Note AS — LETTER THE EIGHTH.

Parallax is the changeable Position of Bodies to different Situations of the Eye. First having found the Quantity of a Degree (i. e. a 60th Part of the Circumference) upon the Earth's Surface, Aratosthenes discover'd that the Magnitude of the whole was easily known; and then from the Moon's horizontal Parallax having given the Radius of the Earth, the Distance of the Moon is soon determined; next by the menstrual Parallax of the Lunar Orbit, the Distance of the Sun is found; and by the Elongation of the inferior Planets, their mutual Distance from each other; and, lastly, from the annual Parallax of the Earth's Orbit, all the other Orbits of the superior Planets are easily found.