ATABLE of the Equation of Days, Shewing how much equal Time is fafter or flower than apparent Time, throughout the Year. Days |January February March April May June Fafter Fafter Fafter Fafter Slower Slower Min.Se M. S.M. S.M. SIM S.M. S. ATABLE of the Equation of Days. Days July Auguft September October November December Falter S.M. S.M. S. M. S. M. M. S.M. S T CHA P. X. Of PARALLAX E S. HE Difference betwixt the true Place of the Sun, or Planet, and its vifible or apparent Place, is called its Parallax. By the true Place is meant the Place where it would appear, among the fixed Stars, if it were feen from the Center of the Earth. The apparent Place is where it does appear, when feen from the Earth's Surface. This Difference is greateft when the Planet is at the Horizon, and gradually decreases as it rifes higher, till, when it comes to the Zenith, it has no Parallax at all. Fig. X. Let C be the Center of the Earth, O the Place of Observation on its Surface. Let QPN represent a Quadrant of the Planet's apparent diurnal Path, and LV Z the Starry Firmament. Now when the Planet is at P, were it to be seen from the Earth's Center, it would appear in the Firmament at T; but to an Obferver, viewing it from the Surface at O, it appears in the Horizon at H. The Arch Arch HT, or the Angle HPT, or OPC, is called its Horizontal Parallax. When the Planet rifes to S, the Arch V R, which is less than H T, measures its Parallax ; at N it has no Parallax at all. These are called Diurnal Parallaxes. The Angle which is formed by a Right Line, drawn from the Earth Fig. XI. to the Sun, and another from the Sun to the Moon, at either of her Quadratures, is called the Menftrual Parallax of the Sun. Dr. Hook and Mr. Flamfeed thought they had discovered an annual Parallax of the fixed Stars, of about 45"; Fig. XII. that is, an Angle produced by two right Lines, drawn from the fame fixed Star, to two oppofite Points in the Earth's annual Orbit. But Mr. Bradley has fince accounted for that supposed Parallax another Way. He conceives it to proceed from the progreffive Motion of the Star's Light*. The Sun or a Planet's Distance from the Earth, and confequently its real Magnitude, is found by its Parallax. For in every Triangle, if two Sides and one Angle, or one Side and two Angles, be known, all the other Sides and Angles are certainly found by Trigonometry. Thus in the Triangle OPC (Fig. X.) the Side OC Philof. Tranfa&t. No 406. 1 2 (which (which is the Earth's Semidiameter) being known, and the Angle at O, which is a right Angle; if the Angle at P can also be found, the Side P C, which is the Planet's Distance from the Earth, is also difcovered *. And the Distance being once known, the true Magnitude may be computed from its apparent Diameter. For, having the apparent Diameter, we can find, by the Help of trigonometrical Tables, what Proportion its real Diameter bears to its Diftance. For let DE be the apparent Diameter, seen by an Eye at F; then as the Radius of a Circle is to the Tangent of the Angle DFE, fo is the Distance F E to the real Diameter of the Object. And from the Diameter, the Magnitude of a Globe is eafily computed. Fig. XIII. Aftronomers have contrived various Methods for finding the Parallax of the heavenly Bodies; for a Specimen of which take the following, from Dr. Keill's Aftronomical Lectures, Lect. XXI. In an Eclipfe of the Moon obferve when both its Horns are in the fame vertical Circle, and then in that Moment take the Altitudes of both Horns: The Difference of these two Altitudes being halfed and added to the leaft, or fubftracted from the * Because as the fine of P is to O C, fo is the radius to P C. greateft, |