On the Lenth of the Year After Varāhamihira’s PanchaSiddhantika

Abstract

By the Siddhantika Period (Aryabhata and Varāhamihira, possibly around AD 550), the position of the aphelion, the nodes (intersection of the orbit of the planet with ecliptic) and the orbital periods of the planets were fairly well determined. Varāhamihira also documented the correct ayanamsha (precession of the equinoxes, the points of intersection of the ecliptic with the celestial equator) of about 0.0141 days per year. Considering this, it is strange that the length of the year was believed to be 365.2584 days as against our presently accepted mean value of 365.2562 days for the sidereal year. This could have been due to one of two reasons: (1) The Earth’s spinning around its axis has slowed down, thereby decreasing the orbital period by 0.0022 days over about 1500 years. Then, any astronomical calculations based on data for more than 4000 years, which might have used the Earth’s rotational period as a unit, have to be re-examined. There appears to be substantial geological evidence consistent with this hypothesis; or, (2) The year might have been determined based on time taken by the Earth to move from aphelion to aphelion (aphelion, mandochcham, is important in astrology). If this was the convention for orbital periods, it is surprising that this information is not recorded nor discussed in relation to the origin of the co-ordinate system, even though we certainly shifted the start of the year from a position near Antares to the current position in Aries.