Indian Science and Technology in the Eighteenth Century: Some Contemporary European Accounts c9.51

c9.51

30. The tables of Tirvalore, which, as we have seen, refer their date to the beginning of the Calyougham, make the siderial year to consist of 365ᵈ, 6ʰ, 12′, 30″; and therefore the tropical of 365ᵈ, 5ʰ, 50′, 35″, which is 1′, 46″, longer than that of De La Caille.⁴⁸ Now, the tropical year was in reality longer at that time than it is at present; for though the siderial year, or the time which the earth takes to return from one point of space to the same point again, is always of the same magnitude, yet the tropical year being affected by the precession of the equinoxes, is variable by a small quantity, which never can exceed 3′, 40″, and which is subject to slow, and unequal alternations of diminution and increase. A theorem, expressing the law and the quantity of this variation, has been investigated by M. De La Grange, in the excellent memoir already mentioned;⁴⁹ and it makes the year 3102 before Christ 40½″ longer than the year at the beginning of the present century.⁵⁰ The year in the tables of Tirvalore is therefore too great by 1′, 5½″.