Indian Science and Technology in the Eighteenth Century: Some Contemporary European Accounts c9.81

c9.81

49. We have mentioned the use that is made of the semidiameters of the sun and moon in these calculations, and the method of ascertaining them, is deserving of attention. For the sun’s apparent diameter, they take four-ninths of his diurnal motion, and for the moon’s diameter, one twenty-fifth of her diurnal motion. In an eclipse, they suppose the section of the shadow of the earth, at the distance of the moon, to have a diameter five times that of the moon; and in all this, there is considerable accuracy, as well as great simplicity. The apparent diameters of the sun and moon, increase and diminish with their angular velocities; and though there be a mistake in supposing, that they do so exactly in the same proportion, it is one which, without telescopes and micrometers, cannot easily be observed. The section of the earth’s shadow, likewise, if the sun’s apparent diameter be given, increases as the moon’s increases, or as her distance from the earth diminishes, and nearly enough in the same ratio to justify the rule which is here laid down.