Precession of the equinoxes

The precession of the equinoxes is the movement of the equinoxes westward along the ecliptic over time. It is caused by the 'precession' of the Earth with respect to the distant stars.

Ancient Indian astronmers varied in their discussions of the precession. Back in Vedic times, it had been observed that the spring equinox had moved from the Nakṣatra Mṛgaśiras, through Rohiṇi, to Kṛttikā.

The Romakasiddhānta mentions it. The Sūryasiddhānta does as well, and gives a rate of movement. It also says that the position would swing back and forth, which is in contrast to other notions that it continuously moves along the ecliptic in one direction.

The astronomers Devacāryā, Vaṭeśvara, Āryabhaṭa II, Muñjāla, and Pṛthūdakasvāmin all give methods of calculating the precession, although some describe it in terms of the precession of the solstice. Nīlakaṇṭha mentions the need to take it into account when making calculations about an eclipse. Parameśvara gives a method of measuring it.

However, Āryabhaṭa I, Brahmagupta, and Lalla are not known to have mentioned the precession, and Bhāskara I disbelieved it.

Follows is a table of various rates of precession as calculated by astronomers. Kuppanna Sastry has written that these calculations are complicated by the fact that the 'Hindu sidereal year' is different from an ordinary sidereal year by 8", and so the calculated angular motion might be misleading.