# Astronomical Dialogues Between a Gentleman and a Lady/Dialogue 7

Within a Day or two, I obtained for the Lady a sight of the Orrery; she desired we might have no other Company but one young Lady more of her Acquaintance; because, said she, I shall ask so many Questions, as perhaps will shew my Impertinence to those who are not acquainted with Things of this Nature, and my Ignorance to those who are.
As soon as the Instrument was taken out of its Case and set upon the Table, she expressed herself mightily pleased with the cleanness and cleverness of the Workmanship of it; for indeed the Outside of it is very rich and beautiful. The Frame is of fine Ebony richly adorned with twelve silver Pilasters, in the form of Cariatides; and with all the Signs of the Zodiack, cast of the same Metal, and placed between them; the Handles were ​also of Silver finely wrought, with the Joints as nice as ever were seen in the Hinges of any Snuff-Box: On the Top of the Frame, Ecliptick and Zodiack.which was exactly circular like the Horizon of a Globe, is a broad silver Ring, on which the Figures of the 12 Signs are exactly engraved; with two Circles accurately divided; one shewing the Degrees of each Sign, and the other the Sun's Declination, against his Place in the Ecliptick, each Day at Noon.
The Nature and Use of these Circles the Lady perfectly understood, from what she had before learned; and therefore in her pleasant way, she began thus:
If so much Art and Expence be bestowed upon the Outside of this curious Machine, I don't doubt but the Inside of it is at least equally curious and useful: And therefore I must desire you, Sir, said she, to begin quickly, and to shew it all to me, as the Man doth the Tombs at Westminster; tho' I hope you won't be always in the same hast, nor imitate his precipitant Manner, and awkward Tone of Speech; but do it slowly and distinctly, allowing me time to think and consider about it, and to ask you all the Questions I have a mind to.
This Silver Plate on which the Signs of the Zodiack, &c. are drawn, represents the Plane of the Great Ecliptick of the Heavens; or that of the Earth's Annual Orbit round the Sun; which as it passes thro' the Sun's Centre, so its Circumference is made by the Earth's Centre's Motion; and which for the better advantage of View and Sight, is here, you see, placed parallel to our Horizon.
The large gilded Ball which stands up, you see, here in the midde, not upright, but making with the Plane of the Ecliptick an Angle of about 82 Degrees, is so placed to represent the Inclination of the Sun's Axis; and which being pretty near the Centre of this Orbit, represents the Sun.
Pretty near, said she; why is not the Sun then exactly in the Centre of that Circle which you call the Earth's Orbit?
No, Madam, said I, nor is that Orbit exactly a Circle; but an Oval or Ellipsis. ​As in this Figure which I will now draw with my Pencil and shew you: Let the Curve Line P O T R represent the Orbit of the Earth revolving round the Sun, which is placed not in C the Centre, but in S, a Point in the longer Diameter, which they call the Focus: The Distance between C and S, is what in the Ptolemaick System, was called the Eccentricity, and expresses how much the Earth's Orbit differs from being a True Circle. And the Contrivance of this Instrument is so admirable, that you will see by and by, when I set it a going, this Eccentricity, and that of the other Planets will be plainly shewn to your Eye, in the same proportion as they are in the Heavens.
Pray, Sir, said she, go on; I find I shall come to understand this better, when I come again to read Dr. Gregory, and Mr. Whiston.
Madam, said I, you see here two little Balls standing upon two Wires, at different Distances from, but pretty near ​the Sun; the innermost of these is designed to represent Mercury, the other Venus.
But why are they placed, said she, upon those two Wires; they stand perking up like the Traitor's Heads upon Temple-Bar; I hope Mr. Rowley hath not discovered that they have committed any late Treason against their Sovereign the Sun.
No, no, Madam, said I, they are very Loyal Planets; the Contrivance is only to bring their Centres to be, sometimes in, and always pretty near the Plane of the great Ecliptick, (and by the by the Plane of their Orbit, always passes thro' the Sun, and intersects the Ecliptick in two Points, which they call Nodes) and this Position is contrived in order to shew us what Appearances they do really exhibit in their several Revolutions round the Sun. For the same Reason you see the Earth and Moon here placed likewise on Wires or Pins, that their Centres may get sometimes actually into, and always be pretty near this Plane of the great Ecliptick; for so the Orbits of all the Planets are really placed in the Heavens.
Madam, said I, that is to represent also the Angle that the Earth's Axis, or that of the Equator, makes with the Axis of the Ecliptick; which latter, in this Instrument, being perpendicular to the Horizon, the Earth's Axis is placed so as to make an Angle with the Plane of the Horizon of 66° 1/2; or dipping down from the Zenith just 23° 30′, which you know is the Angle made by the two Planes of the Equator and Ecliptick. And as the Earth in each of her annual Revolutions round the Sun, always keeps her own Axis parallel to its self; so you will see, by and by, when the Instrument moves, that this Terella, or little Ivory Earth, will do so too, as it takes its Tour quite round the Golden Sun in this Instrument.
I long to see that, said the Lady, very much; but I suppose I must suspend my Inclinations, till you tell me 'tis fit they should be gratified.
You observe Madam, said I, another Wire here, standing close to this Silver Circle, and which hath a Ball upon it, whose Centre is in the Plane of that Circle: This is designed to represent the Moon; and the Silver Circle represents her Orbit round our Earth, the Plane of which always runs thro' the Earth's Centre, and the Figures that are engraved upon it, shew her Age, from one New Moon to another.
Well! said she, this is mighty instructive! I long to see the Earth and the Moon move, but I know I must have patience: I suppose the Moon's Globe being black on one side, and silvered white on the other, is designed to represent her Phases as they call them, of which you have shewed me something before.
See Fig. of the Orrery.'Tis so, Madam, said I; and you will see this Machine so admirably contrived, ​that what I told you of the Moon's monthly Revolution, will shew it self to be in fact true here; for in the Lunula here, will turn round its own Axis, at the same time as it moves in this Silver Orbit round the Terella. And in reality, Madam, I can't blame your eagerness to see the Machine put into Motion, when I see how well you understand it, and know what it ought to do: And therefore you shall be detained no longer, than while I desire you to take Notice of this Hole in the great Brass Plate that covers all the Movement, and of this moveable Index here on the silver Ecliptick. You see there are on the former some Figures engraved; they are the common solar Years: and by taking the Instrument to pieces, it may be set to this present Time: And the Planets, by means of an Ephemeris, may be set to any particular Time also. So that if a Weight or a Spring, as in a Clock, were applied to the Axis of the Movement, so as to make it move round once in just 24 Hours, these Representative Planets, which you see here, would all perform their Motions round the Sun and one another, exactly in the same Order and regular Manner, as their Originals do in the Heavens; and this would then be ​a true Celestial or Astronomical Clock, which would shew the Aspects, Eclipses, and other Phænomena of the Sun and Planets, for ever. But because this would be instructive only in that slow tedious way to such as could have daily recourse to it, Mr. Rowley hath contrived, by Winch or Handle, to turn the Axis swiftly round about, and by that Means to shew all the Phænomena or Appearances in a very little Time, as you shall see I will now proceed to do; for by turning this Handle backward or forward, you may see what Eclipses, Transits, &c. have happened in any Time past; or what will happen for any Time to come, without doing any injury to the Instrument.
I am amazed at the Thought and Contrivance of this Instrument, said she, and I doubt not shall receive a prodigious Pleasure when I see it put into its proper Motions: But pray, Sir, let me first ask you, Are all the Planets here?
No, Madam, said I, (for I see nothing can 'scape your Ladyship's discernment) here are only shewed the Orbits of Mercury, Venus, the Earth, and the Moon; ​for the others are at too great a Distance to be brought into the Instrument, if any tolerable Proportion be observed between its Parts: And indeed, by what you will see of the Motion of these Three Planets, and of the Earth's Satellite, the Moon, you will easily know what the Phænomena of the Superior Planets and of the other Satellites would be, if they could be here shewn; as they cannot well be without embarrassing the Instrument with a vast Number of Wheels more: And it hath almost 100 already.
But now, Madam, I will fix on the Handle, and begin to put the Instrument in Motion.
One entire turn of the Handle answers to the Diurnal Motion of the Earth round its Axis, as you will see by the Motion of the Hour Index, which is placed at the foot of the Wire on which the Terella is fixed; and which you perceive moves once round as I now with my Hand turn the Spindle of the Machine round, after the same manner. You will take Notice also, that the Instrument is so excellently formed, that I can make the Motion tend either way, forward or backward; and turn it about after the same manner, 'till I bring the Earth to answer to any Degree ​or Point of the Ecliptick. As for Instance, I will move it about till I bring the Earth to the first Point in Aries. Then you see, to an Eye placed on the Earth, the Sun will appear to be in the Opposite Point, that is, in the first of Libra.
But Sir, said she, I perceive as you turn the Earth about, the silver Circle on which the Moon's Age is placed, and which I think you said represented her Orbit, rises and falls; What is the meaning of that?
Madam, said I, you know the Moon's Orbit is not exactly in the Plane of the Ecliptick; but makes an Angle with it of between 4 or 5 Degrees: And just so much this Circle rises above and sinks below the great Ecliptick, according as the Moon hath North or South Latitude, and just as much as that Latitude is: And you will observe two little Studds, which are placed in two opposite Points of this silver Circle; they are designed to represent the Moon's Nodes, or the Points of Intersection of her Orbit, with that of the Ecliptick: Of which, more by and by.
You see, Madam, said I, that one entire turn of the Handle is, as I said before, a Natural Day: Now, if you please to take off one of the broadest of your Patches, and make it a Spot upon the Golden Sun there, you shall see that your Patch will move quite round in 25 Days, or 25 turns of this Handle; Sun's Motion round his Axis.and that will shew you how by the Motion of the Spots in the real Sun the Astronomers discover'd he had such a Motion round his Axis, as you shall see Mr. Rowley hath given here to his Representative.
Well, said she, since even my Patches must become Astronomical, I will stick one upon this Fictitious Sun; but I must own I don't love those Spots.Spots upon the Natural one; nor to have any of his Face hid, or his Heat impaired: But shew me to what part of the Sun this Patch is to he preferred.
Twenty seven Turns and a little more than a quarter of one, you shall see, will carry the Moon's Periodic Month.Moon round in her Orbit; in which time you will observe she always turns the same Hemisphere towards the Earth.
Take Notice also, Madam, that now I have just made 12 Turns and an half, which hath carried your Patch to the opposite part of the Sun.
And shall I ever see it again, said she, shall I ever recover the Solar Traveller?
Yes, Madam, said I, you may have it again; but pray keep it for hereafter only for such Uses; and don't replace it on your Face; for I am as angry at Patches in a good Face, as you are at Spots in the Sun; and for your Reason, ​because I would not have any part of it hidden from me.
But the Handle goes on; a Turn or two more will have carried the Moon half round in her Orbit; observe how she moves: 'Tis now 25 Turns, you see, your Patch is come safe about to you; off with it.
No, said she, there it shall stick till we have done, since you won't have it be on my Face any more: I love dearly to see it turn round; and perhaps should I put it on, it may make my Head turn quite round too, as I think it begins to do already without it; but pray turn on your Handle however.
Madam, said I, at the end of 27 Turns and a Quarter, you see I have made the Moon perform her Revolution round the Earth: Mercury is got about a third part of his way; and in 17 Turns more will have finished just half his Revolution. And Venus, you see, will then have advanced a fifth part of her Way, in proportion to the Magnitude of her Orbit: And the Earth also hath traversed in the Ecliptick the Distance of above three Signs.
​And by thus revolving the Earth and Planets round the Sun, you may bring the Instrument to exhibit Mercury, and sometimes Venus, as directly interposed between the Earth and the Sun; and then they will appear as Spots in the Sun's Disk; as I hinted to you before, p. 114. And this Instrument shews also very clearly the Difference between what they call Geocentrick and Heliocentrick Aspects, according as the Eye is placed in the Centre of the Earth or Sun.
Well, said she, I have no Words to express the Pleasure and Satisfaction I receive from this most Curious Engine, nor the Amazement the wonderful Contrivance of it gives me. Were my Fortune but half as great as my Curiosity, I would have one of these Instruments as soon as possibly I could get it, and then without being beholding to any of you He-things, I would turn it about myself, till I made it do all I had a mind to. And I wish now, that I could see the Inside of it; and understand what Numbers of Teeth and Pinions he hath made use of, to produce these various motions.
Well, said she, since I can't have that Satisfaction now, pray proceed to let me know as much of it as you can.
Madam, said I, you will next be pleased to see the Difference between the Moon's Periodick and Synodick Months.Periodick and Synodick Month, and the Reason of it, very plainly here shewn to the Eye: I have now turned the Handle round till I have shewn you just such a Period, as the Time between our first New Moon, when the Earth was in the first Point of Aries, and the present one: and at the Earth's Place in the Ecliptick, where this happens, I will stick this bit of Paper; and turning 271/4 turns of the Handle more, you see, I have brought the Moon again to be exactly interposed between the Earth and ​the Sun; and then you know it will be New Moon to us; but you see the Line of the Syzygy is not right against the bit of Paper, but behind it; and it will require two Days time or two Turns more, before it will get thither.
I think the Reason of that, said she, appears here very plain; because in this 27 Days the Earth advances so far forward in her annual Course, as is the quantity of the Difference in time between the Moon's two Months. But pray, Sir, said she, won't this naturally carry you to shew me how the Eclipses are formed?
Yes, Madam, said I, and that is all which is material, that I have left to shew you.
You know, Madam, the Astronomical Books tell you there can be no Eclipse of either Sun or Moon, but when the Moon is in or near the Nodes: And this will be here very plainly shewn to you by the means of this Thread, of which if you please to take that End, we will extend it so as to represent the Line of the Syzygies: I will turn the Handle about till the next Conjunction of the Moon comes to be in or near the Node, ​or in the Plane of the Ecliptick; and then you shall see there will be an Eclipse of the Sun. You see I have turned the Handle about 27 times; but now the Centres of the Sun, Earth, and Moon are not near in a Right Line, as the Thread shews you; and therefore there will be no Eclipse of the Sun: But you see now at the (a) Full Moon, the Line connecting the three Centres, is very near the Node; therefore there will be an Eclipse of the Moon: (a) After I had turned it round several Times till it hapned so.And (a) now, you see, there is an Eclipse of the Sun; which is Central, when all the three Centres above mentioned come into this Thread thus stretched in the Plane of the Ecliptick; and Total, when the Moon is in her Perigæum, at the greatest Distance from the Sun, and nearest to us.
But in order yet farther to shew the Solar Eclipses, and also the several Seasons of the Year, the Increase and Decrease of Day and Night; and the different Length of each in different Parts of our Earth, Mr. Rowley hath this further elegant Contrivance.
He hath provided this little Lamp to put on upon the Body of the Sun; which casting, you see, by the Means of a Convex Glass, and the Room made a little ​dark, a strong Light upon the Earth; will shew you at once all these things; first how one half of our Globe is always illuminated by the Sun, while the other Hemisphere is in the dark; and consequently, how Day and Night are formed, by the Revolution of the Earth round her Axis; for as she turns from West to East, she makes the Sun appear to move from East to West. And you will please to observe also, Madam, that as I turn the Instrument about in Order to shew you the several Seasons of the Year, and the Length and Decrease of Day and Night, how the Shadow of the Moon's Body will cover some part of the Earth, and thereby shew you, that to the Inhabitants of that part of the Earth there will be a Solar Eclipse.
That is exceeding Plain and Instructive, said the Lady; I have taken Notice of two or three already, as you have whirled the Earth and Moon round the Sun. But pray for what other End do you thus turn it now?
Only to bring it to shew you the Autumnal Equinox, said I, Madam! and then you will plainly see the Reason ​of the Equality of Days and Nights all over the Earth, when she is in that Position.
O! Sir, said she, I thank you; this explains the Figure you drew me before, by which alone I could not get so distinct and so clear an Idea of the Earth's two Motions, as thus shewn me. But now I see, that as the Earth turns round her Axis, just one half of the Equator and all Parallels to it, will be on the Light, and the other half in the Dark; and therefore the Days and Nights must be every where equal: For I see the Horizon of the Earth's Disk now lies parallel to the Plane of the Solstitial Colure.
Excellently well remembred and expressed, said I, Madam. Your Ladyship, I see, hath studied hard since I saw you last in the Country, and we are now sure of you for an Astronomer.
I don't know that, said she, 'tis probable I may never take pains enough to go into the Calculatory Part; but I think every one should be desirous of knowing the Reason of these common things we ​are now upon, and which happen to us every Year. But pray, Sir, go on, and stop when the Earth comes to be in Cancer.
'Tis now got thither, said I, Madam; and you will observe that the Horizon of the Disk, or that Plane which divides the Earth's two Hemispheres, the Enlightened from the Dark one, is now no longer parallel to, but lies at right Angles to the Plane of the great Solstitial Colure: The Earth being now in Cancer, the Sun will appear to be in Capricorn; and consequently it will be our Winter Solstice. And you see plainly, that as I keep turning the Earth round its Axis either way, the entire Northern frigid Zone, or all Parts of the Earth lying with the Artick Circle, are in the Dark Hemisphere; as you see by this little bit of Wafer, which I stick upon the Circumference of that Circle.
Your Ladyship will observe also, that now I remove that bit of Wafer, and place it in the Circumference of that Circle which exhibits the Path of the Vertex of London, how much Longer, in a Diurnal Revolution of the Earth, that will be in the Dark, than in the Light: ​Just such is the disproportion of our Days to our Nights at that time; scarce a third Part.
I see this thing, said she, exceeding plain; and also that the Inhabitants of our North Pole, if any such there are, have not seen the Sun since the 12th of September.
No, nor can't again, said I, Madam, till the Vernal Equinox; for all this six Months they must be condemned to perpetual Darkness. But pray observe, Madam, that as I move the Earth along in its Orbit, 'till it come thither, how the Nights shorten, and the Days lengthen, by Degrees, till they come then to an Equality again on the 10th of March; when our Earth being in the first of Libra, the Sun must appear to be in the first Degree of Aries. And now the Earth's Axis, which you see always keeps parallel to its self, will come again to be in the Plane of the Horizon of the Disk, and consequently the Equator, and all its Parallel Paths will be bissected by that Horizon in every Diurnal Revolution of the Earth; or there will be an universal Equinox all over the Globe.
That it will, Madam, said I; and now you will observe with pleasure, how the Days Encrease, and the Nights Decrease, as the Earth moves on towards Capricorn, where now I will stop it; while you observe that all the Polar Circle is got into the enlightned Hemisphere; as also above two parts in three of the Path of London (h L m f) in Fig. V. and therefore now our Days are at Longest, this is our Summer Solstice, or Midsummer.
Yes, said she, I see it, and understand it perfectly well: But I see withal, that our Days, now at their greatest extent, are going to shorten again, which I will bear as long as I can, that is, till ​you wheel the Earth about again into Aries: But then, if you please, we will leave off, having attended upon the Earth in one entire Revolution round the Sun; and most demonstratively and delightfully seen, how thereby all the Phænomena of the different Seasons of the Year, and the Varieties and Vicissitudes of Night and Day are solved and accounted for.
Pray when you see Mr. Rowley, thank him from me, for this most noble and intellectual Entertainment.
Thus imitated and applied to Mr. Rowley's ORRERY.
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This famous Sphere of Archimedes is mention'd by Cicero and by Ovid: and the former saith, that it shewed the Motion of the Sun, Moon, and Planets. Pliny tells us, that Atlas and Anaximander, both made such a Sphere; as Diogenes Laertius saith Musæus also did. Sextus Empiricus saith it was made of Wood; and Cælus Rhodiginus, that it was of Brass.
FINIS.