[ 113 ] CHA P. III. Selenography, or the Philofophy of the A. 4. PR Moon. RAY what is the Etymology of the Word Selenography? B. It is compounded of Em, the Moon, and Text, a Defcription; wherefore it means a phyfiological Defcription of the Moon. A. You fpeak of the Moon in the fingular, as if there was but one; whereas juft now you numbered our Moon with feveral others. B. Yes, there are feveral other Moons, as Jupiter hath 4 Moons, and Saturn hath 5. Moons; but because we know little more of them than their Number, Motions, and Distances from their primary Planets, we fhall omit them here, and fpeak of them together with their Primaries, by and by. A. Well, fince the Moon is favoured with the Preheminence to be confidered alone; pray what do you firft obferve in her? B. 'Tis found that the Body of the Moon is a large dark, opake, fpherical Body, analogous to our Earth in Matter and Form. A. What Proportion doth the Moon beat to the Earth in its Magnitude and Density of Matter? B. The Bulk of the Moon is to that of the Earth, as 5386333000 to 258445900000, or as 5 to 258; that is, the Earth is about 50 Times (at least) bigger than the Moon; the Denfity of the Moon is to the Denfity of the Earth, as 4891 to 4000, or as II to 9; the Quantity of Matter in the Moon is to that of the Earth, as 1 to 40, or as 1000 to 39788, more nearly; and the Weight of Bodies on the Moon's Surface, is to their Weight on the Earth's Surface, as 34 to 100. A. Pray, Sir, can you tell the particular Dimenfions of the Moon's Body in English Measure? B. Yes: The 'Diameter of the Moon is 2175 English Miles, her Circumference therefore must be 6829 Miles; whence the Superficies of the Moon will contain 14855440 fquare Miles, and her folid Content will be 5386333000 cubick Miles, as before. A. On what do thefe Meafures depend? B. On the Distance of the Moon from the Earth; which fome Aftronomers make 59, fome 60, fome 61, Semi-diameters of the Earth, that is, about 238920 English Miles; whence the Diameter of the Moon's Orb will be 477840 Miles, and Circumference thereof Of the Moon, her Dimenfions, and Motions. 115 thereof 1500418 Miles, which therefore is the Journey the Moon performs every Revolution. A. Then you do allow the Moon to move about the Earth every Day from East to Weft, though you deny this to the Sun. B. The Moon doth, indeed, move about the Earth, and that in about 27 Days, 7 Hours, and 43 Minutes, at a mean Rate: But this Motion is not from Eaft to Weft, but on the contrary, from West to Eaft, every Day about 13 Degrees and 10 Mi nutes. 1 A. How comes it then to appear to do thus ? B. By reafon of the daily Motion of the Earth about its own Axis from West to East once in 24 Hours'; which maketh the Sun, Moon, and all the heavenly Bodies, appear to move the contrary Way from East to West in the fame Time. A. Pray why do we fee the Moon rife and fet above an Hour later every Day than ano ther? B. I fhall make this cafy to conceive: Thus, fuppofe T be the Globe of the Earth, WSEN the Orb of the Moon; let AD represent the Horizon, in which let the Moon be on any Day, at her rifing and fetting, in E and W: Now, because the Moon moves every Day about 13° 10' from Weft to East, let that Distance be reprefent ed ed in the Moon's Orb by WE and EH; therefore at the fame Time, on the following Day, the Moon will be under the Horizon at H, at her rifing, and above the Horizon at F, at her fetting the Day before: Where the Surface of the Earth muft turn from A to B, and from D to C, before the Moon will be again in the Horizon at rising or fetting; which Motion from A to B, or from D to C, takes up above an Hour's Time more or lefs every Day. See Fig. XXI. A. Sir, I thank you; I perfectly underftand you, and the meaning of this Phanomenon: Pray hath the Moon any other Motion? B. Yes: She revolves about her own Axis juft in the fame Time fhe performs one Revolution about the Earth, viz. in 27 Days 7 Hours 43'. A. Pray how are you fure of that? B. Because at the fame Times we always fee the fame Face or Side of the Moon ; but this could not happen, unless a proper Motion about its Axis turned every Day juft fo much of the Moon's Body to the Earth, as her periodical Motion turns from it. A. This must be a wonderful Harmony and Correfpondence of Motions, indeed! But what is the Form of the Moon's Orbit? B. The Moon's Orbit is elliptical, but is perpetually changing, and never continueth the fame Species, or of the fame Figure. A. How Of the Moon's Phafes, is inhabited, 117'A. How fo, pray ? B. The Caufe of the Inequalities of the Motions of the Moon, and the Form of her Orbit, is the different Attractions of the Sun and Earth, the Eccentricity of her Orb, and the Obliquity of the Axis of her daily Motion. A. I think you agree the Moon receives all the Light the fhines with from the Sun. B. Yes, he does fo; and by Reflexion. conveys it to us in the Sun's Abfence. A. Pray, Sir, give me leave to ask what is the Reason why fome Parts of the Moon's Face look dark, and others light? B. The bright Parts of the Moon's Body are the more eminent Parts of Land, which reflect the Light of the Sun, as Hills, Mountains, Promontories, Ilands, &c. and the dark Parts of the Moon are thought to be Seas, Lakes, Rivers, Fens, &c. by some and by others, they are faid to be fhaded Valleys, Caverns, Pits, &c. But in Truth they represent both Water and shaded Places; for neither of thefe reflecting Light, must appear dark and not bright, ¿ A. Then you fuppofe the Moon to be inhabited, I find; I have heard there be fome of that Opinion. B. Yes, undoubtedly; to what End else can serve the Distribution of Land and Water, Mountains and Valleys, Caverns, Pits, I 3 &c. |