# Astronomical Dialogues Between a Gentleman and a Lady/Dialogue 5

​The Lady plied her Telescopes, and pursued her Astronomical Studies with great Application and Success; and after some time, when I had the Honour to wait upon her again, she took me out into the Summer-house in the Garden, and then began thus with me.
Sir, said she, you have already taken a great deal of Pains to gratify a Woman's Curiosity; but I must beg you to indulge me yet a little farther, and to afford me a Lecture upon another Point; about which, as I am ashamed to trouble ​you, so I should be afraid to ask you, but that you have been so kind already, as to help me to get rid of many Fears and Terrors, too incident to our Sex: And if you can ease my Mind of this remaining Dread, I shall think you can do me a signal piece of Service.
You must know I have been tumbling over those Books of Astronomy, which you have bid me read; and tho' there be very many Things that I don't understand fully at present, yet there are some also that I know enough of, to be put into the Vapours by them.
The Affair of Comets, Sir, with their grisly Beards and horrid Tails, fright me almost out of my Wits: For god-sake therefore, tell me, as plainly as you can, whether my Dread is well grounded; Do they really forbode all manner of Mischief to Mankind, as well as do a great deal, when they come among us? What are they? Are their Motions natural, and accountable by Mathematical Calculation, as those of the Planets? Or are they miraculously sent hither as the Messengers of God's Wrath, and as the Executioners of his Judgements upon sinful Mankind?
I'm glad to hear you say so, said she, and I begin a little to be comforted: But pray go on, and compleat my Cure; for I don't care to be drown'd or burnt up by one of these extravagant Ramblers a Comet, before I am aware.
O Madam, said I, I perceive where you have been dipping; I will therefore give you the most satisfactory Account I can.
'Tis very true, Madam, said I; for that great Comet which appeared here in the Year 1680, (and which I saw, and very well remember, tho' then but a Boy) went so near to the Sun, as to acquire a Degree of Heat above 2000 times as great as that of red-hot Iron: And if its Body was about the Size of our Earth, as it was judged to be, it won't be cool again this Million of Years: And yet it pleased God, that that Comet went away from us, without doing us any sensible Harm, that I know of; and so little do I fear being hurt by any of them, that I could almost wish another would appear, to help us to compleat the Theory of their Motions.
Nay, said she, if you that know so much of them are not afraid of them, I'm sure I won't be so for the future: Pray therefore, Sir, proceed and tell me what you can of the Number, Motions and Appearances of these Comets, how ​their Beards and Tails are formed; and how you account for the most eminent of their Appearances.
Madam, said I, there have within this last 400 Years appeared to this part of the World but 24 Comets, (how much greater a Number there may be God knows, and perhaps subsequent Observations may discover more.) And of these according to the Observations of Dr. Halley and the other Astronomers, three of them have had their Orbits, and Appearances so very like, and the Times of their appearing so very equal, that they have judged it very probable that those 3 Comets which successively appeared as three were in reality but one or the same Comet appearing at three several Times.
And the like they are inclined to judge of two others; that they also are but one, appearing at two different Times.
That great Comet that appeared here in the Years 1680, and 1681, was seen before in our Hemisphere, A.D. 1106; once before, about the Year 532; and also 44 Years before our Saviour's Birth: and therefore they conclude the Time of its Periodick Revolution round the Sun to 575 Years.
​The Orbits of these three are described in Fig. IV.The Time of the Revolution of another Comet, which they judge will appear again A.D. 1758, is 75 Years: Another, which probably may be seen here again, A.D. 1789, makes its Ellipsis round the Sun in 129 Years.
What Bigness do you take these Comets to have been of Sir, said the Lady.
Madam, said I, they are generally of the size of the rest of the Planets, and have Atmospheres about them like our Earth: But then as all our Planets move pretty nearly in the Plane of the Earth's Ecliptick, these Comets are tied to no such Rules; for the Planes of their Orbits have very different, nay, almost all manner of Directions and Positions, and their Motions are all manner of Ways; some from East to West, others from West to East, some from South to North, and others a quite contrary way, &c. And yet their Motion is equable enough, and shews us this great Point; that as there can be no such solid Orbs as was imagined in the Ptolemaick System; so there can be neither any such thing as a Plenum, and no such subtile Matter as the Cartesians have invented to solve their Hypotheses: ​But we may fairly conclude, that all the vast Spaces both between and beyond the Planetary System, are filled with no Matter capable of making any considerable Resistance to their Motions, but rather are an immense Void, or Vacuity.
I think that is a very probable Conclusion, said she; for if there were any quantity of resisting Matter, it must always obstruct a little, and by degrees must make very sensible Alterations in the Planets Motions; which I don't find to have been in Fact discovered; but sure, Sir, these Comets must go off to vast Distances from the Sun?
Yes, Madam, said I, and therefore they are still more unfit than any of the other Planets, to be inhabited by such kind of Beings, as those of human Race; for the middle Distance of the Great Comet that appeared in 1680, was more than 5000 millions of Miles from the Sun; as its greatest Distance was above twice as much; and yet its least Distance was not above a 20,000th part of its greatest: so that in its whole Revolution, it would be subject to such Extremities, as that its greatest Degree of Light and Heat to its least, were above 400 millions ​to one. And yet notwithstanding this immense Extension of its Ecliptick Orbit, the Great and Allwise Architect of the Universe hath probably so adjusted the Centrifugal and the Centripetal Forces, that it doth not quite leave the Sun, tho' it go so far from him, but returns again towards him, and revolves round him in a determinate Period of Years. None of the Orbits of any of these Comets yet known, are in or near the Plane of the Earth's Ecliptick; and therefore in their Ascent from the Sun, tho' heated never so much by him, yet they won't come near enough to our Earth to burn us, or affect us with any sensible Heat; and therefore, Madam, your Fears of being burnt in your Bed by a Comet, I hope will vanish for this time.
Well, said she, and so they will; but I love to know the Reasons of things as well as any a Man of you all. But pray, Sir, what are the Heads, Beards, and Tails of these Comets?
Madam, said I, the Bodies of Comets are probably in Substance like our Earth; fixt, solid, and compact: Their Tails are probably long and very thin trains of Smoak and Vapours, emited ​from the heated or enkindled Body, Head, or Nucleus, as some call it, after their Perihelion, or after their having been at their nearest Distance to the Sun; for then it hath been observed, that the Tails of all Comets have appeared largest and longest. In the Lexicon Technicum, under the word Comet, you will find a great deal said about the Phænomena of Comets, their Beards, Tails, &c. from Sir Isaac Newton, and other Authors; and there you will likewise find Conjectures about their Use in the Planetary System.
Sir, said she, I shall have recourse to those Books with a great deal of Pleasure, and will trouble you no farther now with my Enquiries: I see Company appearing, let us forget our Astronomy a while, and trifle with them as agreeably as we can.