Indian Science and Technology in the Eighteenth Century: Some Contemporary European Accounts c9.82

c9.82

50. The historian of the Academy of Sciences, in giving an account of M. Le Gentil’s *Memoir*, has justly observed, that the rule described in it, for finding the difference between the true and apparent conjunction, at the time of a solar eclipse, contains the calculation of the moon’s parallax, but substitutes the parallax in right ascension for the parallax in longitude;⁷² an error which the authors of this astronomy would probably have avoided, had they derived their knowledge from the writings of Ptolemy. From this supposed parallax in longitude, they next derived the parallax in latitude, where we may observe an application of the doctrine of similar triangles; for they suppose the first of these to be to the last in the constant ratio of 25 to 2, or nearly as the radius to the tangent of the inclination of the moon’s orbit to the plane of the ecliptic. We have here, therefore, the application of another geometrical theorem, and that too proceeding on the supposition, that a small portion of the sphere, on each side of the point which the sun occupies at the middle of the eclipse, may be held to coincide with a plane touching it in that point.