DIALOGUE XIV. On the Ufe of the STELLATED PLANETARIUM Euphrofyne. INCE I laft faw the Planetarium, I obferve you have made an Addition to it; pray, what means that Hoop about it, with fo many fmall Stars? Cleon. That is to anfwer the Purpose I have often mentioned to you, of fhewing more plainly and directly the Manner in which the Planets muft neceffarily appear to us to go forwards and backwards, and at Times be Stationary, among the Stars in that Part of the Heavens through which they pafs; which Stars are thofe engraved on the Hoop, and this ftellated Zone will therefore fitly represent the artificial ZODIAC of the Planets. Euphrof. You have indeed often promised me the Pleafure of those Appearances in this Way, which I prefume will be more natural than those in which I have already viewed them. But I obferve, you have only the three Planets next the Sun, in the Planetarium now. Cleon. They will be fufficient for the prefent; and you will take Notice that the Wire which paffes from the Earth through the Planet Mercury to the ftarry Zodiac, does represent a Ray of Light, by which the Planet is seen by the Eye; and as the Planet moves on the Inftrument it will point out the apparent Place of the Planet among the Stars, just as a Ray of Light fhews the apparent Place of the real Planet in the Heavens. Euphrof. I fee the Congruity of fuch a Representation; and I farther obferve, you have placed these three Planets at their proper or proportional Diftances from the Center of the Sun. Cleon. I have placed them at their true Diftances that you may fee the Times employed in their direct and retrograde Motions, and their ftationary Afpects are the very fame in the Planetarium as in the Heavens, or in the Ephemeris, where the faid Motions are all minuted down very punctually. Euphrof. I fhall be pleased with repeating this aftronomical Praxis in a different Way from that of the Geocen tric Planetarium, though that delighted me much; efpecially as I apprehend this will le more natural, and perfect: But as the Ephemeris will be neceffary, I will go and fetch it Here it is, Cleonicus, how are we to proceed with it? Cleon. I will tell you-give me the Book-Let us now propose to find when the Planet Mercury next begins to appear direct in Motion, when Stationary, and when retrograde; and how long a Time each of thefe Phanomena will take up. Euphrof. This will be very entertaining; but, pray, will it not be required to find the true Places of the Earth and the Planet for this Purpose? Cleon. It will, and that you can eafily do for any given Time; now the Time when every Planet begins to appear direct in Motion, is marked with the Letter D, and when retrograde, with the Letter R; and therefore may be eafily obferved at any Time, and fhewed by the Stellated Planetarium. Euphrof. Well, I long to try the little Skill I have in a Solution of this Problem. The Planet Mercury I fee you propofe for the Subject, but which Ephemeris must I ufe for this Purpose, this for the old Year 1755, or must we have one for the prefent new Year 1756 ? Cleon. It will be beft to make use of both, because the Planet Mercury begins to be direct in Motion the 4th Day of December 1755, as there you fee in the Table, in the Column for Mercury. Euphrof. I do fee the Letter D placed there, to denote it-It is then in the 23° 44′ of Scorpio m, feen from the Earth. Cleon. Very good, my Euphrofyne. Now let us place the Earth upon its proper Stem or Arm, and juft over the 24th Day of December, 1755; and you obferve a fine long moveable Wire in the middle Part of its Surface. Euphrof. I do.But what Purpose is that Wire to answer? Cleon. It will pass through the Body of the Planet Mercury, when placed on its Arm, and its End will conftantly pass by and among the fixed Stars on the Hoop or Zodiac; and fo it will reprefent the Ray of Light, by which the Planet is feen by us, and by which its apparent Motions will be determined and shewn among the Stars in the Heavens juft as they appear to us. |