# Astronomical Dialogues Between a Gentleman and a Lady/Dialogue 4

​Not long after this, the curious Lady attack'd me again, thus; I have been considering, said she, the amazing Subject we discoursed upon the last Time, and am prepared now to talk with you about the Earth and the Moon, and the different Magnitudes and Motions of each; and of this I find it previously necessary to have some Knowledge, or else my Enquiries into those of the other Planets, will not give me sufficient Satisfaction: Pray, Sir, how many Miles is the Diameter of our Earth reputed to be, by the Astronomers?
Something less than 8000, Madam, said I; and because I know you will expect it, I must tell you, that we attain this Discovery thus: Both in England and in France, a Measure in Length hath been taken upon the Earth's Surface, under one and the same Meridian, or, in a right Line running exactly North and South, till by accurate Instruments it ​was found, that the Pole was raised or depressed exactly one Degree. This, the Mathematicians of both Nations agreed in to be almost 70 Miles, English: And there being 360 Miles in a Degree, that Number, multiplied by the former, gives you the Number of Miles in the whole Circumference of a great Circle on the Earth, or how many Miles it is round our Globe; and then, by the Principles of Geometry, they know, that something more than one Third of that must be the Earth's Diameter. I don't trouble you, Madam, with the exact Numbers, nor the Multiplication and Division, but you may depend upon it, that the round Number of 8000 Miles, is pretty nearly the Earth's Diameter, tho' something too much: And the half of this, viz. 4000 Miles, is the Earth's Semidiameter.Semidiameter, or the Distance from the Surface to the Earth's Centre, a Number, or Measure, much used by Astronomers.
I Thank you, Sir, quoth she, for this; the Knowledge of this single Point, will I see carry me a great Way, when I come to read Astronomical Authors: But, pray, Sir, go on and oblige me with a further Account of this Earth: Do you think it really turns round its ​Axis, as you have found the Sun to do by its Spots?
Yes, Madam, said I, and as there is nothing more easy and simple than this Motion, so it accounts for the Appearances of Cause of Day and Night.Day and Night in an easy and natural Manner; for as this Earth revolves from West to East in exactly 24 Hours Time, it makes the Sun appear to do so from East to West in the same Time; and makes it Day to those Places on its Surface, which are turned towards the Sun, and Night to such as are in the opposite Parts; as you see, Madam, if I set this Globe into the Sun's Light, it will illuminate but one Half of it, and the other Half will lie in the Shadow; but as I turn the Globe round its Axis, all Parts of the Earth's Surface painted upon it, will come successively into the Light, as the opposite Parts go, after the same Manner into the Dark.
I Grant you, Sir, said she, this is a very natural and easy Way of accounting for the Vicissitudes of Day and Night; and so short and unembarassed in comparison of the other wild Notion, which makes the Sun, and all the Region of the fixed Stars, to revolve round us in 24 Hours, ​that it recommends itself to us, at first Sight, as agreeable to the other Proceedings of Nature, if we could but get rid of our Prejudices, so as to conceive it possible to be done, without our perceiving it. But can we travel above 1000 Miles in an Hour, and not be sensible of it?
As easily as 10 in a Ship, Madam, said I; where, let the Vessel move never so fast forwards, if it were not for the Tossings and Shocks which the Resistance of the Water and Waves make, and for the Rustling and Bustle that the Wind makes in the Sails, you would perceive no Motion at all in the Ship, but judge it to be perfectly at Rest; and if another Ship lay at Anchor by you, you would judge that to move backwards, and not your self forwards. And much more will this appear plain, if you, consider that with the Earth's Motion round its Axis, the Air, and all the Atmosphere moves along with you, and doth not resist you, as is the Case in the Motion of a Ship. But indeed, the greatest Wonder in this Case is, that we are not all whirl'd off into the Air, like Dirt from a Wheel, or Drops of Water from a twirling Mop, or Stones parting from a Sling.
Madam, said I, your Ladyship's thinking of them now is proper enough; for tho' made to expose another Matter, they will illustrate what we are upon:
Why this is very true, said she, of those dirty Drops, and I can't imagine why 'tis not so with us; for I don't know any thing that fastens us down to the Earth, but our firm Inclinations to this World, which I believe yet hath no Centripetal Force.Physical Power to keep our Bodies annexed to its Surface. Pray, how do you account for this Difficulty?
Well, said she, a new World of Knowledge opens and dawns upon me! I ​begin to see a thousand Things, of which I had no Notion before; and I believe the Motions of the heavenly Bodies, after this, won't appear such abstruse unintelligible Things as I imagined them to be: But, pray, Sir, explain this a little further, with regard to the Moon.
You must know, Madam, said I, that this Gravitation of a Planet towards any Central Body, decreases vastly, as the Distance from that Centre increases; and therefore the Moon being about 60 of the Moon's Distance. Earth's Semidiameters, or 240000 Miles distant from us; her Gravitation towards the Earth, will be 3600 time less than that of a Cannon-Ball shot out of a Gun on or near the Surface of our Globe; and the great Creator hath so wonderfully contrived it, that her Centrifugal Force, or her Endeavour to fly off from the Earth, is exactly equal to her Gravitation thither; and this keeps her in her Orbit, as it doth all the Planets in theirs, as I said before.
O wonderful and happy Adjustment! said she, for I perceive, if the Moon's Gravity towards the Earth were much abated, she would run out of her Orbit, and leave us; and if the opposite Force ​were much lessen'd, she would, in a little time, tumble down upon us: Am I right, Sir, in this Conclusion?
Exactly, Madam, said I, and I perceive I need not say much more to you upon this Head, except it be to tell you, that if the Centrifugal Force were taken away from the Planets, and that only the Power of Gravitation towards the Sun remain'd, they would all soon fall down to him, and our Earth would get down thither in about 64 Days and 10 Hours time.
But I think, Madam, we are gotten to the Moon a little too soon, having not yet quite done with the Earth, whose Annual Motion round the Sun therefore, let us next consider: By which all Increase and Decrease of Day and Night, and the Changes and Seasons of the Year are made.
And can you give me any good Reasons, that I can understand, to believe this Annual Motion of the Earth, said she?
I Think, Madam, said I, there is in Astronomy a plain Demonstration for the Motion of the Earth round the Sun; but it will be too remote for your present Knowledge of these Matters: However, ​I think 'tis a very good Argument for its being so, that this Way there is a Parity and Agreement with the other Proceedings of Nature, which is very suitable to the Wisdom, Easiness and Conciseness observed by the Divine Being: For it being now agreed, that the Sun is the Centre of all the other Primary Planets, and that we are placed in such a due Distance from the Sun, between the Orbits of Venus and Mars, as answers to the Time of those Planets Revolution round the Sun; and since 'tis also agreed, that the other primary Planets, as well as Mars, Venus, and Mercury, do, in their several Orbits, revolve round the Sun; what Reason can possibly be assigned, why the Earth should not do so too? since they are Earths likewise as our Planet and the Moon are, and consequently our Earth must be as capable of moving round the Sun, or any other Centre, as they or she are.
I own, said she, that 'tis much more natural, orderly and harmonious to suppose it so, and therefore I will lay aside all Prejudice, and believe it with a good Astronomical Faith.
Madam, said I, if you will observe what Fontenelle saith, very justly, of Na​ture, that She is always magnificent in the Design, but frugal in the Execution of it, you will never believe that the Sun and fixed Stars turn all round us in 24 Hours; when you reflect, that the bare Motion of the Earth round its Axis will answer all your Ends that are to be served by the other. That would be just as absurd, as for a great Architect to contrive, with vast Expence and Machinery, a Kitchen-Grate, that should revolve round a Spit, in order to roast a Wheatear or a Wren; but never so much as dream of a Way to turn the Spit round.
'Tis monstrous, said she, as well as ridiculous, and as I told you before, I won't believe one Word about it: I see, that the more plain and intelligible things are, the more they are valuable; and that Obscurity and Mystery are usually the Effects of Ignorance, and want of Skill either in the Operator, or the Explainer. But, Sir, will you give me leave then to step to the Moon.Moon, and ask you a few Questions about her, for I can't put those fine Lines of Butler out of my Head:
And I know a good deal of his Meaning in them; as that the Moon borrows her Light from the Sun, &c. but I could be glad if you would explain a little upon that Matter, and upon her Motion round the Earth; after we have at Night examined her Face by the Telescope: Is this a good Time to look at her?
Yes, Madam, said I, a very lucky one, for she is now increasing, and not quite full; we shall see her Mountains more distinctly, and the Light of the Sun move from one Hill to another.
The Evening, according to our Wishes, proved very clear and fair, and the Lady was mightily pleased with the Face and Appearance of the Moon thro' the Glass; and having also the Day before been reading a little in Mr. Huygens's Cosmotheoros, or his Celestial Worlds discover'd, or Conjectures about Worlds in the Planets; and in Mr. Whiston's late ​Book, called Astronomical Principles of Religion; she was prepared to ask me some very proper Questions, and began thus:
Pray, Sir, said she, is not our Earth a Moon to the Lunar People, as well as she is to ours?
Yes, Madam, said I, and a most useful and a glorious one too; and we may in some Measure perceive that our selves, by the Light which our Earth reflects upon the Moon before he is just new, and for some Time after; for doubtless that is the only Light that then renders her visible to us: And when you consider that the Light of our Earth, consider'd as a Moon, will be thirteen times greater than that of the Lunar Light to us, it won't appear strange, that its Reflection on the Moon should render her then dark Body visible to us. However, this Terrestrial Light, when the Earth appears at Full to the People in the Moon, is not above a 3600th Part of the Sun's Light there, as the Light of our Full-moon to that of the Sun, shining upon us, is about As 1 To 48000.
Just in the Time, Madam, said I, that she is revolving round the Earth; which I will explain to you presently: But, first, it will be proper to inform you, that the Figure of the Moon not being exactly globular or spherical, but a little oval, or like that of an Egg, her longest Diameter (which exceeds her shortest by about 200 Foot) would, if you suppose it extended so far, pass thro' the Centre of our Earth: And hence it is that we see always the same Face of the Moon obverted towards us, and that this is not hinder'd by her Motion round her Axis, this familiar Instance will shew you. Please to sit still, without turning your self, while I walk round you; you will then see plainly, that if I keep my Face always towards one and the same Point ​of the Compass, while I am moving round you, when I come a Quarter of my Circle, my right Side, and not my Face, will be towards you; when I have gone half Way round you, my Back; and when I'm gotten three Quarters, my left Side will be turned towards you; but if, as I move in my proper Orbit round you, I always keep turning towards you, as indeed I can't help doing, you will then always observe me beholding you with the same Face of Respect and Esteem.
You Men, said she, are not like those constant celestial Lovers; for you seldom continue your Respect for above a Revolution or two: however, you may now stop in your Circular March, continued she, for I see the thing plain, and that the Reason why we see always the same Face of the Moon, is because she moves round her Axis in the same Time that she performs her Circle round the Earth: But, pray, let me know something more of the Manner of her Motions.
Madam, said I, the Moon revolves continually from West to East, and that pretty nearly in the same Circle which we call the Ecliptick; but not exactly so, sometimes running 5 or 6 Degrees above ​it to the North, and sometimes below it to the Southward: She doth not also keep always the same Distance from the Earth; as appears by her Diameter, which when we come to measure, we find sometimes considerably larger than at others; she moves swifter in the Syzygys, as they call them, that is in their Conjunctions with and Oppositions to the Sun, than she doth at her Quadratures, or when she shews just half of her Face.
Well, said she, I perceive now that her Motion is so irregular, that some Comparisons, which have been made with her, are not quite groundless: But this Part I fancy I shall get over by my Books, and I think I know also, that the Reason why she appears full, is, because she is then opposite to the Sun, who shines full upon her; and we lose Sight of her in what we call the New Moon, because she is then between us and the Sun, or in Conjunction with him; and 'tis easy to see also that all her other Changes and Appearances, or Phases, as I remember you call them, are accountable from her being in some intermediate Position between new and full. But, pray Sir, why have not we an Eclipse of the Sun at every new Moon, and one of the Moon at every Full?
I Conceive what you say, said she; so that there can be no Eclipse of either Sun or Moon, unless the Moon be in the Ecliptick as well as the Earth, because the Sun's Light will go by or besides the Earth or Moon.
You have it exactly right, said I, Madam, in general; all that I need tell you further is, that if the Moon have but a very little Latitude an Eclipse may happen; or if she be at the Time of the Conjunction with, or Opposition to the Earth, in or near the Nodes, as they call it, that is, the Points where the Moon's present monthly Circle crosses the Ecliptick. And this falling out commonly twice in every Synodical Month, or Lunation, as we call it, there would be an Eclipse of the Sun ​and Moon both, if the Earth could stay about the Nodes, and did not proceed on in her Orbit all this while, or change her Place in the Ecliptick forward on. However, within the Compass of the Year, there happen usually four notable and almost total Eclipses, somewhere or other, two of the Sun and two of the Moon. But your Ladyship will please to consider, that there is in the Nature of the thing a great deal of Difference between these two kind of Eclipses: In the Lunar Eclipse there is a real Loss of the Moon's Light, and it is also in the whole the same, from whencesoever it is seen, not being changed by the diverse Position of the Spectator on any Part of the Earth's Surface, whether he be in the Equator, or at the Poles.
But in an Eclipse of the Sun, there being no real Loss of the Sun's Light, but only an Interception of part of it, from coming to our Eyes, by the Interposition of the Moon's Body; this Eclipse must appear different according to the different Places on the Earth, from which a Spectator may observe it; for tho' to that part of the Earth to which the Centre of the Moon is then interposed between the Observer's Eye, and the ​Fig. III.Sun's Centre, it will be total, yet it will be but a partial one to all other Places, and none at all to the remote ones, as you will easily see by Plate III.
I Thank you for explaining the Nodes to me, said she, and these Phænomena of Eclipses; and by it you have now saved your self the Trouble of answering many Questions: And I perceive, for the future, I shall get to underſtand these kind of Things and their Terms of Art, pretty well: My Lexicon Technicum is a ready Help to me in time of need, and I believe the Doctor composed it out of a peculiar Regard to our Sex; I'm sure we are very much obliged to him for it. But have we any thing further to say about this Vagrant, the Moon, of whom Dryden speaks a little coarsely, methinks, in his Translation of Ovid:
Pray, Sir, said she, cease your high Strains, or else I shall begin to think, as Hudibras expresses it, That
had last Night some Effect on my Teacher's Head.
Madam, said I, between you both, I own I am now and then a little flutter'd.———But your last Verses mentioning the Tides, put me in Mind of another great Influence and Use the Moon hath over this Earth, besides the great Light she gives us; and that is, that he is the principal Cause of our Tides, which are so beneficial to us, in keeping our Ocean sweet by their Motion; and in helping the Navigation of our Ships in Rivers, and Places of the Sea, near the Shores. But I will refer you to the Lexicon Technicum, under the Word Tides, for an Account of it, where you may receive full Satisfaction, with regard to this Affair; for​merly so very unaccountable, but now very clear and intelligible.
I Can't leave, said she, this inconstant Planet, whom your ill-natur'd Wits have so often made the Resemblance of our Sex, without asking you a Question or two more; What is that you call the Hunter's Moon? I have heard the jolly Fox-Chacers talk much about it.
Madam, said I, what they mean by it, I don't understand; but I suppose it must be some very long Moon, which shines a great while about the hunting Season, and so hath become eminent that Way, tho' this can't always so happen: But if you will please to go to the Globe yonder, you will easily see, that when the Moon is in Cancer, her Sweep, or the Ark which the makes above our Horizon, will be vastly larger than when she is on the other opposite Side of the Equator in Capricorn; and if it then happen to be full Moon, or pretty near it, (as will be the Case next October) the enlighten'd Ark of her Motion above the Horizon will be very large and conspicuous, in comparison of what she will run when her full shall happen to be in Capricorn. But the Moon's ​Latitude won't much help to account for this Phænomenon; for that is greater at her Quadratures, i.e. when the appears an Half-Moon, than at the Syzygies, as we call them, i.e. at New or Full Moon; or when the Sun, Moon, and Earth are all nearly in one Right Line.
I Think I apprehend this, said the Lady; and now for the rest of my Questions. What is the Diameter and Magnitude of this Planet? What is her Motion? What Proportion doth she bear to our Earth? And do you think she is inhabited as we are?
Her Diameter, Madam, said I, we reckon to be almost 2200 Miles; in Degrees, when taken with an Astronomical Instrument, it is about 32 Min. 12 Seconds, which is nearly the same with the Sun's apparent Diameter, for that is but 31 Min. 27 Seconds; her Magnitude, or rather her Mass, or the quantity of Matter in her, with regard to the Earth, is about 1/40 part; but then the Density of her to that of the Earth, Is as 9 To 5; so that if she hath any Inhabitants, as I take it to be highly probable all the Planets have, they can't be of such Constitutions as we are. ​And tho' some of our Astronomers have lately concluded from some Observations made in Eclipses of the Sun, that she hath an Atmosphere, or Air about her like our Earth; yet 'tis probably of a very different Nature from ours, without any Clouds, Rain, Hail, or Snow; because, whenever our Air is clear, we can always discern the Moon's Face, with, as well as without a Telescope, to be bright, clear and distinct: Which I think could not well be, if her Atmosphere were like ours.
The exact time of her Periodical Revolution round the Earth, is in 27 Days, 7 Hours, and 43 Minutes, and this is call'd her Periodical Month; in which Course she runs about 2200 Miles in an Hour. But her Synodical one, as they call it, or the Time from New Moon to New Moon, is 29 Days, 12 Hours, and 3/4 of an Hour.
Pray, said the Lady, what occasions this Difference of above 2 Days and 5 Hours, between these two kinds of Lunar Months?
The Reason, Madam, said I, you will easily apprehend; and 'tis this: While the Moon is revolving round the Earth in ​her Periodick Month, the Earth it self is moved on in her Orbit round the Sun almost an entire Sign, or one twelfth part of the Ecliptick: and therefore that Point in the Moon's Circle or Orbit, where the last Conjunction with the Sun was made, will now be gotten too far to the Westward: and therefore she cannot come again to a Conjunction with the Sun 'till after 2 Days and about 5 Hours; which Time must be pass'd before the Moon can have exhibited all her Phases.
I hope, said she, I shall get to conceive this a little better by degrees; but pray let me go on now; and ask you a Question or two more: I have been thinking, that the Inhabitants of the Moon must have one thing very odd and strange; and that is, that to one half only of their World our Earth, which I am apprised must appear as a Moon to them, can be visible: So that their other Hemisphere will be for ever deprived of the Advantage of a Moon's Light.
O! Madam, said I, if your Speculations lead you into such Depths, we have you safe for an Astronomer; and I don't doubt but that will lead you also into the Study of such other Parts of Mathema​ticks as, the more you know of them, the more you will find them necessary. And I could now tell you a great many surprising things about the Appearance of our Earth to the Inhabitants of the Moon; but I will not deprive you of the Pleasure of reading them your self: you will find them fully enlarged upon at the End of Dr. Gregory's Astronomy, which is lately translated into English; which you will find among those Books, that, according to your Commands, my Bookseller sent you last Night from London.
Very well, Sir, said she; I shall be impatient till I get some further Knowledge of that Matter. But we will now take our Leave of Mrs. Moon; and, if you please, go down, as you call it, towards the Sun: and from thence ascend See Fig. IV.again; taking the rest of the Planets in their Order, according as this Diagram here represents them; which you have kindly drawn for me; and which you call a Scheme of the Solar System. Pray therefore, good Sir, tell me as much as you Mercury.think I can understand, about Mercury, the neareft Planet to the Sun.
Mercury, Madam, said I, is a Planet whose Diameter we reckon to be ​about 2700 miles; and therefore he is about two thirds of the Earth's Magnitude. His Distance from the Sun is about 32 millions of miles; and his mean Distance from us, about 22000 of the Earth's Semidiameter, or 88000000 miles, according to Cassini's Numbers. He revolves round the Sun in something less than 88 days, with the Velocity of 100000 miles in an hour: which is almost as fast again as the Earth travels: for we don't go above 56000 miles in that time; and yet that is making pretty good speed too; for that don't want much of a 1000 miles in a Minute, or 15 miles in a Second; or in that space of Time in which you can distinctly pronounce one, two, three, four. And yet however amazingly swift this may seem, 'tis crawling like the American Ignavus, or Beast called the Sluggard, in comparison of the Velocity of the Rays of Light, which certainly move about 180000 miles in that Time.
What! in a Second? said she: Let me see—; why, that is almost 50000 miles while I can say the word Light. For godsake stop a little, or you will make me perfectly giddy: my Head will turn quite round. What! have you and I then been travelling almost 2000 miles together ​this Morning, and I knew nothing of the matter?
'Tis even so, Madam, said I; and you see we move easily: But if you please I will go on. The Heat of the Sun there, is probably 7 times (Mr. Huygens saith 9 times) as great as with us in the hottest Summer; which is, I believe, enough to make Water to boil. You will easily see therefore that his Inhabitants cannot be such as we are; for our Bodies could by no means bear such a Degree of Heat.
Our Ancestors Bodies, said she, I believe could not but by our drinking so much scalding Tea and Coffee as we now do, I should think we are preparing ourselves to go and live there: And I suppose our famous Fire-Eater came from thence. There can be no Fluids sure in this Fiery Planet, much less Dense, than that which bears his Name; and no doubt all things else are Dense there in the same Proportion, or else the Sun would rarify him, and send all his Furniture off in Fume, Smoak and Vapour.
Well! said the Lady, as much as I hate frozen Zones and bitter cold Weather, I think this Mercurial World to be worse in the other Extream; so I will never wish for a Voyage thither.
Well, said she, I shall not much trouble myself to enquire after him; but I remember a saw him very plain and distinct, during the Total Darkness, in the last Eclipse of the Sun; and that shall satisfy my Curiosity, till some other Opportunity offers it self. But pray Sir, doth the Telescope shew us any thing remarkable about him?
Only Madam, that he hath Phases, as we call them, like those of the Moon, and sometimes appears full, and sometimes horned, like her; which you will easily conceive must be the Case of any Globe of Earth illuminated by moving round ​the Sun, and changing its Position, with regard to him, and to our Eyes. It hath not been yet discovered by any Spots or Marks upon him, that he revolves round his Axis, nor consequently what the Position of that Axis is, tho' 'tis probable he performs that Motion in a certain and determinate Time, as the rest of the Primary, and I believe all the Secondary Planets do. Venus and our Earth must needs appear very bright and large to the Inhabitants in this Planet, and the former will seem 6 or 7 times larger than she doth to us, which will help to supply the want of a Moon to him in the Night. But there is one more very remarkable Phænomenon of him, and that is, that as his Orbit is within ours, he must sometimes get between us and the Sun, and then he appears like a little black Spot in the Face of that Luminary, and may very easily be observed and distinguish'd by a Telescope.
O, I am mightily pleased with this, said the Lady, and shall I ever see him in that Position?
I Hope you will many a time, Madam, said I, for he will be there in April 1720, and in October in 1723, which ​is but a little while hence; and he will also be there again in May 1761.
Well, said she, I will then have a full look at him, if I live so long; and in the mean time let this Herald of the Gods ramble on as he pleases; and let us talk next about Venus.Venus.
But whither am I running? Pray Sir, stop me a little, and tell me some serious Astronomical Things about this celebrated! Planet.
The Distance of Venus, said I, Madam, from the Sun is about 60 Millions of Miles; and by some Spots which the Telescope hath discovered in her Face, she appears to have a Revolution round her Axis: The Time of which seems to be ​about 23 Hours. But neither Cassini at Paris, nor our Mr. Hook here, tho' they plainly saw the Spots to move, were able, positively and expresly to determine the Time of her Diurnal Rotation round her Axis; tho' the former takes it as I said before, to be in about 23 Hours; and therefore that will be the Length of her Natural Day. Her Motion in her Orbit around the Sun, is performed in a little above 224 Days, and her Motion in an Hour is about 70000 Miles.
That's pretty fair, said she, too for a Lady; but I am glad she doth not fly quite so fast as the last Whirlegig Mercury, however: But pray Sir, go on.
This Planet, Madam, said I, Mr. Huygens takes to have a large Atmosphere, which reflects so strong and glaring a Light, that her Body is rarely seen clear and distinct. She also hath Phases like the Moon; as was before observed of Mercury; she hath no Satellites, Attendants, Moons, or Secondary Planets moving round her, because as you very justly observed a while ago, Mercury and she being so near the Sun, have no occasion to be enlightned by Moons, as our Earth, Jupiter and Saturn have. Indeed Cassini, in the Years ​1672, and 1686, with a Telescope of 34 Feet, fancied he saw a Satellite moving round her, whose Diameter was about a quarter part of that of Venus: And Dr. Gregory thinks it not improbable, that this might be really a Moon to this Planet, and takes the reason of its not being usually seen, to be, the unfitness of its Surface to reflect the Rayes of Light: But as no subsequent Observations have confirmed this, I look upon it no more than a Conjecture. Neither she nor Mercury ever come so much as into Quadrature with the Sun, much less to an Opposition to him; and indeed, their utmost Elongation from him, as we call it, or greatest Distance East or West from the Sun, never amounts to above 2 Signs; Mercury not going above 28, and Venus never above 48 Degrees from the Sun. She is much about 40 times larger than our Earth; if, as some say, her Diameter be 7 times as long as that of our Planet: And the Light and Heat of the Sun, is about 4 Times as great as it is with us.
I'm heartily sorry, said the Lady, that 'tis so; for I would fain have had this beautiful Planet to have been inhabited by just such fine Gentlemen and Ladies as we have here; but I find 'twont do; ​the Women wou'd be there all as swarthy as Gibsies, and fry and sweat like Negroes in Africa: Out upon it! I'm afraid I shall find never a Planet fit to be inhabited by such People as you and I are.
Madam, said I, take Care; you are falling in with the Astrologic Whimsies; one would think you had read Athan. Kircher's Iter Extaticum, which agrees with your Wishes as to Venus, Mercury, and Jupiter; but he makes Mars all Smoke and Fire, and Saturn nothing but dull Lead, Dirt and Nastiness, as you will find when you come to look over Mr. Huygen's Planatary Worlds, which I have ordered the Bookseller to send you.
You are always cautioning me against Astrology, said she, and I must thank you for it. But I have heard that their beginning with that Study, hath made some Men become good Mathematicians, and even Astronomers: Shall I name them to you, Sir; you have forgot what you have told me of some of your Friends. But enough, let us proceed, and before we have quite done with this warm Dame, will you please to tell me, why she is sometimes our Morning, and sometimes our Evening Star?
And under both those Denominations, said she, I think the Poets make her change her Sex, and turn He-Thing, as if she could not be as useful when of our Gender, as of yours; for thus, forsooth, Mr. Dryden Compliments the Changling:
And so that blind Creature Milton cries,
Bright Hersperus that leads the starry Train, &c.
Marry come up indeed! Can nothing but Men serve you? Sure we have had Women every way as well qualify'd to be Morning or Evening Stars as any bearded Tyrant of you all.
Madam, said I, this is only owing to Custom, which hath made it the Mens ​Province to write Books and make Verses, and so they Compliment themselves: But however, you may be pretty easy, when you reflect, that we usually call the most useful things She's: Our Saxon Ancestors and our plain honest Country Folk, now call the Sun himself, that Father and Governor of all the Planets, She; and so we agree to call Guns and Fowling Pieces; nay, our Sailors are so well bred, and such Lovers of your Sex, that they call a Ship She, tho' she be a Man of War.
Well! said she, this is some kind of Atonement and Satisfaction; and therefore at your desire, I will for this Time forgive the Gossips of Phosphorus and Hesper; but if they should attempt to make a Man of the Moon, I will never pass it by, for I can hardly be reconciled to those that place a Man in that Planet. Venus in the Sun.But have you any thing further to tell me about Venus?
Only, Madam, that She also sometimes, like her Neighbour Mercury, hath appeared like a Spot in the Sun; as you will easily conceive she may, when you consider that the Orbit of the Earth includes her's within it; and that therefore she must be sometimes, tho' very seldom, ​between our Eye and the Sun, and then she will appear like a Spot in the Sun's Disk. The next time that Venus will be seen in the Sun, will be May 26. 1761. a little before 6 in the Morning; I wish your Ladyship Health and Happiness till the Time of that Observation, and that you may be then well enough to get up to see it.
O! Sir, said she, I can rise betimes in a Morning, for a lesser Occasion than this, and I design to see that surprizing Appearance, if it please God I live so long: But methinks 'tis a little ungratefully done of the Moon and these lower Planets, said she, thus to Eclipse him, or deprive him of any of his Light, when they receive all theirs from him; tho' I'm almost afraid our Earth doth so too; for since she is a Moon to the Moon, it must often be interposed between the Sun and Moon, and therefore for a Time deprive the latter of the Light of the former.
Upon my word, Madam, said I, you begin to run great Lengths, and go deep into the very Heart of Astronomy: And if you will please to read Dr. Gregory's Comparative Astronomy, in the Place I before recommended to you, you will be ​glad to see how rightly you have reasoned. Shall we proceed next, Madam, to talk about what they call the Superior Planets;Mars. and in particular about Mars, who next occurs in Order?
Yes, said she, we must take him in his Way; but I hope you Astronomers han't such terrible shocking things to say of him as the Poets have. Mr. Dryden, I remember, gives such a Description of Him and his Temple, as when I read it, chill'd me with Horror; and what is worse still, after he had enumerated all manner of Slaughters, Famines, Plagues, &c. he adds this:
No, Madam, said I, we give him no such Power of doing Mischief in our Hypotheses; but make him as calm and as gentle as any of the Planets.
Very well, said she, then begin, and say what you please of him.
Well! said she, that is pretty good marching too, for a Man in Armour. Sir, pray go on.
Madam, said I, by some Spots which have appeared in him, the Time of his Diurnal Revolution, is by Mr. Huygens settled exactly at 24 Hours 40 Minutes; and the Motion of those Spots hath also discovered that this Axis hath very little or no Inclination to the Plane of his Orbit; and therefore the Martial Inhabitants will have no sensible difference between Summer and Winter. Huygens thinks that the Colour of the Earth in him is blacker than that of Jupiter, or the Moon. His Light and Heat is twice, and sometimes thrice, as weak as what we receive from the Sun. When he is in his Quadratures, as they call it, that is in the middle between his Conjunction with, or Opposi​tion to the Sun, he appears a little gibbose, and to a good Glass almost bissected; but when at Full, perfectly round and distinct. The Telescope hath not yet been able to distinguish any Moon, or Satellite's moving round him; but that will not be a demonstrative Reason that there are none at all: for as they are at a great Distance from us, so they may be but small, and reflect but a weak and small Light, and therefore may not be visible. The Proportion of Heat and Light in this Planet, in comparison of ours, is not much above half.
O! said she, for all he looks so red, then I perceive the Planet is not so fiery as the Poets feign the God of War to have been. Pray, Sir, let us go on to Jupiter.
These secondary Planets suffer 4 kinds of Eclipses. (1.) When they are within the Shadows of their Principal. (2.) When the primary Planet is between them and us. (3.) When they are between their Primary one and us; for then 'tis difficult to distinguish of 2 Luminous Points one from the other. (4.) When they interpose between one another and our Eye, so as to hide one another from our Sight; which indeed happens but very rarely. And all these Attendants or Satellites, as well as Saturn's, like the Moon, the Earth's most obsequious humble Servant, do always turn their Faces towards their Lords the primary Planets, about whom they revolve, and on whom they wait.
This, said she, exhibits a good Image of respect and regard in Servants and Attendants; I wish our Earthy ones would imitate the Celestial.
Madam, said I, the Times of the Periodical Revolutions of Jupiter's Moons round him, are as follows:
The Innermost moves round him in 1 Day and 18 Hours, the second in ​Days 13 Hour, the third in 7 Days and almost 4 Hours, and the outermost in 16 Days and 161/2 Hours. In the Lexic. Technicum, you will find a good deal more about this Planet, and how the Eclipses of his Satellites are calculated; and thence arises an easy way to find the Longitude on Shore: But I doubt it is not practicable at Sea. If this Evening happen to be clear, as (it promises well) I will shew you the Planet with his Attendants around him.
I shall long to see that Sight, said she; but pray go on, and tell me more about this noble Planet.
His Diameter, Madam, said I, is above 80,000 Miles; and the Quantity of Matter in him is about 220 times greater than that of our Earth; and his Distance from the Sun about 424 millions of Miles: He revolves round his own Axis in 9 Hours and 56 Minutes, and about the Sun in 11 Years and 10 Months: And so large is his Orbit, that he moves after the rate of about 24,000 Miles in an Hour.
This Planet, said she, makes a great Figure by the largeness of his Bulk, and the grandure of his Attendants; but pray ​what kind of Temperament hath the Air of Jupiter? I doubt it must be much colder than ours, and then I shall never desire to be a Jovian.
Madam, said I, the Heat and Light of the Sun can't be above a 27th Part of what we enjoy here, and therefore it must be very dark, dismal, and cold living there; and the weight of all Bodies will be double to what they are on our Earth.
Nay! said she, if the People be twice as heavy, and almost 30 times as cold as we are, even let them live by themselves for me, I'll never hanker after going thither, but content myself with some Jovial Friends here in our dirty Planet, as Dr. Burnet called it; but I shall never have much value for his Judgement any more, that represented Jupiter as the Pattern of the fine Antedeluvian World. But pray, Sir, What Distance may these 4 Moons of Jupiter be from his Body?
The nearest, Madam, said I, is about 130,000 Miles from the Planet; the second about 364,000, the third 580,000, and the fourth or outermost is about a million of Miles distant from him.
Madam, said I, I see you don't only advance in Astronomical, but even in Physical or Natural Knowledge: that Speculation about the Tides of Jupiter is curious and new, and will be worth a further pursuit. But if you please we will now go on with our Planet's Phænomena. You see by the Figure of Jupiter, that besides a famous Spot by which his Diurnal Motion was determined, there are appearances in him like Swathes or Belts, as they call them: These they take to be moveable, and to be formed by the Clouds of this Planet, which seem, like our Trade Winds, to lie in Tracts parallel to the Equator of Jupiter.
And if these are really Clouds, says she, won't it be a proof of Jupiter's ​having a vaporous Atmosphere about him, like that of our Earth.
It will doubtless shew, said I, Madam, that he hath something round him like our Air: but its Texture, Gravity, and Elasticity may notwithstanding be vastly different from that of Ours; but if by it you mean to insinuate that he is inhabited, I entirely agree with you; for I take it, that such an Apparatus as the making four Moons to revolve about, and to enlighten him; (as five such there are also moving round Saturn, besides his Ring) I take this, I say, to be a Demonstrative Proof of both these Planets having some kind of Inhabitants, who have Eyes to stand in need of Light, as well as other Senses proper for their Natures: For we never find Nature doing any thing in vain, but ordering all things with the most consummate Wisdom; and we must never believe she would form Moons, where there are no People to be lighted by them.
Do you think, said she, that our Earth can be seen by the Jovial Inhabitants?
No Madam, said I, by no means.
This, Madam, said I, I think is all that is very remarkable about this famous Planet, except one thing more, which is indeed very considerable and surprizing: And that is this; that by the Eclipses of Jupiter's Satellites, made by the Interposition of his Body between them and our Eye, it hath been discovered that Light is in its Motion Progressive, and not Instantaneous, but that it takes up a determinate Time to come from Jupiter to our Eyes: For they have observed that these Eclipses happen sooner than they ought to do, by Calculation, when our Eye by the ​Annual Motion of the Earth, meets the Rays of Light reflected from them, whether at their last going out of the Sun's Light into Jupiter's Shadow, or at their first coming into that Light afterwards; and these Eclipses evercome too slow for the same Calculation, when we are going from those Rays; and this is always in that Proportion, which implies that the Rays of Light go from the Sun to our Eyes in about 71/2 Minutes of Time: And on this Calculation it was, that what I told you before about the prodigious Velocity of the Rays of Light, was founded.
I shall look a little further, said she, into this Affair some other time; Saturn.but pray let us now go on to talk about Saturn.
That outermost Planet in our System, Madam, said I, is at a very great Distance from the Sun, about 777 millions of Miles; and the Time of his Revolution round him, is about 30 Years, or more exactly speaking, in 10759 Days, 6 Hours, and 36 Minutes: And yet so very large is his Orbit, that he moves at the rate of about 18000 Miles an Hour; his Diameter is about 61000 Miles; and with regard to the Quantity of Matter in him, 'tis about 94 times as great as that ​of our Earth; but his Density is not much above a 7th part of that of the Matter of our Planet. And as to Light and Heat, 'tis probable that he hath not above a 90th part of what we enjoy by the Sun. Indeed in order to supply this great Defect of the Sun's Light, occasion'd by so great a Distance, our All-wise Creator hath furnish'd him with Five Moons or Attendants; the largest of all which, and which is the only one that is commonly seen, is the 4th in order from his Body; and he bears the name of the Hugenian Satellite, because first discover'd by Mr. Huygens. These Satellites of Saturn revolve round him in the Plane of his Ring (of which Ring I shall speak presently) and so their Circles make the same Angle with his Orbit, that the Plane of his Ring doth, which is about 31 Degrees. But the Orbit of Saturn himself is nearly coincident with the Plane of our Earth's Ecliptick, as are indeed the Orbits of all the primary Planets. It doth not yet, I think, appear, that Saturn hath any Diurnal Revolution round his own Axis; the Time of his Periodick Motion round the Sun, I gave you before; and those of his Satellites are as follows: The Innermost of these Moons revolves round Saturn in one ​Day, 21 Hours and 20 Minutes, and is distant from him about 146,000 Miles. The second is distant from him about 187,000 Miles, and performs his Revolution in 2 Days, 17 Hours and 40 Minutes. The third's Revolution takes up 4 Days, 13 Hours and 45 Minutes, and he is distant from the Centre of Saturn about 263,000 Miles. The Hugenian Satellite is about 600,000 Miles from him, and moves round him in 15 Days, 22 Hours and 40 Minutes. The last is 1,800,000 Miles distant from Saturn, and takes up 79 Days, 22 Hours in revolving round him.
'Tis highly probable that there may be more Satellites than these five moving round this remote Planet; but their Distance is so great, and their Light may be so obscure, as that they have hitherto escaped our Eyes, and perhaps may continue to do so for ever; for I don't think that our Telescopes will be much farther improved.
But the most surprizing and unparallel'd Phænomenon of all, in this Planet, is that which we call his Ring; which appears nearly as the Figure represents it,Vid. Fig of Saturn. in an ordinary Telescope: 'Tis a vast Body of Earth, as is most probable, of ​perhaps 7 or 800 miles in Thickness, which at the Distance of about 21000 miles from Saturn's Body, and with just as great a Breadth, is placed in a circular Arch, round about the Planet, in Figure much like the great Wooden Crane-Wheels, in which Men or Horses walk, to raise Goods, or to draw Water. 'Tis placed exactly over the Equator of Saturn, and is not any way contiguous to his Body, nor supported by any thing. The Surface of this Ring is not rough and full of Hills and Protuberances, as that of the Moon in most places is; but even and plain, as it is in those Regions of the Moon, which some, because of their great Evenness, have judged to be Seas.
The Thickness of the Ring, comes not into Astronomical Observation, appearing but as a Line. And tho' the two broad Surfaces of the Ring reflect a good deal of strong Light, yet the marginal Surface of it, or its Edge or middle Part between the two eminent Surfaces, reflects hardly any at all. The Plane of the Ring is inclined to that of the Ecliptic, with an Angle of about 31 Degrees; and this Inclination in the Course of one entire Revolution of Saturn round the Sun, hath some Variation; being twice ​greatest, and twice least of all. And this occasions the Planet sometimes to appear without any Ring at all, as when the Sun happens to be in the Plane of the Ring; and at other times, with Ansæ, as they call them, or Handles only; when but little of the Surface of the Ring can be seen: And at all other times the Ring will appear in an Oval Form, which sometimes will be more, sometimes less oblong.
I suppose, said the Lady, it is at that critical Time when the Ansæ only appear, that Saturn puts on the Figure which Hudibras makes Sydrophel give him, that is, that its like a Tobacco-Stopper.
That is but a mean Ridicule, said I, Madam; but I perceive it hath some Use; for it impresses itself, and the Thing, stronger on the Memory, than perhaps a more just and serious Description would have done. But your Ladyship will soon be above these little Helps: And you will receive a great deal of Pleasure, Madam, by reading what Dr. Gregory hath written about this Ring, in his Discourse of Saturn, and in his Comparative Astronomy, so often recommended to you; where the most consi​derable Phænomena of this Ring, and of the Satellites, as they appear to an Eye supposed to be placed in Saturn, are explained and accounted for; or you may consult the Lexicon Technicum.
I will, said she, attach myself heartily to that Book, as soon as I can: And after we have view'd this Planet with our Telescope, which I will sit up any time of the Night to do, if you can afford me your Assistance. For these two superior Planets have so many Wonder attending them, that I grow seriously amazed; and long to understand a little more of them, and to contemplate these wonderful Works of our great Creator. And indeed what a vast Field of Thought, what a new World of Speculation, do these Discoveries open to us! How empty and starv'd is a Mind unfurnish'd with such glorious Ideas!
What a rich Fund of Images is treasured up here to embellish our Poetry? And yet I don't remember to have met with many Allusions taken from these Things, except in a late Copy of Verses presented to her Grace the Dutchess of Bolton, where after the Poet had said a great many fine and just things ​of her, I now remember these Lines; the Beauty and Propriety of which, did not at first strike me so much as they do now, since I have been conversant with these Speculations.
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But now, Sir, said she, if you please we will leave off, unbend, and go to our Tea.