In following Years the bearded Corn enfu'd
From Earth unafk'd, nor was the Earth renew'd:
From Veins of Vallies Milk and Nectar broke,
And Honey fweated thro' the Pores of Oak.

DIALOGUE XIII.

On the Ufe of the GEOCENTRIC PLANETARIUM by Means of an Ephemeris.

WE

Cleonicus.

E have now finished our Survey of the Planetary Syftem, and I am well affured, my Euphrofyne, you understand the general Phænomena thereof very well; but fomething yet remains to be understood upon this Head, and I am willing to hope will give you as much Pleasure as any Part of our paft Speculations, as it is of a practical Nature, and what you are to perform your own Hands.

with

Euphrof. I long to know what it is, Cleonicus, as every new Idea gives me new Delight.

Cleon. What I mean is, what I fome Time ago hinted to you, viz. That it is not enough for you to know the fpeculative Part of Aftronomy, but also that you learn to find the Places and relative Afpects of the Planets, from an EPHEMERIS, for every Day of the Year.

Euphrof. And do you think I am capable of doing this, Cleonicus?

Cleon. Yes, what fhould hinder you, when you are put into a proper Method for it?

Euphrof. But I suppose I must first learn the Ufe of that Book you call an Ephemeris.

Cleon. You must.Here it is.Look at it. Euphrof. It is but fmall indeed, but it is filled with fuch a Number of Characters, Figures, and Columns, that at first View I know not what to make of it.

Cleon. That is every One's Cafe in a new Attempt;

G 3

when

when you confider each Article of the Ephemeris feparately you will find it eafy enough to understand.

Euphrof. But firft tell me why this little Book is called an Ephemeris?

Cleon. Because it is a Diary or daily Account of the Motions or Places of the Planets, as they appear to the Eye, placed both in the Sun, and at the Earth, throughout the Year.

Euphrof. But why do you fuppofe the Eye to view the Planets from the Sun, fince that is impoffible?

Cleon. Because on that Suppofition you can obferve or affign for any Time the true Places of the Planets in the Ecliptic; and this true Place feen from the Sun, is called the Heliocentric Place of the Planets, for the Name of the Sun in Greek is Helios; but if the Planets are viewed from the Earth (called in Greek, Ge) then their Places and Motions are called Geocentric.

Euphrof. I understand you, I believe, pretty well; I fee on the Right Hand Page, for every Month, a small Table with the Title of The Heliocentric Motions; as there, on the Top, for the Month December, 1755.

Cleon. Very well, my Euphrofyne. You obferve that Table divided in 7 Columns; in the firft is placed every 5th Day of the Month beginning with the firft. On the Top of each of the other Columns, the Character of the Planet and of the Sign in which it is on the first Day of the Month; and the Figures in the respective Columns fhew the Place in Degrees and Minutes which the Planet is in, as feen from the Sun precisely at 12 'Clock at Noon, on the feveral Days in the firft Column; for

you

you must know, the Aftronomers always begin their Day at Noon.

Euphrof. This is all fo plain, that I can't but apprehend it; and thus for Inftance, on December the ift, at 12 o'Clock, I obferved the Heliocentric Place of Saturn (↳) is in 29° 26' of Capricorn (v); of Jupiter (4) in 4o: 22' of Libra (~); of Mars (♂) 24°: 33 of Gemini (¤); of the Earth () 9° : 3′ of Gemini (11); of Venus (8) in 29° 6' of Sagittarius (4); and of Mercury () in 18° 40' of Cancer (5); and fo for any other Day there specified.

Cleon. Very right, my Euphrofyne, I fee there will be no Difficulty about the other Table of the Geocentric Motions, where the Place of the Sun and each Planet is given in their proper Columns, as viewed from the Earth, for every Day at Noon.

Euphrof. But befides thefe, there are several other Tables and Columns on the fame Page; pray, what is their Use?

Cleon. We fhall fee that hereafter, Sifter; the Heliocentric and Geocentric Motion will be fufficient at this Time to confider; and because we want to know conftantly in what Part of the Ecliptic the Planets are, their Geocentric Places are fhewn for every Day, as you fee for the Month December *.

* Tables of the Heliocentric and Geocentric Motions or Places for December 1755, are here added from Parker's Ephemeris, left the Reader fhould not have the faid Book at Hand; and one Month only is fufficient for an Example of its Ufe.

G4

Euphrof. Well, let us not spend Time about Things fo cafy; I fee that for every Day of the Month, the Places of the Sun and all the Planets are fhewn in the Ecliptic, as they appear from the Earth; but pray, what depends on all this?

Cleon. A very pleafant Aftronomical Praxis; by which you may at any Time be able to entertain yourself in a moft rational and agreeable Manner, that is to fay, you may

in a Minute or two, reprefent to yourself the true Appearance of the planetary System, just as it really is, in the Heavens for any Day you please; by affigning to every Planet its proper Place in its Orbit.

Euphrof. This will be doing great Things indeed, Cleonicus; but how am I to go about it? I long to know

Cleon. For this Purpofe, my Euphrofyne, I have prepared for you what I fhall call a GEOCENTRIC PLANETARIUM, which you here fee on this large Sheet of Pafteboard, where you obferve the Orbits of all the Planets are drawn at their proper Distances from the central Sun, in the fame Proportion as in the Syftem itself; and in the next Place, you fee the Planets themselves all moveable in their Orbits, on Paper Labels, about the Center of the Sun. Next, you fee the Orbit of the Earth divided into its 12 Signs and Degrees, which we shall call the Solar Ecliptic; in this, the True or Heliocentric Places of the Planets are eftimated. And, in the laft Place, about the Earth you fee a fmaller Circle of the Signs, moveable about its Center, in which the Planets viewed from the Earth, must all appear, and which for that Reason I fhall call the Geocentric Ecliptic *.

I have added this Plate, with its Appendix, that fuch Gentlemen and Ladies as have not the Opportunity of a real Planetarium, may in a great Measure fupply the Want of it by this Geocentric Planetarium, efpecially as fomething of this Kind has been always wanting to give Youth a Tafte of the Pleasure of practical Aftronomy.

In order then to prepare this Inftrument for Ufe, you proceed in the following eafy Method.

1. Cut the eighth Plate out of the Book, and pafte it on a Sheet of Card Pafte-Board.

2. Then with a Pair of drawing Compaffes, make the Orbit. of Mars a compleat Circle on the Pafte-Board. Then cut out the Whole, of a circular Form, a little beyond the Orbit of Mars.

3. In the Appendix to this Plate, there are engraved the Labels of Mercury, Venus, the Earth, and Mars, which are to be cut out clofe to the black Lines; thefe, together with the Face of the Sun, are to be placed on the Center of the Planetarium on a String, by which they may be tied clofe down, yet freely moveable about the Center, fo that the Center of the Planet on the Label may be always in the Orbit. Note, the Label of the Earth must be uppermoft, and the Face of the Sun on that.

Cut out the Geocentric Ecliptic, so that it may confist only

of