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Fig. LXXV.

The Semidiameter of the s Earth 1650 Log.

3.217484

Moon 446 Log..

2:༨༣༡༢༢༨

The Logar. Difference

0.568149

Multiply by

To the Logar. of 50.63

1.704447

So that the Earth is greater than the Moon 50 times, and

parts.

III. Of the Semidiameters, and Proportions of the other Primary
Planets to the Earth.

From the accurate Obfervations of Hugenius, Gaffendus, and. Horrox are determined.

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From hence may be gathered, That the Body of Saturn is
Greater than the Earth 298 times.

The Body of Jupiter 577 times..

But the Barth is Greater than the other Three: For it

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Fig. T LXXV.

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GEOMETRICAL ASTRONOMY,

SHEWING

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. I.

How to lay down (in Plano) the Theory of the Earth, or

other Planet.

F

OR the performance hereof you muft have a Scale of Chords, and a Scale of 1000 equal Parts to the fame Radius: (Or, rather, a Sector, with fuch Scales upon it.) Then,

I. How to defcribe and divide the Ecliptick.

With 60 deg. of your Scale of Chords, defcribe a Circle; which divide into 12 equal Parts, for the 12 Signs, noting them with their refpective Characters, V, 8, II, &c. This Circle thus deforibed and charactered, reprefents the Ecliptick Circle in the Heavens; whofe Centre is O, 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 4 phelial Points of each Planet, are thefe which are exhibited in this little Table.

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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. © ; and that fhall be the Aphelial Line of

MARS.

For the Aphelial Line of the Earth: The EARTHS Apbelial Point being in 7.00 deg. of Capricorn, from that Point draw a Right-line to the Centre, as the Line o 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 h, 4, 3, they will be found by the numbers in this little Table. For,

If from the Aphe-Sto the Centre be: 1000,√946
Sthe Excentric Point fhall be 954
Sfrom the Aphelial Point 2915

lial Point of

Wherefore, take the length of the Aphelial Line Ap. 3. 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. &, upon the Aphelial Line of Mars, to X, fo fhall X be the Excentric of MARS;. upon which Point (at the diftance of Ap. X) if you defcribe a Circle, as Ap.,,, that Circle fhall 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

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