ASTRONO M Y. 
?Jty, by which it is retained in its place, is fuppofed to be 
uniform throughout the whole Sun, it will really recede 
from the centre more at the equator than at any ot the 
parallels; and hence the Sun’s diameter will be greater 
through the equator than through the poles; that is, the 
Sun’s figure is not perfectly fpherical, but fpheroidical. 
As to the apparent motion ot the Sun, as teen from the 
Earth, the fir'll.and mod obvious phenomenon is, his daily 
riling in the eaft, and his letting in the weft; after which 
the Moon and (tars appear, (till keeping the fame wefterly 
courfe, till we lofe tight of them altogether. Thefe ap¬ 
pearances give rife to what is called the apparent diurnal 
motion of the heavens. This cannot be long obferved, 
before we mult alfo perceive, that the Sun does not always 
rife exactly at the fame point of the heavens, his motions 
deviating conliderably at particular feafons from thofe they 
perform at other times. Sometimes we perceive him very 
high in the heavens, as if he would come diredlly over our 
heads; at other times he is almolt funk in the fouthern part 
of the heavens. If we commence our obfervations of the 
Sun, for inftance, in the beginning of March, we flrall find 
him appear to rife more to the northward every day, to 
continue longer above the horizon, to be more vertical, or 
higher, at mtd-day ; this continues till towards the end of 
June, when he moves backward in the fame manner, and 
continues this retrograde motion till near the end of De¬ 
cember, when he begins again to move forwards. It is 
this change in the Sun’s place, that occalions him to rife 
and fet in different parts of the horizon at different times 
of the year. It is from hence that his height is fo much 
greater in fummer than in winter. In a word, the change 
of the Sun’s place, as it regards the Earth, is the caufe of 
the different length in the days and nights, and the vicifil- 
tudes of the feafons. Hence a knowledge of the Sun’s 
apparent motion is abfolutely necelfary, in order to form a 
true idea of the phenomena of the heavens. If on an even¬ 
ing w'e take notice of fome fixed liar near the place where 
the Sun fets, and oblerve it for feveral fucceflive evenings, 
we (hall find that it approaches the Sun from day to day, 
till at laft it will difappear, being effaced by his light, 
though but a few days before it was at a fufficient dillance 
from him. That it is apparently the Sun which approaches 
the liars, and not the liars the Sun, is plain, from this 
j-eafon; the liars always rife and fet every day at the 
fame points of the horizon, oppofite to the fame terreftrial 
objects, and are always at the fame dillance from each 
other; whereas the Sun is continually changing both the 
place of its riling and fetting, and its dillance from the 
liars. The Sun advances nearly one degree every day, 
moving from welt to eaft ; fo that in 365 days we lee the 
fame liar near the fetting Sun, as was obferved to be near 
him on the fame day in the preceding year. In other 
words, the Sun has returned to the place from whence he 
fet out, or made what we call his annual revolution. 
Thofe who are not accullomed to aftronomical calcula¬ 
tion, will be furprized at the real magnitude of this lumi¬ 
nary ; which, on account of its dillance from us, appears 
to the eye not much larger than the Moon, which is only 
an attendant on our Earth. When looking at the Sun, we 
are viewing a globe, whofe diameter is 890,000 Englilh 
miles; whereas the Earth is not more in diameter than 
7,970 miles: fo that the Sun is about 1,392,500 times 
bigger than the Earth. Thus as it is the fountain of light 
and heat to all the planets, fo it alfo far furpaffes them in 
its bulk. If the Sun were every where equally bright, 
his rotation on his axis would not be perceptible ; but by 
means of the fpots, which are vifible on his furface, w e 
are enabled to difcover this motion. And by this motion 
we likewife difcover not only the time the Sun employs in 
turning round his axis, but alfo the inclination of his axis 
to the plane of the ecliptic. 
By the Sun’s atrnofphere, is occafioned that appearance 
which is termed the zodiacal light. This light is feen at 
fome feafons of the year, either a little after fun-fet, or 
a little before fun-rife. It is faintly bright, and of a whi- 
tilli colour, refembling the milky-way. In the morning 
it becomes brighter and larger, as it rifes above the ho¬ 
rizon, till the approach of day, which diminimes its fplen- 
dour, and renders it at lad invifible. Its figure is that of 
a fiat or lenticular fpherofd, feen in profile. The direc¬ 
tion ol its longer axis coincides with the plane of the Sun’s 
equator; but its length is fubjeft to great variation, fo 
that the dillance of its fumniit from the Sun varies from 
forty-five to 120 degrees. It is feen to the befi advantage 
about the iolftices. It was firit defcribed and named by 
Caflini, in 1683 ; but it had been noticed by Childrey, 
about the year 1650. 
Several particulars of the Sun, related by Newton, in- 
his Principia, are as follow : j. That the denlity of the 
Sun’s heat, which is proportional to his light, is feveii 
times as great at Mercury as with us ; and therefore our 
water there would be all carried off' and boil away: for 
he found by experiments of the thermometer, that a heat 
but feven times greater than that of the fun-beams in 
fummer, will ferve to make water boil. 
2. That the quantity of matter in the Sun is to that 
in Jupiter, nearly as 1100 to 1 ; and that the diftance of 
that planet from the Sun, is in the fame ratio to the Sun’s 
femi-diameter. 
3. That the matter in the Sun is to that in Saturn, as 
2360 to 1 ; and the diftance of Saturn from the Sun is in a 
ratio but little lefs than that of the Sun’s femi-diameter. 
And hence, that live common centre of gravity of the Sun 
and Jupiter is nearly in the fuperftcies of the Sun; of 
the Sun and Saturn, a little within it. 
4. And by the fame mode of calculation it will be found,- 
that the common centre of gravity of all the planets, can¬ 
not be more than the length of the folar diameter diftant 
from the centre of the Sun. This common-centre of gra^ 
vity he proves is at reft ; and therefore though the Sun, 
by reafon of the various pofitions of the planets, may be- 
moved every way, yet it cannot recede far from the com¬ 
mon centre of gravity, and this, he thinks, ought to be 
accounted the centre of our world. Book iii. prop. 12. 
5. By means of the folar fpots it hath been difeovered, 
that the Sun revolves round his own axis, without moving 
confiderably out of his place, in about twenty-five days, 
and that the axis of this motion is inclined to the ecliptic 
in an angle of 87° 30' nearly. The Sun’s apparent dia¬ 
meterbeing fenfibly longer in December than in June, the 
Sun mull be proportionably nearer to the Earth in-winter 
than in fummer; in the former of which feafons there¬ 
fore will be the perihelion, in the latter the aphelion: and 
this is alfo confirmed by the Earth’s motion being quicker 
in December than in June, as it is by about one-fifteenth 
part. For, finee the Earth always deferibes equal areas 
in equal times, whenever it moves fwifter, it mud needs 
be nearer to the Sun: and for this reafon there are about 
eight days more from the Sun’s vernal equinox to the 
autumnal, than from the autumnal to the vernal. 
6. That the Sun’s diameter is equal to 100 diameters 
of the Eaj-th ; and therefore the body of the Sun mult 
be 1,000,000 times greater than that of the Earth. Mr. 
Azout affures us, that he obferved, by a very exadt me¬ 
thod, the Sun’s diameter to be no lefs than 21' 45" in his 
apogee, and not greater than 32' 45" in his perigee. 
7. According to Newton, in his theory of the Moon, 
the mean apparent diameter of the Sun is 32' 12". The 
Sun’s horizontal parallax is now fixed at 8" . 
8. If we divide 360 degrees (the whole ecliptic) by the 
quantity of the folar year, it will give 59' 8", See. which 
therefore is the medium quantity of the Sun’s daily mo¬ 
tion : and if this 59' 8" be divided by 24, we have the 
Sun’s horary motion equal to 2' 28": and if this laft be 
divided by 60, it will give his motion in a minute, See. 
And in this way the tables of the Sun’s mean motion are 
conftrufted, as placed in books of aftronomical tables and 
calculations. 
