14. The Hindus assumed that the earth was the centre of the visible universe and was itself stationary, the sun revolving round it like a superior planet. They divided the great circle of the sun's path into twelve zodiacal signs, each of 30° of arc, the first of which was called Mēsha; but prior to this by many centuries they had divided the same circle into 27, or sometimes 28, nakshatras or lunar asterisms, observing the moon's path in the heavens and allowing roughly an asterism to each day of her average journey round the earth in the course of a sidereal month. The first point of the first sign Mēsha is the point from which all celestial longitudes are measured, its employment corresponding in this respect to the first point of Aries in European astronomy, though it is not at all in the same position. This first point of Mēsha corresponds to the point at the end of the nakshatra Rēvatī and the beginning of the nakshatra Aśvini. (See below under "Nakshatras.") It is considered to be an immovable sidereal point, according to some authorities coincident with the principal star of the nakshatra Rēvatī, but according to the Sūrya-Siddhānta lying 10′ to the east of that star. The star has been identified with ζ Piscium, a star lying almost on the ecliptic, which was found to be a convenient point from which to take measurements. It will be observed that, since the first point of Mēsha is fixed and immovable, the Hindūs have not as a rule paid attention to the precession of the equinoxes. The Hindū solar year is therefore always considered as sidereal unless special mention is made of its being treated as tropical. The first point of Mēsha is considered by the Sūrya Siddhānta to have coincided with the vernal equinox in A.D. 499, by almost all the other authorities in A.D. 522, but by the Siddhānta Śirōmaṇi in A.D. 527. In the year of coincidence the sidereal signs were exactly the same as the tropical signs, or the signs whose initial points are calculated with reference to the precession of the equinoxes.[1]
The sun, passing in its annual course completely round from and to the sidereal First Point of Mēsha, enters and leaves in turn each of the twelve signs. This complete revolution is of course due to the sidereal revolution of the earth about the sun, and its length in time constitutes the length of the solar sidereal year. (The tropical year is separately considered below under the head of "Tropical Saṁkrāntis.") The solar sidereal year is the ordinary solar year of the Hindus. It is divided into twelve months, the length of each of which, its time measurement being calculated by the time occupied by the sun in passing completely through one sign or 30° of arc, differs from the others in consequence of the earth's orbit being an ellipse. The moment of the sun's entering each of the signs in turn marks the astronomical beginning of a true or apparent solar month, and this "coming together" of the sun and the sign is called a "saṁkrānti." Since it is evident that the sun's entry into each sign in succession may be calculated by his mean longitude at the moment of such entry as well as by his true longitude, we have sometimes to calculate the moments of occurrence of mean saṁkrāntis as well as of true or apparent ones. But throughout this volume, unless clearly designated and distinguished as "mean," the word "saṁkrānti" means a true or apparent sidereal saṁkrānti, i.e., the junction of the sun in its course with the first point of each sign as calculated by the sun's (earth's) true motion and without reference to the precession of the equinoxes. When the precession of the equinoxes is taken into account the saṁkrānti is called a "tropical saṁkrānti." The true sidereal day is the time occupied by a meridian on the earth's surface in completing one revolution about the earth's axis. A solar sidereal month is a month of thirty sidereal days. But neither of these is used in the Hindū
- ↑ For an excellent notice of the nakshatras reference may be made to Whitney's Oriental and Linguistic Studies, s.v., "The Lunar Zodiac." Also to many passages in his notes to E. Burgess's translation of the Sūrya Siddhānta, especially I., 27–34, p. 14.
