3. Inclinaspanned by an imaginary line called the celestial equator, tion of the which is in the same plane as the earth's equator, and as it to the were, a vast concentric circle. The points in the celestial equator. 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 231° 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 231 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. the moon All the planets move round the sun in orbits whose 4. The planes are slightly inclined to that of the earth, the plane of orbits of Mercury having the greatest inclination of 6°. The plane and of the moon's orbit round the earth is also inclined at 5° 9′ planets. 5. The signs of the zodiac. to the ecliptic. The orbits of the moon and all the planets must necessarily intersect the plane of the earth's orbit on the ecliptic at two points, and these are called the nodes of the moon and each planet respectively. In consequence of the inclination being so slight, though the course of the moon and planets is not actually on the ecliptic, they are all so close to it that they are included in the belt of the zodiac. Thus the moon and all the planets follow almost the same apparent course on the zodiac or belt round the ecliptic in the changes of position resulting from their own and the earth's orbital movements with reference to what are called the fixed stars. As the sun completes his circuit of the ecliptic or zodiac in the course of a year, it followed that if his course could be measured and divided into periods, these periods would form divisions of time for the year. This was what the ancients did, and it is probable that the measurement and division of time was the primary object of the science of astronomy, as apart from the natural curiosity to ascertain the movements of the sun, moon and planets, when they were looked upon as divine beings controlling the world. They divided the zodiac or the path of the sun into twelve parts, and gave to each part the name of the principal constellation situated on, or adjacent to, that section of the line of the ecliptic. When they had done this and observed the dates of the sun's entry into each sign or rashi, as it is called in Hindi, they had divided the year into twelve solar months. The following are the Hindu names and meanings of the signs of the zodiac: II THE SANKRĀNTS 261 The signs of the zodiac were nearly the same among the Greeks, Egyptians, Persians, Babylonians and Indians. They are supposed to have originated in Chaldea or Babylonia, and the fact that the constellations are indicated by nearly the same symbols renders their common origin probable. It seems likely that the existing Hindu zodiac may have been adopted from the Greeks. Sankrants. The solar year begins with the entrance of the sun into 6. The Mesha or Aries.1 The day on which the sun passes into a new sign is called Sankrant, and is to some extent observed as a holy day. But the Til Sankrant or entry of the sun into Makara or Capricorn, which falls about the 15th January, is a special festival, because it marks approximately the commencement of the sun's northern progress and the lengthening of the days, as Christmas roughly does with us. On this day every Hindu who is able bathes in a sacred river at the hour indicated by the Joshis of the sun's entrance into the sign. Presents of til or sesamum are given to the Joshi, owing to which the day is called Til Sankrant. People also sometimes give presents to each other. 7. The nakshatras or constel the moon's The Sankrants do not mark the commencement of the Hindu months, which are still lunar and are adjusted to the solar year by intercalation. It is probable that long before lations of they were able to measure the sun's progress along the path. ecliptic the ancients had observed that of the moon, which it was much easier to do, as she is seen among the stars at night. Similarly there is little reason to doubt that the first division of time was the lunar month, which can be remarked by every one. Ancient astronomers measured the progress of the moon's path along the ecliptic and divided it into twenty-seven sections, each of which represented roughly a day's march. Each section was dis 1 Owing to the precession of the equinoxes, the sidereal year is not the same as the solar year, being about 20 minutes longer. That is, the sun passes a particular star a second time in a period of 365 days 6 hours and 9 minutes, while it passes the equatorial point in 365 days 5 hours 48 minutes 49 seconds, this latter period being the solar year. The difference is due to slight changes in the direction of the earth's axis, which change the position of the celestial equator and of the equinoctial point where the sun crosses it. It is not clear how the Hindus get over this difficulty, but the point does not affect the general account. |