ASTRONOMY. 
will (hew us its pofition, and through what (tars it will 
pafs, which will therefore direCt us very accurately where 
to look for it. Hence it will be mod vifible, oxteris pa¬ 
ribus, when the ecliptic makes the great eft angle with the 
horizon. On October 6, 1684, M. Facio perceived the 
point A distinctly terminated, the angle of which was 
a6i°. M. Eimmart obferved the lame on January 13, 
1694. In 1683, when M. Caflini firft obferved it, S A was 
fifty or lixty degrees, and IO about eight or nine degrees. 
In 1686, and 1087, S A extended from ninety to 103 de¬ 
grees, and IO was about twenty degrees. On January 6, 
1688, S A did not appear to be above forty-five degrees, 
but the horizon was then filled with fogs, and Venus ihone 
very bright. The appearance therefore depends upon the 
ftate of the atmofphere, and fituation of the planets, which 
may produce light enough partly to obfeure it. IO has 
fometimes been extended to thirty degrees. M. Pingre, 
in the torrid zone, has obferved S A to be 120 degrees. 
The thicknefs I O ought to appear different at ditferent 
times of the year, becaufe the Earth will be in a different 
fituation in refpeCt to its edge. 
M. Facio conjectured, that this appearance arifes from 
a collection of corpufcles encompalling the Sun in the form 
of a lens, reflecting the light of the Sun. M. Caffini fup- 
pofed that is might arife from an infinite number of pla¬ 
nets revolving about the Sun ; fo that this light might owe 
its exiftence to thefe bodies, as the milky way does to an 
innumerable number of fixed ftars. It is now, however, 
generally fuppofed, that it is matter detached from the Sun 
by its rotation about its axis. The velocity of the equa¬ 
torial parts of the Sun being the greateft, would throw 
the matter to the greateft distance, and, on account of the 
diminution of velocity towards its poles, the height to 
which the matter would there rife would be diminifhed ; 
and, as it would probably fpreada little fideways, it would 
form an atmofphere about the Sun fomething in the form 
of a lens, whofe feCtion perpendicular to its axis would 
coincide with the Sun’s equator. And this agrees very 
well with obfervation. There is however a difficulty in 
thus accounting for this phenomenon. It is very well 
known, that the centrifugal force of a paint of the Sun’s 
equator is a great many times iefs than its gravity. It does 
not appear, therefore, how the Sun, from its rotation, can 
detach any of its grols particles. If they be particles de¬ 
tached from the Sun, they muff be lent off by fome other 
unknown force ; and in that cafe they might be fent off 
equally in all directions, which would not agree with the 
obferved figure. The caufe is probably owing to the Sun’s 
rotation, although not immediately to the centrifugal force 
arising therefrom. 
Of the FIXED STARS. 
No part of the univerfe affords fuch exalted ideas of the 
ftmehire and magnificence of the heavens, as the confide- 
ration of the number, magnitude, motion, nature, and 
diftance, of the-fixed ftars. We admire indeed, with pro¬ 
priety, the vaft bulk of ourown globe ; but, w'hen we con- 
fider how much it is furpaffed by moft of the heavenly bo¬ 
dies, what a point it degenerates into, and how little more 
even the vaft orbit in which it revolves would appear, 
when feen from fome of the fixed ftars, we begin to con¬ 
ceive more juft ideas of the extent of the univerfe, and of 
tiie infinity of creation. The unbounded (pace appears 
filled, at diff erent diftances, with thefe Harr.; each of which 
is probably a Sun, with attendant planets rolling round it. 
In this view, what, and how amazing, is the ftruCiure of 
the univerfe ! 
With refpeCl to the dijiances of the fixed liars, they are fo 
extremely remote, that we have no distances in the plane¬ 
tary fyffem to compare to them. Their immenfe diftance 
appears from hence, that they have no fenfible parallax ; 
that is, that the diaineterof the Earth’s annual orbit, which 
is nearly 190 millions of miles, bears no fenfible propor¬ 
tion to their diftance. 
M. Huygens (Cofmotheor. lib. iv.) attempts to deter.- 
407 
mine the diftance of the ftars, by making the aperture of 
a telefcope fo fmall, as that tiie Sun through it appears no 
larger than Sirftis; which lie found to be only as 1 to 
27,664 of his diameter when feen with the naked eye ; fo 
that, were tiie -Sun’s diftance 27,664 times as much as it is, 
it would then be feen of the fame diameter with Sirius, 
And hence, fuppofing Sirius to be a fun of the fame mag¬ 
nitude with our Sun, the diftance of Sirius w 1 be found 
to be 27,664 times the diftance of the Sun, or 345 million 
times the Earth’s diameter. Dr. David Gregory invefti- 
gated the diftance of Sirius, by fuppofing it of the fame 
magnitude with the Sun, and of the fame appar.nt dia¬ 
meter with Jupiter in oppofition : as may be feen at large 
in his Aftronomy, lib.iii. prop. 47. Cailiai (Mem. Acad. 
1717), by comparing Jupiter and Sirius, Mien viewed 
through the fame telefcope, inferred, that the diameter of 
that planet was ten times as great as that of the (lar ; and, 
the diameter of Jupiter being 50", he concluded that the 
diameter of Sirius was about 5" ; fuppofing then, that the 
real magnitude of Sirius is equal to that of the Sun, and 
the diftance of the Sun from us 12,000 diameters of the 
Earjh, and the apparent diameter of Sirius being to that 
of the Sun a.s 1 to 384, the diftance of Sirius becomes equal 
to 4,608,000 diameters of the Earth. Thefe methods of 
Huygens, Gregory, and Caflini, are however conjectural 
and precarious; both becaufe the Sun and Sirius are fup¬ 
pofed of equal magnitude, and alfo becaufe it is fuppofed 
the diameter of Sirius is not determined with iuflicient ex¬ 
amine fs. 9 
Mr. Michell has propofed an enquiry into the probable 
parallax and magnitude of the fixed ftars, from the quan¬ 
tity of light which they afford us, and the peculiar cir- 
cumftances of their fituation. With this view lie fuppo- 
fes, that they are, on a medium, equal in magnitude and 
natural brightnefs to the Sun; and then proceeds to in¬ 
quire, what would be the parallax of the Sun, if he were 
to be removed fo far from us, as to make the quantity of 
light, which we fhould then receive from him, no more 
than equal to that of the fixed ftars. Accordingly, he af- 
fumes Saturn in oppofition, as equal, or nearly equal, in 
light to the brighteft fixed ftar. As the mean diftance of 
Saturn from the Sun is equal to about 2082 of the Sun’s 
femidiameters, the denfity of the Sun’s light at Saturn 
will coniequently be lefs than at his own Surface, in the 
ratio of the fquare of 2082 or 4,334,724 to 1. If Saturn 
therefore reflected all the light that falls upon him, he 
would be lefs luminous in that fame proportion. And, 
befides, his apparent diameter, in the opposition, being 
but about the 103th part of that of the Sun, the quantity 
of light which we receive from him muft be again dimi¬ 
nished in the ratio of the fquare of 105 or 11,025 to 1. 
Confequently, by multiplying thefe two numbers together, 
we (ball have the whole of,the light of the Sun to that of 
Saturn, as the fquare nearly of 220,000 or 48,400,000,000 
to 1. Hence, removing the Sun to 229,000 times his pre- 
fent diftance, he would ftill appear at lead as bright as Sa¬ 
turn, and his whole parallax upon the diameter of the 
Earth’s orbit would be lefs than 2": and this muft be af¬ 
firmed for the parallax of the brighteft of the fixed ftars, 
upon the fuppofttion that their light does not exceed that 
of Saturn. 
By a like computation it may be found, that tire diftance, 
at which the Sun would afford us as much light as we re¬ 
ceive from Jupiter, is not lefs than 46,000 times Iris pre- 
fent diftance; and his whole parallax in that cafe, upon the 
diameter of the Earth’s orbit, would not be more than 
nine feconds; tiie light ot Jupiter and Saturn, as feen 
from the Earth, being in the ratio of about 22 to 1, when 
they are both in opposition, and fuppofing them to reflect 
equally in proportion to the whole of the' light that falls 
upon them. But, if Jupiter and Saturn, inftead of re¬ 
flecting the whole of tire light that falls upon them, fhould 
really refleCt only a part of it, as a fourth, or a Sixth, which 
may be the cafe, the above diftances nruft be increafed in 
the ratio of 2 or 2% to i, to make the Sun’s light no more 
•3 than 
