Precession of the equinoxes

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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. [1]

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.[2]

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.[3]

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.[3] Nīlakaṇṭha mentions the need to take it into account when making calculations about an eclipse.[4] Parameśvara gives a method of measuring it.[5]

However, Āryabhaṭa I, Brahmagupta, and Lalla are not known to have mentioned the precession, and Bhāskara I disbelieved it. [3]

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.[2]

Equinox precession rates
Source Date Rate given Calculated period
in years
Calculated angular motion
per year
Sūryasiddhānta[3] circa 500-1100 AD 600 per mahāyuga 7200 54"[2]
Muñjāla[3] 900s AD 199,669 per kalpa ~22,000 59".9
Pṛthūdakasvāmin[3] 900s AD 189,411 per kalpa ~23,000 56".82
Modern[6] 1992 - 25,982 50".3

See Also

References

  1. The Amateur Astronomer's Introduction to the Celestial Sphere, William Millar, Cambridge University Press, 2006. pg 101-107
  2. 2.0 2.1 2.2 Indian Journal of Historic Science, 9, 1974, p 35-37, Kuppanna Sastry, as quoted in Template:Refia, p xxxiv-xxxv
  3. 3.0 3.1 3.2 3.3 3.4 3.5 Template:Ref-indian-astronomy, p. xxxiv-xxxv
  4. Template:Refia2.8.1
  5. Template:Refia14.6.1
  6. Astrophysical Formulae, Kenneth R. Lang, Springer-Verlag, 1980, and Astrophysical Data: Planets and Stars, Kenneth R. Lang, Springer-Verlag, 1992, as cited in The Amateur Astronomer's Introduction to the Celestial Sphere, W. Millar, 2006, p. 103