20

Parallaxof theEarth’sSemi dia-meter.

Fig. y.

The INTRODUCTION: j<

Circle from its Circumference, the Earth’s £Diameter will be found to be 6872 Miles, jand its Semidiameter 3436 Miles. The jParallax of the Earth’s Semidiameter, or the fAngle under which it is seen from a certain ^Planet, may be found by comparing the jtrue Place of the Planet, as it would be seen jfrom the Center of the Earth, (which is jknown by Computation) with its apparent }Place, as it is seen from some Point on the jEarth’s Surface. Let CZA be the Earth, jZC its Semidiameter, 0 some Planet, and ,BHT an A rch of a great Circle in the Heavens, jat an infinite Distance. Now the Planet 0 twill appear from the Earth’s Center, C, in the jPoint of the Heavens H;bu t a Spectator from -the Point Z upon the Earth’s Surface,will feethe fame Object 0 in the Point of the Hea- ■vens B j and the Arch B H the Difference is ,equal to the Angle B0H—Z0C, the ,Parallax ; which being known, the Side C 0 ;the Distance of the Planet from the Center ;of the Earth, at that Time, may be easily ,found. Now if this Distance of the Planet ,from the Earth be determined, when the ;Centers of the Sun, the said Planet, and ofthe Earth, are in the fame right Line, wehave the absolute Distance of the Planet’s :Orbit from the Earth’s in known Measure:Then it will be, As the relative Distance be-twixt the Earth’s Orbit and that of the Planet,is to the relative Distance of the said Planet

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