Bharatiya Jyotihshastra (History of Indian Astronomy), translated by R. V. Vaidya v2.c12.s5.48.3

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.