GEOMETRICAL.. ASTRONOMY, SHEWING 2 A Plain and Eafie Way to Project Theories of the Planets (in Plano :) And by them to find their true Places, both in Longitude and Latititude, at any time. And their Distances from the Sun or Earth. CHAP. Ι. How to lay down (in Plano) the Theory of the Earth, or other Planet. OR the performance hereof you must have a F Chords, and a Scale of 1000 egual Parts to Scale of the fame Radius: (Or, rather, a Sector, with such Scales upon it.) Then, I. How to describe and divide the Ecliptick. With 60 deg. of your Scale of Chords, describe a Circle; which divide into 12 equal Parts, for the 12 Signs, noting them with their respective Characters, V, ४, II, &c. This Circle thus deforibed and charactered, represents the Ecliptick Circle in the Heavens; whose Centre is, the fixed place of the Sun. In this Ecliptick Circle, every of the Planets hath a Point, which Fig. is called the Aphelion of that Planet: For when the Planet is in that LXXVI. Point, it is at its greatest distance from the Sun. Now, the A phelial Points of each Planet, are these which are exhibited in this little Table. Lll 2 The From the Aphelial Point of the Planet (as in our Example MARS,) 0.35 de. draw a Right-line to the Centre, as the Line Ap. 8; and that shall be the Aphelial Line of MARS. For the Aphelial Line of the Earth: The EARTHS Aybelial Point being in 7.00 deg. of Capricorn, from that Point draw a Right-line to the Centre, as the Line Ap., and that shall be the Earth's Aphelial Line, in the Theories of all the Planets. III. To find the Excentric Points of any of the 3 Superior Pla-nets 5, 4, 3, they will be found by the numbers in this little Table. For, If from the Aphe lial Point of h2to the Centre 4 be the Excentric Point shall be 954 28 Sfrom the Aphelial Point 2915 Wherefore, take the length of the Aphelial Line Ap. 8. in your Compaffes, and open the Sector in 10, and 10 of the equal Parts. Then, if from the Sector you take 915, (the Excentric of Mars) it will reach from the Aphelial Point Ap. 8, upon the Aphelial Line of Mars, to X, so shall X be the Excentric of MARS;, upon which Point (at the distance of Ap. & X) if you describe a Circle, as Ap. 5, 6, 8, that Circle shall be the Orbit of the Planet MARS, in which it moveth. For the Aphelial and Excentric Points of the EARTH in the Theories of the three Superiour Planets, they will be at the Numbers exhibited in this little Table. For, The Fig. The distance of the (101) And the distance of the (099 LXXVI. Aphelial Point of the from the Centre, shall be in the Theory of Excentric of the from 186 the Aphelion of the 2 shall be in the Theo 611ry of Wherefore, Out of your Sector, (it being still opened to the former distance) take the distance of the Aphelial Point of the Earth (for MARS 611) and set it (upon the Earths Apheliat Line) from the Centre to Ap., so shall the point Ap. be the Aphelial Point of the Earth - And the distance of the Excentric of the Earth, from the Aphelial Point of the Earth being 600, take 600 out of the Sector, and fet it from the Earths Aphelial Point Ap. downwards towards the Centre, and that shall be the Centre whereon to defcribe the Orbit of the Earth on the Theory of MARS. Which Point falling fo near the Centre O you may describe the Earths Orbit upon the Centre it felf; the difference being only! Parts: And in the Theories of the other two Superior Planets, the difference wil be much less; for in the Theory of Saturn it will be Parts: And in the Theory of Jupiter, but Parts: So that in small Theories they need not be much regarded. 1 4. For the Aphelial and Excentric Points of the Earth, and the two Inferior Planets and, the Tables following will ex-hibit. |