gohvr system ; and shall next describe the 
bodies that compose it. 
Of tbs Sun. The Sun was long considered, 
from its constant emanation of heat and 
light, as an immense globe of fire. When 
viewed through a telescope, several dark 
spots are visible on its surface, which are of 
various sizes and duration. From the mo- 
tion of these spots, the Sun has been found 
to move round its axis in twenty-live days, 
which is two days less than its apparent revo- 
lution, in consequence of the Earth’s motion 
in its orbit in the same direction ; and its axis 
is found to be inclined to the ecliptic, in an 
angle of about eighty-two degrees and a 
half. 
Various opinions have been formed re- 
specting these spots ; they have been con- 
[ sidered as opaque islands in the liquid igneous 
matter, and by some as pits or cavities in 
the body of the Sun. But from whatever 
: cause they may arise, they evidently adhere 
to its surface : for if one of them appears upon 
the eastern limb or edge of the Sun’s disc, 
it is seen to move thence towards the 
western edge in about thirteen days and a 
half, then the spot disappears, and in about 
the same time, it is seen again upon the 
eastern edge, and so continues to go round, 
completing its apparent revolution in twenty- 
seven days, during one half of which time 
we see it on the disc of the Sun, aud during 
the other halt it disappears, which could 
not happen if the spots did not adhere to the 
Sun. The following particulars respecting 
the Sun are given by sir Isaac Newton. 
1. That the density of the Sun’s heat, 
which, is proportional to his light, is seven 
times as great in Mercury as with us, and 
that water there would be all carried off in 
the shape of steam ; for he found, by experi- 
ments with the thermometer, that a heat 
seven times greater than that of the Sun’s 
beams in summer will serve 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 distance of that planet from the Sun 
is in the same ratio to the Sun’s semi-diameter; 
consequently, that the centre of gravity of 
the Sun and Jupiter is nearly in the super- 
ficies of the Sun. 
3. That the quantity of matter in the Sun 
is to that in Saturn as 2360 to 1, and that the 
distance of Saturn from the Sun is in a ratio 
but little less than that of the Sun’s semi-di- 
ameter. And hence the common centre of 
gravity of Saturn and the Sun is a little within 
the Sun. 
4. By the same method of calculation it 
will be found, that the common centre of 
gravity of all the planets cannot be more 
than the length of the solar diameter distant 
from the centre of the Sun. 
3. The Sun’s diameter is equal to 100 di- 
ameters of the Earth, and therefore its mag- 
nitude must exceed that of the earth one 
million of times. 
6. If 360 degrees (the whole ecliptic) is 
divided by the quantity of the solar year, it 
will give 59' 8" which therefore is the 
medium quantity of the Sun’s apparent daily 
motion ; hence iris horary motion is equal to 
2' 27". By this method the tables of the 
Sun’s mean motion are constructed as found 
in astronomical books. 
Of the inferior planets. Mercury being 
Vol. 1. 
ASTRONOMY. 
the planet nearest to the Sun, and the least 
in magnitude, is very seldom visible. It 
never appears more than a few degrees from 
the Sun’s disc, and is generally lost in the 
splendour ol the solar beams. On this ac- 
count, astionomers have had few opportuni- 
ties of making accurate observations upon 
it ; no spots have been observed upon it 
consequently the time of its rotation on its 
axis is not known. Being an inferior planet, 
it must shew phases like the Moon, lig. 7 ; 
and it never appears quite full to us. It is 
seen sometimes passing over the Sun’s disc 
which is called its transit. 
Venus is the brightest and largest to ap- 
peal ancc of all the planets, and is distin- 
guished from the rest by her superiority of 
lustre. It is generally called the Morning 
or Evening Star, according as it precedes or 
follows the apparent course of the Sun. 
Some have thought that they could discover 
spots upon its disc ; but Dr. Herschel has not 
been able to see them; consequently, the 
time of rotation round its axis is not known. 
\ onus also appears with phases ; and transits 
sometimes take place, which are of very 
great importance in astronomy. 
The elongation of any planet is its apparent 
distance from the sun. 
An inferior planet is at its greatest elon- 
gation, when a line drawn from the Earth 
through the plahet is a tangent to the orbit 
of the planet ; when the planet is at M (lig. 
4.) being in conjunction with the Sun, it has 
no elongation; as it moves from M to V, 
its elongation increases till at V, when EV 
drawn from the earth to the orbit of the 
planet is a tangent to that orbit, its apparent 
place in the ecliptic is C, and its elongation 
is S C, which is the greatest it can have, for 
in passing from V to N it decreases, and at 
N it is nothing. From N to U it in- 
creases, and at U the elongation is again at 
the greatest. 1 his will hold equally in ellip- 
tical .:s in circular orbits. If the orbits of 
the planets were circular, the distance of 
each from the Sun would be to the Earth’s 
distance, as the Sun at its greatest elongation 
to the radius, that is, as V S to E S. By ex- 
amining the figure it will be seen, that the 
interior planets are never in opposition to 
the Sun, and are never in quadrature. For 
in opposition, the Earth is between the Sun 
and the planets, which can never happen 
when the orbit of the planet M G B is in- 
cluded within that of the Earth. They are 
never in quadrature, because the greatest 
angle of elongation is contained by S E and 
E \ , and it the angle S E V was a right 
angle, E V would be a tangent at E the 
Earth’s orbit : but it is a tangent, as has been 
seen, to an orbit less than that of the Earth; 
it therefore makes an angle with S E; less 
than a right angle. Plence the reason that 
the inferior planets never appear far from 
the Sun ; and as the orbit of Mercury is in- 
cluded vvithin that of Venus, the former must, 
when visible, always appear nearer to the 
Sun, than the latter. We may also observe 
that the apparent velocity of Venus is great- 
est at the times of conjunction. Since the 
plane of her orjiit is oblique to the Earth, 
those parts of it which are viewed by a spec- 
tator directly, will appear longer than other 
equal parts viewed obliquely. Of course, 
the motions ot the planet, if uniform, will 
appear unequal. 
1C(J 
The time when an inferior planet will 
come again into a given situation with respect 
to the Sun and the Earth, mav be thus found. 
Whilst Venus performs one revolution, the 
Earth, whose periodical time is longer than 
that of Venus, will not have completed its 
revolution. Before Venus and the Earth 
can be again in the inferior conjunction, 
Venus must, therefore, besides its entire 
revolution, describe an arc equal to that 
which the Earth lias passed over : conse- 
quently, the number ot degrees passed over 
by each, or their angular motions, in the 
same time, will be reciprocally as their peri- 
odical times ; that is, as the periodical time 
of the Earth is to the periodical time of 
Venus, so is the angular motion of Venus 
(which is equal to four right angles added to 
the angular motion of the Earth between two 
inferior conjunctions) to the angular motion 
ol the Earth in the same time ; whence (El. 
V. 17.) as the difference between the peri- 
odical times of the Earth and Venus, is to 
the periodical time of Venus, so are four 
right angles, or 360°, to the number of de- 
grees over which the Earth passes in her 
orbit from one inferior conjunction to an- 
other. This is only true upon the suppo- 
sition that the planets moved in circular 
orbits, in which case the following general 
rule w ould apply to the finding the time from 
conjunction to conjunction, or from oppo- 
sition to opposition, of any two planets. 
“ Multiply their periodic times together, 
and divide the product by their difference, 
and you have the time sought.” For let 
P = the periodic time of the earth, p = that 
of the planet (suppose an inferior), t — time 
“160° 
required: then PI 1 day” 360°; th« 
angle described by the eartli in 1 day : for the 
360'’ . 
same reason — is the angle described by 
the planet in 1 day: hence • — is 
• P P 
the daily angular velocity of the planet from 
the Earth. Now' if they set out from con- 
junction, they will return into conjunction 
again, after the planet has gained 360°: hence 
3CO° 360. „ „ P p 
-f- T- 360 •••• lda >-:'=p^. For 
• i> P 
a superior planets = - v 
When the inferior planets are passing from 
their greatest elongation V. (fig. 4.) through 
N their superior conjunction, to their great- 
est elongation U, they appear to a spectator 
on the earth to move from west to east ; for 
when the planet is at G it will appear to have 
moved from C to H, and w'hen at A and B 
it will appear to have passed from H through 
L to a and b ; of course the motion of the 
planet is direct, or from west to east; but 
while it moves from U to V, its motion will 
appear to us retrograde or from east to west ; 
for when it has passed from U to J and K, it 
will appear to have moved in the heaven* 
from D through d to b and a, that is, from 
east to w r est When the inferior planets are 
at their greatest elongation, they appear sta- 
tionary ; because when the planets' are at U 
and V, the line drawn from E the Earth to 
the planet, is a tangent to the orbit, W'hich st S 
nearly coincides with a small arc of the curve, 
that a spectator at the Earth cannot distin- 
guish the tangent from the curve, when the 
