Of Comets

Besides the Planets, which all move round the Sun from West to East near the Plain of the Ecliptic; there are other surprising Bodies in the System call’d Comets, whose Motions are perform’d in every different Plains, and in all manner of Directions, both direct and retrograde.

These Bodies have appeared in all Ages, but being much more swift in their apparent Motions than the Planets, generally remain visible but a very short Space of Time.

The ancient Philosophers took them for Meteors,¹ but the Moderns finding them to have have no diurnal Parallax, have concluded they must be far above the Moon; and now by many Observations compared together, and Returns of some, they nowo by many Observations compared together, and Returns of some, they are at last found to be of the same Nature with the Planets, and subject to the same Laws of Motion, but in vast excentric Orbits, yet in every Part describing Areas proportional to the Times.

Sir Isaac Newton has demonstrated,
That the periodic Time of any Comet,
Is to the periodic Time of any Planet,
In a sesquiplicate Proportion of their principal Axis.

That is, if the Axis of a Comet’s Orbit, were to the Axis of a Planet’s Orbit, as 9 to 1, the periodic Time of the Comet would be to the periodic Time of the Planet, as \(\sqrt[9]{9}\) to 1, or as 29 to 1. Hence a Comet moving in Orbit 9 times the Axis of Jupiter, will be 324 Years in performing one Revolution round its Orbit.

And the Velocity of a Comet thus moving in an Ellipse, is to the Velocity of a Planet moving in a Circle at the same mean Distance from the Sun, nearly in a subduplicate Proportion of double the Distance of teh Planet, to the Distance of the Comets, or as \(\sqrt{2}\) to 1.

The Orbs and Bodies of the Comets appear thro’ Telescopes, to be surrounded with very dense Atmospheres, especially in those returning from the Neighbourhood of the Sun; at which Time they generally emit vast Tails, sometimes like fiery Beams, and frequently much brighter than the Milky Way. These Tails are always turn’d from the Sun, an doften extend to 30, 60 and 90 Degrees in Length.

There are many Opinions concerning the Substance and Matter of these Tails; but the Generality of Astronomers in the present Age agree, that they are produced and form’d of a thin Vapour arising from the Head or Nucleus, and reflecting the Sun’s Light.

The Heads of all the late Comets, since the Year 1500, have ever exceeded 8^min. or 12^min. and the Nucleus, or Star, is generally \(\frac{1}{10}\) or \(\frac{1}{15}\) of the whole Diameter; near the Sun the Nucleus are the Least, being there the most obscur’d the Smoak, but the Light of the Head generally increases from the Earth towards the Sun, and decreases from the Sun to the Earth.

Flamstead in the year 1682, observed the Comet’s Head, to be 2^min. in Diameter, but the Nucleus found to be no more than 12^2ds. and the Head of the Comet in the Year 1665 was above 6^min. and the Nucleus less than Jupiter.

The apparent Motions of Comets in general are very unequal, insomuch, that Heveleus in July, 1683, observed a Comet to move 40^min. and 45^min. per Day, which in September following, he found moved 5^deg. per Day.

Sir Isaac Newton having demonstrated, that Comets might move in such vast eccentric Ellipses, that near the Sun; Parabolas (of a more easy Solution) might be Use of instead of them: The late Dr Edward Halley, from a Collection of such Observations as could be depended upon, fram’d the following Table, being the Result of a prodigious deal of Calculations.

Dr Halley was of Opinion, that the Comet of 1682, was the same appeared in the Year 1607, and before in the Year 1531, concluding its Period to be 75½ Years. That the Commet of 1661, might be the same that appear’d 1532, and its Period 129 Years; and that the great Comet of 1680 must have been the same with that of 1106, which likewise appear’d at Julius Cæsar’s Death, and also in the Consulate of Lampadius and Orestes, consequently its Period will be about 575 Years; and hence the transverse Diameters of these three Comet’s Orbits, will be found respectively, ∛75×75, ∛129×129 and ∛575×575 times greater than that of the Orbis Magnus.

Hence the greatest Distance from the Sun.

Their mean Distances.

And their mean Velocities per Hour.

The Stars are looked upon by all modern Astronomers, to be but no other than the great Globes of Fire like the Sun, promiscuously distributed through the Mundane Space, and may very possibly be the Centers of other Systems of Planets like ours, since we have no Reason that can contradict it, and many that may induce us to believe it.

Their various apparent Magnitudes are intirely the Effect of an unequal Distance, as will manifestly appear from the Scheme of their Disposition, where the same Stars DEFGH, altho’ their Globes be represented equal, appear respectively of different Magnitudes, as is evident from the different Points of View AB and C in equal Arches of Vision RS, TV and OQ. EFGH, as seen from D, appear like Part of the Pleiades, a bright Knot of Stars and DK from C, as the double Star of Castor, &c.

The ingenious Mr Huygens, from the Analogy of apparent Light, having contracted the Sun’s Image, by Means of a small Hole at the End of his Telescope, to the Appearance of Syrius, = 4^3ds. found, that the Sun’s Distance to that of Syrius was nearly, as 1 to 27664; hence the Distance of Syrius in round Numbers from us, will be at least 2,213,120,000,000 Miles, and consequently, according to the Velocity of Light, if Syrius was to be totally extinct before the last Rays could reach the Earth, the Star itself would have been put out 142 Days.

Its hardly possible for a thinking being to view this vast Creation of glorious Bodies, whose astonishing Extent no human Eye can ever discover, or the Mind comprehend, without being struck with an awful Dread and Admiration of the great Creator’s Wisdom and amazing Power; and altho’ the Existence of himself, and manner of acting from his own divide Nature can never come under our Observation, or into the finite Sphere of our weak Cognoscence; yet his Ubiquity and Omniscience, have ever been acknowledged to be the highest Degree of Adoration, and his State always allowed to be perpetually the same; the only one perimitive and independent, the sole supream God, original Maker and Disposer of all created Things; of infinite and eternal Majesty, and of universal Dominion. In the old Heathen Worship, he is called Jupiter or Jove, and Virgil finely expresses their Notions of him as in the following Lines.

—————The Heav’n and Earth’s compacted Frame,
The flowing Waters and the starry Flame;
And both the radiant Lights, one common Soul
Inspires and seeds and animates the Whole:
This active Mind infus’d thro’ all the Space,
Unites and mingles with the mighty Mass.
Hence Birds of Air and Monsters of the Main,
And Men and Beasts the Breath of Life obtain.

Dryden’s Virgil.

The Julian Account of Time, commonly call’d the old Stile, takes its Name from Julius Cæsar, and was first settled² 45 Years before Christ.

In this Account of Time, the Year is supposed to consiste of 365^days. and 6^hours. but the odd Hours not being taken Notice of annually, every four Years amount to a Day; and hence every fourth Year by the Addition of this one Day more than in the common Year of 365, becomes a Bissextile or Leap Year of 366^days. Now the true solar Year successfively consisting of only 365^d. 5^h. 49^min. 16^2ds. here is an over-reckoning of 10^min. 44^2ds. yearly, and this annual Variation in every 134 Years, producing one Day to be rejected in the Reason why the vernal Equinox, or the Sun’s Ingress to Aries is now on the Tenth of March, which in Julius Cæsar’s Time was on the 24th, having changed as follows,

Sun in Aries

The new Stile or Gregorian Account of Time, takes its Name from the Pope Gregory the XIIIth, who finding the Julian Account to be erroneous, resolved upon a Reformation of it, and begun it in the Year 1582.

The Pope finding the vernal Equinox at the Time of the Council of Nice, to have been on the 21st of March look’d no farther back into past Time, and resolved upon fixing it there for the future. To which End he order’d 10 Days of the present Year 1582 to be omitted, which was accordingly done, by calling the 5th of October of the 15th, by this Means the Sun’s following Ingress into Aries, fell on the 21st of March, which must have otherwise been on the 11th.

And to prevent such like Errors for the future, he ordered 3 Days in every 400 Years to be reducted thus,

The 29th of February, (being the Day added in the Julian Account, to make Leap Year) at the End of the three Centuries successfively was to omitted, at the End of the fourth retained.

Thus before the 29th of February 1700, the Difference betwixed the two Stiles was but 10 Days, and since, now is 11; and in the 18 Century will 12, &c.

Thus far I have thought proper to acquaint the Reader with such Particulars in Astronomy, are most generally inquir’d after by those who are not profess’d Mathematicians; and these I take to be the most useful and entertaining; but those who are desirous of perusing this Science farther, may find all the Improvements that have been made in this Study for many Ages, ingeniously sum’d up, explain’d and demonstrated in the learned Works of our indefatigable Countrymen Flamstead, Newton, Gregory and Halley, in which they may compleat their Knowledge of all that has, or possibly may be done this Way.

Finis.

1. Aristotle was of Opinion that the Comets were nothing but sublunary Vapours, or airy Meteors.
2. Britain being then a Roman Province received this Account, and has kept it ever since.