Page:Cyclopaedia, Chambers - Volume 2.djvu/807

 SUN

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SUN, Sol, in Aftronomy, the great Luminary which enlightens the World, and by his Prefence, conftitutes Day. SeeDAY.

The Sun is ufually reckoned among the Number of 'Planets $ but that he ought rather to be numbred among the fix 'd Stars, will be fhewn in its Place. See Star and Planet. .

According to the Copernican Hypofhefis, which is how oenerally received, and which has even Demonstration on its Side, the Sun is the Centre of the planetary and cometary Syftem ; round which all the Planers and Comets, and our Earth amon< the reft, revolve, in different Periods, according to their different Diftanccs from the Sun. See this Motion I/luf rated and ^Dentoiiftratedunder the Article Planet.

But the Sun, though thus eas'd of that prodigious Motion,, whereby the Ancients imagined him to revolve daily round our Earth ; yet is he not a perfectly quiefcent Body.

From the Phenomena of his Macuke or Spots, it evidently appears, that he has a Rotation round his Axis; like that of the Earth whereby the natural Day is meafured ; only flower. Some of thefe Spots havemade their firft Appearance near the Edge or Margin of the Sun, and have been feen fome time after on the oppofite Edge ; whence, after a Stay of about 14 Days, they have re-appeared in their firft Place, and taken the fame Courfe over again 5 finifhing their entire Circuit in 27 Days time ; which is hence deduced to be the Penod of the Sim's Rotation round his Axis. This Motion of the Spots, is from Eafi to Weft, whence we conclude that

of the Sinn to which the other is owing, to be from"?

For the various Appearances of the Solar Spots, their Caufe, ^c. fee Macule and Spots.

For the annual Motion of the Sun round the Earth ; 'Tis eafily fhewn by Ailronomers, That the annual Motion of the Earth will decafion fuch an Appearance, though it be demon- ilratcd that there is no fuch Thing.

A Spectator in the Sun, woulcl fee the Earth move from Weft to Eaft, for the fame Reafon as we fee the Sun move from Eaft to Weft, And all the Phenomena refulting from this annual Motion, in whichsoever of the Bodies it be,, will appear tbe fame from either. Let S, for Inftance (Tab. Aftronomy, Fig. 39.) reprefent the Sun % A-B CD the Earth's Orbit, which it paries through from Weft to Eaft, in the Space of a Tear.

Now, a Spectator in S, viewing the Earth at A, will refer at to the Point of the Sphere of the Stars, T : When arrived in B, the Spectator will fee it, as in rhe Point S ; when in C, as in the Point, §& till after its whole Circuit, it will he again feen in T- Thus will the Earth appear to defcribc the whole Ecliptic, and to pafs, fucceftively, from Sign to Sign.

"Stippofe, now, the Spectator removed from the Sun to the Earth, which imagine in C ; the Distance of the hVd Stars, we have Oiewn^ is fo vaft, that that ot the Sim is but a Point to it. The Spectator, therefore, now fituate on the Earth, will fee the fame Face of the Heavens, the fame Stars, f£c. as before; the only Difference will be, that as before he imagined the Earth in the Heavens, and the Sun in the Centre ; he will now fuppofe the Sun in the Heavens, and the Earth in the Centre'.

The Earth, therefore, being in C, the Spectator will fee the Sun in T ; and the Spectator being carried along with the Earth, and partaking of his annual Motion, will not perceive either his own Motion, or that of the Earth ; but obferving the Sun, when the Earth is arrived at D, the Sun will be feen at S. Again, while the Earth proceeds to A, the Sim will feem to have moved through the Signs S, SI, and itf! : And while the Earth defcribes the Semi-circle ABC, the Sum will appear to have moved in the concave Surface of the Heavens, through the Six Signs, ^, Tit, t, ^, ™, ^. So that an Inhabitant of the Earth will fee the Sun go through the fame Orbit or Circle in the Heavens, and in the fame Space of Time, as a Spectator in the Sun would fee the Earth defcribe the fame.

Hence arifes that apparent Motion. of the Sun, whereby he is feen to advance, infeniibly, toward the eaftern Stars : In- fbmuch, that if any Star near the Ecliptic, rife any Time with the Sun 5 after a few Days, the Sun will be got more to the Baft of the Star, and the Star will rife and fet before him.

For the feveral Phenomena resulting from the Sun'5 appa- rent Motion, or the Earth's real Motkn, as tbe "Diverfay of "Day and Night, of Seafons, &c. ; See Earth.

Nature, Properties, Figure, &c, of the Sutf;

i° As the Solar Spots are fometimes found to ftay Three Days longer behind the Sun, than they fpend in pafnng over the Hemifphere vifible to us ; _ we eafily deduce, that they don't adhere to the Surface of the Sun, but are at fome Diftance therefrom.

2 As the Spots frequently rife and vanifh, even in the midft of the Sun's Diik 5 and undergo feveral Changes, both

with regard to Bulk, and Figure, and Denfity • it follow?, that they frequently rife, de novo, about the Sw, and are again diffipated.

3^ Hence it fhould follow, that they are form'd out of the Exhalations of the Sun ; and are no other than Solar Clouds. See Vapour, Cloud, &c.

4 Since, then, Exhalations proceeding from the Sm t rife above him, and Hop at a certain Altitude ; 'tis. evident there is fome Fluid incompaffing the Sun, to urge the Exhalations to rife; and this Fluid mult be denfer at bottom, and rarer a top^ like our Atmofphere. See Atmosphere.

5" Since the Spots frequently diffolve and difappear in the middle of the Sun's Diffc ; the Matter of the Spots, that is, the Solar Exhalations, fail back again to the Sun : Whence it follows, that there muilarife various Alterations in the Sun's Atmofphere, and the Sun himfelf. SeeR.AiN, Hail, Me- teor, i$c.

6" Since the Revolution of the Spots round the Sun is found very regular, and the Spots very near the San; it follows, That the Spots do not revolve round the Sun ; but that the Sun, together with his Atmofphere, wherein the Macula; are, move round their common Axis, in an Interval. of about 27 Days; and hence it is, that the Spots near the Limb, being viewed obliquely, appear narrow and oblong.

1° Since the Sun, in every Simarion, appears like a cir- cular Diik; its Figure, as to Senie, mutt be Spherical 5 though we fhall hereafter mew, That it isreally Spheroidical,, Befides the Macula; or dark Spots, feveral Authors make mention of Facula; or Spots brighter than the reft of the <SWs Disk ; and rhofe generally' larger, and very different from the Macula:, both in Figure^ Duration, &c.

Thefe Facu:a;, Kirchcr, Scheiner- $c. take to be Erup- tions of Flames; and hence take Occafion to reprefent the Face of the Sun as full of Volcano's, &c. But Huygens, ufing the bell Telefcopes, could never find any liich Things, though he has fometimes fpied certain Places in the Macula; themielves, more lucid than the reft. Biit thefe do not feem owing to any kindled Matter, which were fcarce con- fident with their Duration, and their frequent Change into Macula; ; but to the Refraction of the Suns Rays througU the thinner Exhalations, when as the Groffer, in their Neighbourhood, intercept the fame. SeeFACuLiE.

8° That the Subftance of the Sun, is Fire, we thusprove; The Sun hhines, and his Rays, collected by concave Mirror^ or convex Lens's, burn, confume, and melt the rnoft folid Bodies, or el(e convert them into Afh.es, or Glafs. Where- fore, as the Force of the folar Rays is diminifb.ed by their Divergency; in a duplicate Ratio of the Diftances recipro- cally taken ; 'tis evident, their Force and Effect is the fame, when collected by a burning Lens or Mirror, as if we were at fuch Diftance from the Sun, where they were equally denfe. The Sun's Rays, therefore, in the Neighbourhood of the Sun, produce the fame Effects, as might be expected from the molt vehement Fire ; confequently, the Sim is of a fiery Subftance. See Fire.

Hence it follows, That its Surface is every where Fluid ; that being the Condition of Flame. See Flame.

Indeed, whether the whole Body of the Sun be Fluid, as fome think, orSulid, as others ; we don't determine: But as there are no other Marks, whereby to diftinguifh Eire from other Bodies, but Light, Hear, a Power of Burning* Confumingj Melting, Calcining and Vitrifying; we don't fee what fhould hinder, but that the Sun may be a Globe of Fire like ours, inverted with Flame.

9 Since the Macula: are form'd out of the folar Exhala- tions ; it appears, that the Sun is not pure Fire, but that there are Heterogeneous Particles mix'd along with it.

io° The Figure of the Sim is a Spheroid, higher under the Equator than about the Poles, This we prove thus ; The Sun ha.s a Motion about his own Axis, and therefore the folar Matter will have an Endeavour to recede from the Centres of the Circles wherein it moves; and that, with the greater Force, as the Peripheries of the Circles are greater, But the Equator is the greateft Circle, and the reft, towards the Poles, continually decreafe ; therefore the Iblar Matter, though at firft in a ipherical Form, will endeavour to recede from the Centre of the Equator, further than from the Centres of rhe Parallels.

Confequently, fince the Gravity whereby it is retain'd in its Space, is fuppofed to be uniform throughout the whole Sun ; it will really recede from the Centre, more under the Equator than under any of the Parallels. And hence the Sun's Diameter, drawn through the Equator, will be greater than that patting through the' Pole, i, e. the Sun's Figure is hot perfectly Spherical, but Spheroidical. See Spheroid* For the Paradlax of the Sun ; fee Parallax.^ For the Suns Diftance : As the Determination thereof depends on that of 'the Parallax ; and as the Sun's Parallax is not found without a long, operofe Calculus ; So, Aftrono- nomers don't agree much about either of them.

The mean Diftance of the Sun from the Earth, fome make 7490 Diameters of the Earth, others iocoo, others 12000 9

others