Bharatiya Jyotihshastra (History of Indian Astronomy), translated by R. V. Vaidya v2.c12.s5.48
Bhāratīya Jyotiḥśāstra (History of Indian Astronomy), translated by R. V. Vaidya v2.c12.s5.48
- Language
- English
v2.c12.s5.48.1
a concave hemispherical sun-dial, having a vertical style in the centre, by means of which the interval included between sunrise & sunset was divided into twelve equal parts. It is probable that by the use of the gnomon the Chaldeans succeeded in obtaining an approximation to the length of the solar year, but there is not the smallest reason to suppose that they employed it in determining any other of the fundamental facts of astronomy. Indeed, they do not seem to have made observations at all for the purpose of forming materials to serve as the groundwork of future reasoning. They simply contented themselves with noting the more remarkable phenomena as they occurred, and hence deducing a few rough conclusions of a general nature. It would appear, however, that by comparing together the Chaldean records of eclipses some of the Greek mathematicians ascertained with considerable accuracy several periods relating to the motion of the moon. The earliest astronomical observation as having been made by the Greeks previous to the establishment of the *Alexandrian* school, is a determination of the summer solstice by Meton, in the year 430 A.C. The instrument, termed a heliometer, which was used by Meton on this occasion was, in all probability, no other than a modification of the gnomon.
v2.c12.s5.48.2
The date of this solstice has been chosen for the epoch of the Metonic cycle of nineteen years.* A new era commenced in the history of astronomical observation when *Alexandria* became the capital of the civilized world. Under the liberal patronage of the Ptolemies a magnificent building was erected, in which were deposited circular instruments for determining the positions of the heavenly bodies, and every facility was given to astronomers for prosecuting a continuous series of observations. Timocharis and Aristillus are the earliest individuals mentioned in connexion with this school. These astronomers appear to have flourished about the year 300 A.C. Ptolemy (150 A.D.) cites several of their observations. Among these are the declinations of a few of the principal stars and the records of eclipses. It does not appear that they were acquainted with any method for determining the right ascensions of the stars. Eratosthenes (275 B.C. circa) one of the Alexandrian astronomers, determined the obliquity of the ecliptic to be 23° 51' 19". It is manifest that neither the distances of the stars from the equator nor the obliquity of the ecliptic could have been determined even roughly without the use of instruments. In treating of the obliquity of the ecliptic, Ptolemy describes an instrument for determining the meridional altitude of the sun.
v2.c12.s5.48.3
It was composed of two concentric circles, placed exactly in the plane of the meridian, one of which revolved within the other about their common centre. The inner circle carried two small prisms attached to the opposite extremities of a diameter, and when the sun was on the meridian, it was turned round until the shadow of the upper prism fell exactly upon the lower one. An index, affixed to the latter, then marked upon the graduated limb of the outer circle the meridional altitude of the Sun. It was, in all probability, by means of an instrument of this construction that Eratosthenes determined the altitude of the Sun at each of the solstices & hence deduced the obliquity of the ecliptic. Ptolemy cites a passage from Hipparchus which shows that at Alexandria they used to ascertain the passage of the Sun through the equinox by means of a circular ring of metal disposed in the plane of the equator—the shadow of the upper half being watched until it fell upon the inner or concave surface of the lower half. It is not by what means the earlier astronomers determined the declinations of stars. Whatever credit may be due to the earlier astronomers of the Alexandrian school for the sound principles of observation which they appear to have practised, it is to Hipparchus alone that the establishment of astronomy, as a science of calculation based upon observed facts, is to be attributed.
v2.c12.s5.48.4
He determined the length of the year to be—365d—14g—48p as against the former measure of 365d—15g.