The INTRODUCTION:

under which the Semidiameter of the Earth’sOrbit is seen from that Planet. In Marsthe Angle A rf S (which is the visible Elon-gation of the Earth seen from Mars , orthe Parallax of the Annual Orb seen fromthat Planet) is about42 Degrees; and there-fore the Earth is always to the Inhabitantsof Mars, either their Morning or Evening iStar, and is never seen by them so far dis- jtaut from the Sun, as we fee Venus. Thegreatest Elongation of the Earth seen fromJupiter, being nearly equal to the AngleA If S is about 11 Degrees. In Saturn theAngle A h 8 is but 6 Degrees; which isnot much above £ Part of the greatestElongation we observe in Mercury. Andsince Mercury is so rarely seen by us, pro-bably the Astronomers of Saturn (except ■they have better Opticks than we have)have not yet discovered, that there is sucha Body as our Earth in the Universe.

The Parallax of the Annual Orb, or thegreatest Elongation of the Earth’s Orbit seenfrom any of the superior Planets, being,given; the Distance of that Planet from the:Sun, in respect of the Earth’s Distance fromhim, may be found by the fame Methodsas the Distances of the inferior Planets were.Thus, to find the Distance of Mars fromthe Sun, it will be as the Sine of the An-'gle S cf A is to the Radius, so is the Dis-tance A S (the Distance of the Earth from

the