dary Planets, that thofe of Jupiter will appear fituated on each Side of him, juft as I have reprefented them in this little Scheme. Euphrof. Indeed! Can you predict and defcribe their Order and Pofition for any given Time fo exactly, before it comes to pafs? But let me prove you Cleon. You fhall. See Jupiter near South-Weft I'll direct the Telescope towards him fee his Moons It is fixed; -I Euphrof. I fee them plainly as you have reprefented them-Two on the Left Hand, and two on the Right; one of which latter is near to Jupiter, the other at the greatest Distance from him- They appear exceeding plain indeed and lie very nearly in a ftrait Line. Jupiter himself fhines extremely fair and large have loft Sight of one of the Moons-and the others are going after―They feem to have a quick Motion to the Right, and will foon all disappear. Well, they are a pretty Sight, and worthy the Curiofity of any Perfon. But let me now view the Moons of Saturn? Cleon. Saturn you fee yonder about South-Eaft, in a very good Situation for our Purpose. I'll turn the Telescope about, and direct it to him.-It is now fixed, Euphrofyne; fee Saturn his Ring, and two of his Moons all in View. Euphrof. I fee them but the Moons are fo very fmall, I can but just discern them-They are both on the Left Hand of the Planet - Saturn fhines very ferenely — His Ring is a Prodigy never enough to be admired; with this Telescope I can plainly fee the dark Sky between him and the Ring-But he haftens from my Sight.-I have feafted my Eyes fufficiently with the pleasing Wonders, and wish that my Sex in general had the Happiness of tafting these Pleasures along with me. Cleonicus, I thank you; take Care of the Glass, and let us now prepare to go home. Cleon. I am glad you are pleased with the Sight you have had of these Moons; a better Opportunity could not have offered. But the Air feems now cold, and we will return home as you request. Euphrof. And let us walk apace; and as we walk anfwer me a few Queftions concerning thefe Moons: And in the first Place, why do thofe of Jupiter appear all in a ftraight Line, in a Situation Eaft and Weft? Cleon. This is owing to their moving all in such Orbits round their Primaries as lie right before the Eye, and whose Planes are nearly in the Plane of the Earth's Orbit; for then Circles will be projected into their Diameters by the Eye, and all Bodies in fuch Circles will seem to move in that right Line. Euphrof. I can fcarce tell what you mean. Cleon. I will explain myfelf by-and-by by Candlelight, and fhew you how Bodies moving in fuch Circles as the Moons do, will appear to move in a straight Line from East to West, while they defcribe that half Circle which is nearest to you; and then to return back again in the fame ftraight Line, while they pass through the remote Half of their Orbits. Euphrof. If this be the Cafe, then each Moon appears to go a certain Distance from Eaft to Weft, and from Weft to Eaft, in each Revolution in its Orbit. Cleon. Very true; and from thence alfo 'tis evident, that the remotest Moon may fometimes feem nearest to Jupiter, and the contrary; fince the apparent Places of all are conftantly changing in a direct Line to and from the Bodies of their Primary. Euphrof. I obferve this Line in which they feem to move paffes through the Center of their Primary, therefore I fuppofe the Moons do fometimes appear to pass over his Body, do they not, Cleonicus? Cleon. Yes, Sifter, they do; and then they appear on the Body of their Primary, fometimes like bright Spots, and fometimes like dark ones. Euphrof. Pray, how can that be? Cleon. They naturally appear bright by the reflected Light of the Sun, and their appearing dark is owing to fome very large Spot or Spots on the Hemifphere next to us, or from fome Quality of the Surface not apt to reflect the folar Light. Euphrof. The Moons, I apprehend, pafs over the Body of Jupiter, as they pafs from Eaft to Weft, or defcribe the nearest Half of their Orbits. Cleon. You apprehend the Matter right; and you will as eafily conceive, that as they move from Weft to Eaft, or describe the remoteft Semi-circle, they will pafs behind the Body of their Primary, and fo will be eclipfed from our Sight at that Time. Euphrof. I do, Cleonicus; and fhould be glad if I could fee any of them thus eclipsed. Cleon. I will take an Opportunity one Evening ere long, to fhew you fuch an Eclipfe of these Moons, which you must know is two-fold: For (1.) the Satellite may be hid from our Sight by the Body of the primary Planet, and this is called an Occultation thereof. And (2.) the Satellite may be cclipfed by entering into the Shadow of their Primary, and this is called an Obfcuration of those Moons, or moft properly, an Eclipfe of them. Euphrof. Why then, fince the Time of their Revolutions are fhort, I fuppofe that one or other of them muft frequently disappear. Cleon. Very frequently; you often fee one or more of them miffing; and Mr. Molyneux tells us he viewed Jupiter A. D. 1681, November 2, at Ten at Night, and obferved a total Difappearance of all his Moons at once: Jupiter, fays he jocofely, appeared folitary, and as it were deferted by his Guards, and a bold Lucian might have pulled him headlong from his Throne without Refistance. Euphrof. Well, that must be a very wonderful Sight, truly. But our Walk is at an End for To-night, I fee; and therefore fhall fufpend what I have further to query till To-morrow, and thank you for the Pleasures of this Evening. Cleon. Such Pleafures are mutual, I feel them in as large a Degree as you can do, my Euphrofyne; which we fhall now conclude with the following Lines of Sir Richard Blackmore. We've now beheld bright Planetary Jove, Sublime in Air, thro' bis wide Province move; And round him turn, as round the Earth the Moon. With ling'ring Labour finishes his Year. |