Page:Cyclopaedia, Chambers - Volume 1.djvu/425

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Jpollonitts Myndianus was the firft who took Comets for regular Srars ; and ventur'd to foretel, that one day the Pe- r,ods and Laws of their Motion would be difcover'd.

AUronomers, however, are ftili divided on that Head : $c-ivton, Flamjlead, Halley, and the UngUjh Ailronomers, $£c. Teem fatisfy'd of the Return of Comets : CaJJini, and others of the French, think it highly probable $ de la Hire, and others, oppofe it.

Thofe on the affirmative Side, fuppofe the Comets to de- fcribe Circles prodigiouily eccentric, infomuch as we can on- ly fee 'em in a very fmalJ part of their Revolution : Out of this, they are loft in the immenfe Spaces $ hid not only from cur Eyes, bur our Telefcopes. That little part of their Circle next us, M. CaJJini, &c. have found to pafs between the Orbits of Venus and Mars.

For tie Reafons of the Return of Comets, M. CaJJini gives thefe that follow.

r. In conlidering the CouiTe of the Comets, with regard to the Fix'd Stars, they are found to keep a confiderable time in the Arch of a great Circle, /. e. a Circle whofe Plane pal*- 1 fes thro' the Centre of the Earth : Indeed, they deviate a little from it, chiefly towards the end of their Appearance ; but this Deviation is common to them with the Planets.

2. Comets, as well as Planets, appear to move fo much the falter as they are nearer the Earth ; and when they are at equal Diftances from their Perigee, their Velocities are near- ly the fame.

By fubtrafting from their Motion the apparent Inequality of Velocity occaiion'd by their different diitance from the Earth, their equal Motion might be found ; but wc fhould not be certain this Motion were their true one; in regard they might have confiderable Inequalities, not diitinguifhable in that [mall Part of their Orbit viiible to us. 'Tis, indeed, probable, their real Motion, as well as that of the Planets, is unequal in it felf : and hence we have a Reafon why the Obfer*aticns_made during the Appearance of a Comet can- not give the jufl Period of their Revolution. See Period.

3. There are no two different Planets whofe Orbit cuts the Eclyptic in the fame Angle, whofe Nodes are in the fame Points of the Eclyptic, and whofe apparent Velocity in their

} Perigee is the fame : Of confequence, two Comets feen at different Times, yet agreeing in all thofe three Circumltan* ces, can only be one and the fame Comet.

And this were the Comets of 1577 and rtf8o obferv'd to do j and thofe of 1652, and i 8 : Not that this exact agree- ment in thefcCirctun (lances is absolutely neceffary to determine 'em the fame Comet. M. CaJJini finds the Moon her felf irre- gular in 'cm all : Accordingly, he is of Opinion, there are fe- veral which difagrce herein, yet may be accounted the fame.

I'he great Objcttious againjl the Return of Comets, are, the rarity of their Appearance, with regard to the number of Revolutions affign'd to 'em.

In 1702 was a Comet, or rather the Tail of one, feen at Rome, which M. CaJJini takes to be the fame with that ob- ferv'd by Arifiotle, and that fince feen in 166% ; which would imply its Period to be 54 Years. Now, it may feem ft range, that a Star which has fo fhort a Revolution, and of confequence fuch frequent Returns, fhould be fo feldom feen. Again, in April of the fame Year, 1702, a Comet was ob- ferv'd by MofT Siancbini and Maraldi, fuppofed by the latter to be the fame with that of 1664., both by reafon of its Motion, Velocity, and Direction. M. de la Hire took it to have fome relation to another he had obferv'd in 1698 5 which M- CaJJini refers to that of KJ52 : On this Suppofltion its Period appears to be 43 Months, and the Number of Re- volutions between 11S52 and 11598, fourteen : But 'tis hard to fuppofe, that in this Age, wherein the Heavens are fo nar- rowly watch'd, a Star fhould make 14 Revolutions unper- ceiv'd : efpecially fuch a Star as this, which might appear above a Month together ; and of confequence be frequently difengag'd from the Crepufcula.

For rhis, reafon M. CaJJini is very referv'd in maintaining the Hvpothefis of the Return of Comets, and only propofes thofe for Planets, where the Motions are eafy and fimple, and are folv'd without {training, or allowing many Irregu- larities.

M. de la Hire propofes one general Difficulty agaihft the whole Syttcm of the Return of Comets, which would feem to hinder any Comet from being a Planet 5 and 'tis this: that, by the Difpofition neceffariiy given to their Courfes, they fhouid appear as fin all at firft as at laft; and always increafe, till they arrive at their greatett. Proximity to the Earth : or, if they fhould chance not to be obferv'd as foon as they be- come vifible, for want of Attention thereto $ at leaft, 'tis hnpoffible bur they muff; frequently /View themfelves ere they have arriv'd at their full Magnitude and Brightncfs : But he adds, that none were ever yet obferv'd till they had arrived at it.

But the Appearance of a Comet in this prefent Month of OHobcr 1729, while yet at a great diftance, fo as to be too fmall and dim to be view'd without a Telefcope, may ferve

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to remove this Obftacle, and fet the Comets, Ail!, on the iame footing with the Planets.

Sit Ifaac Ne-zvton fopperies, that as thofe Planets which are neareft the Sun, and revolve in the leaft Orbits, are the fmalleft ; fo among the Comets, fuch as in their Perihelion come neareft the Sun, are the fmafeft, and revolve in leffr Orbits.

Dr. Halley has given us a Table of the Agronomical Ele- ments of all the Comets that have been yet obferv'd with due care ; whereby, whenever a new Comet lhali appear, it may be determin'd, by comparing it therewith, whether it be any of thofe which have yet appear'd ; and consequently its Period, and the Axis of its Orbit determin'd, and its Return foretold.

For there are many things in the Comet of 1531, obferv'd by Appian, which intimate its being the fame with that of 1687, obferv'd by Kepler and Longomontanus ; and which Dr. Halley himlelf again obferv'd in i<5Si. All the Ele- ments agree, and there is nothing contradicts the Opinion, but that inequality in the periodic Revolution ; which, how- ever, he thinks, is no more than may be accounted for from phyfical Caufcs : no more, in effect, than is obferv'd in Saturn ; the Motion of which Planet is fo difturb'd by the reft, efpecially Jupiter, that its Period is uncertain for feve- ral Days together : To what Errors then may not a Comet be liable, which rifes almoft four times the height of Saturn; and whofe Velocity, if but a little increas'd, would change its Elliptic Orb into a parabolic one ?

What further confirms the Identity is the Appearance of another Comet in the Summer of 1451?; which, tho obferv'd by none with accuracy, yet by its Period, and the manner of its Tranfit, he concludes it to be the fame : and thence ven- tures to foretel its return in the Sear 1758. See Solar Syf- tem, and the Plate it refers to ; -where the Orbits of the fe- deral Comets are delineated, and their 'Periods, fo many of them as are known, exprefs'd.

To determine the Place and Courfe of a Comet.

Obfcrve the diftance of the Comet from two Fix'd Stars, whofe Longitudes and Latitudes are known : from the Dif- tances thus found, calculate the Place of the Comet by Trigonometry, after the manner deliver'd under Planet.

By repeating the Obfervations and Operations for feveral Days fucceffively, the Courfe of the Comet wiil be had.

To determine the Courfe of a Comet mechanically, and •without any Apparatus of Inflruments. The following in- genious Method, by a Thread, we owe to Longomontanus : Obferve four Stars round the Comet, fuch, as tliat the Comet may be in the Interlcction of the right Lines that join the two oppofite Stars ; which is eafily found, by means of a Thread plac'd before the Eye, and extended over againft the Stars and Comet.

Suppofe, 11. g. the Comet's place in the Heavens A, (Tab. Jftronomy, Fig. 23.) between the four Stars, E, C, D, E; where the Line joining the Stars B and D, paffes 'thro' the Body of the Comet ; and foof the Lines palling thro' C and E.

On a Globe, wherein thefe. four Stars are found, extend a Thread thro' B and D, and another thro' C and E ; the Point of Interferon will give the place of the Comet. This Practice being repeated for feveral Days, the Comets Courfe will be had on the Globe ; which Courfe will be found to be a great Circle : from any two Points whereof, it will be eafy to find its Inclination to the Eclyptic, and the Place of the Nodes ; only by obferving where a Thread ftretch'd thro' the two Points cuts the Eclyptic.

To determine the Parallax of a Comet. See Parallax.

COMITIA, an Affembly of the Roman People, either in the Comitium, or Campus Martins ; for the Election of Ma- gistrates, or for confulting on the important Affairs of the Re- public. See Campus Martins, &c.

There were certain Days fix'd for thefe AlTemblies, call'd Tlies Comitiales ; mark'd with a c in the Calendar of Julius Qefar.

Comitial Affemblies held for the Eleflion of Confuls, were called Confular Comitia : in like manner, the other Comitia took their Name from the Officer to be created ; whether a Tribune, a Pontiff, jEdile, gjc.

There were three Kinds of Comitia, viz. Curiam, Centnri- ata, and fributa ; fo diftinguifh'd, from the manner where- in the People voted, and gave their Suffrages, vie, by Cu- riceor Parifhes, Tribes or Centuries. See Curije, Tribe, iyc.

Authors make the difference between Comitia and Concha to confift in this ; that in the former the -whole People were call'd together, in the latter only a part.

The Comitium, or Place where the Comitia were ordina- rily held, was a large Hall in the Roman Forum : It was a long time open at top ; on which account, the Affemblies were often interrupted by the ill Weather : 'Twas firft co- ver'd over in the time of the fecond Punic War. SeeFoRu.M.

Rofinus obferves, that the Confuls and Tribunes were not created in the Comitium, but the Campus Martins.

The Word comes from the Verb Coeo, or Comeo, to go to- gether.

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