178 
ASTRONOMY. 
their motion round the sun, and the true 
figure ot their orbits. These particulars were 
left for the determination of Newton, who has 
discovered the paths they describe, and the 
laws to which they are subject. Their revo- 
lutions are now known to be performed in 
very long ellipses, whose lower focus is very 
near the sun ; being governed throughout by 
the same law, of describing equal areas in 
equal times, which is known to regulate the 
motions of all the other bodies in the system. 
By observations of the parallax it is also 
found, that at their fiivt appearance they are 
nearer to us than Jupiter; whcnce.it is con- 
cluded that they, are in general less than that 
planet ; for if they were as large they would 
be seen at as great a distance. In their 
motions round the sun they are also subject 
to the same irregularities as the planets ; but 
as their orbits are extremely eccentric, those 
variations are much more considerable. 
When they are near the sun their motion is 
very rapid, and iii the more distant parts ot 
their orbits extremely slow ; so that their , 
vicissitudes, in this respect, are as much in 
the extreme, as what they must undergo ■ 
from heat and cold. . . 
When a comet arrives within a certain ; 
distance of the sun, it emits a fume or vapour : 
called its tail ; which shows that they contain 
a portion of matter considerably more rare 
and 'volatile than any on the earth; for the 
•tail begins to appear when they are yet in 
a higher, and consequently a colder region 
than Mars. In every situation of the comet, 
the tail is always directed to that part of the 
heavens which is nearly opposite to the 
sun ; and it is always greater alter the comet 
has passed its pevihelium, than duiing its 
approach towards it, being greatest of all 
when it has just left that point. 
The celebrated Dr. Halley, from the 
theorv of Newton, has calculated tables for 
determining the orbits of the comets, which 
in several instances have been found agree- 
able to observation. That remarkable comet 
which was observed in the year 1(330, is shewn 
to be (he same with the one that appeared in 
the year 1106, its period being 575 years. 1 he 
distance of this comet from the sun when in 
its perihelium, was to the distance of the earth 
from die sun, as 6 to 1000; its heat, therefoie, 
at that time, was to the heat ot the summer s 
sun with us, in the ratio of about 28000 to 1 ; 
so that, according to Newton, the surface of 
this comet, supposing it be composed ol a 
matter like our earth, would have acquired a 
heat about 2000 times greater than that of 
red-hot iron; which is- so intense, that va- 
pours, exhalations, and every volatile matter, 
must have been immediately consumed and 
dissipated. 
In order to describe the course of a comet, 
let ABC (tig. 19) be the very long orbit of 
a comet, in one of whose foci S is placed 
the Sun ; the ap helium in A; the perihelium 
in P. The comet is not visible to us but when 
it approaches towards B, and during the time 
which it passes the arc front BtoC of its orbit. 
But the time is considerably shorter than that 
which it employs to pass the other portion of 
its orbit CAB, for these two reasons: first, 
because the arc B C is much, shorter than 
the arc CAB; and in the second place, be- 
cause the comets, like the other planets, are 
slower in their course while they depart far- 
ther from the Sun; and on the contrary they 
are swifter as they approach the sun. It re- 
quires much less time to pass over the portion 
BC of their orbit which is visible to us, than 
the other portion CAB. 
As the great eccentricity of the orbits of 
comets renders them very liable to be dis- 
turbed by the attraction of the planets and 
other comets, it is probably to prevent too 
great disturbances from these, that while all 
the planets revolve nearly in tire same plane, 
the comets are disposed in very different 
cones, and disposed all over the heavens ; 
that when at their greatest distance from the 
Sun, and moving slowest, they might be re- 
moved as far as possible out of the reach ot 
each other’s action. 
The number of comets belonging to our 
system is unknown; but from the acountsof 
the antients, and the more accurate observa- 
tions of the moderns, it is ascertained that 
more than 450 had been seen, previous to 
the year 1771; and when the attention of 
astronomers was called to this object, by the 
! expectation of the return of the comet of 
1759, no less their seven were observed in 
the course of as many years. Hence we may 
reasonably conclude, ‘that their number is 
considerably beyond any estimate tnat can 
be made from tire observations of which we 
are now in possession. 
The Antient Const ellationr. 
Of ilie fixed Stars. The fixed stars 
are so called, because they are observed 
not to change their places in the hea- 
vens as tire planets do. they appear of 
an infinite variety of sizes, yet for conve- 
nience it is usual to class them into six or 
seven magnitudes: thus they are called stars 
of the first, second, &o. magnitude. 1 o the 
naked' eye they appear innumerable, but this 
is only a deception, caused probably by the 
refraction of our atmosphere. The whole 
number of stars visible to the naked eye is 
about 3 1-86 ; but seldom above one-third of 
that number can be seen by most people. 
The antients, that they might tire better 
distinguish the stars with regard to their situ- 
ation in the heavens, divided them into se- 
veral constellations, that is, systems of stars, 
each system consisting of such as are near each : 
other. And to distinguish these systems from 
one another; they gave them the names of 
such men or things as they fancied the space 
they took up in the heavens represented. 
To these, several new constellations have 
been added by modern astronomers. 
The following table contains the firm is of 
the constellations, and the number of stars 
observed in each by different astronomers. 
Ursa minor 
Ursa major 
Draco - 
Cepheus 
Bootes, Arctophilax 
Corona Borealis 
Hercules, Engonasin 
Lyra - 
Cygnits, Gatlin a 
Cassiopeia 
Perseus - 
Auriga - 
Serpentarius, Ophiuchus 
Serpens - 
Sagitta - 
Aquila, Vullur 
An tin o us 
Delphinus 
Equulus, Equi sectio 
Pegasus, Equus 
Andromeda 
Triangulum 
Aries - 
Taurus - 
Gemini - - 
Cancer - - - 
Leo - 
Coma Berenices 
Virgo 
Libra, Chela 
Scorpio - 
Sagittarius 
Capricornus 
Aquarius - 
Pisces 
Cetus 
Orion 
Eridanus, Fluvius 
Lepus 
Canis major , 
Canis minor 
Argo Navis 
Hydra 
Crater 
Corvus 
Centaurus 
Lupus 
Ara 
Corona Australis 
Piscis Australis 
The Little Bear 
The Great Bear 
The Dragon 
Cepheus 
Bootes - - - 
The Northern Crown 
Hercules kneeling - 
The Harp 
The Swan 
The Lady in her Chair 
Perseus 
The Waggoner 
Serpentarius 
The Serpent - 
The Arrow 
The Eagle ? 
Antinous \ 
The Dolphin 
The Horse’s Head - 
The Flying Horse - 
Andromeda 
The Triangle 
The Ram 
The Bull 
The Twins 
The Crab 
The Lion ? 
Berenice’s Hair $ 
The Virgin 
The Scales 
The Scorpion 
The Archer 
The Goat 
The Water-bearer - 
The Fishes 
The Whale - 
Orion - 
Eridanus, the River 
The Hare 
The Great Dog 
The Little Dog 
The Ship 
The Hydra 
The Cup 
The Crow 
The Centaur 
The Wolf 
The Altar 
The Southern Crown 
The Southern Fish 
8 
7 
12 
24 
S5 
29 
73 
87 
SI 
32 
40 
80 
13 
4 
51 
35 
23 
18 
52 
*54 
8 
8 
8 
21 
_ 
29 
28 
45 
113 
10 
11 
17 
21 
19 
18 
47 
81 
_ 
13 
26 
37 
55 
29 
29 
46 
59 
14 
9 
40 
66 
29 
15 
40 
74 
18 
13 
22 
64 
5 
5 
5 
18 
- 
15 
n 
23? 
19$ 
71 
10 
10 
14 
18 
4 
4 
6 
10 
20 
19 
38 
89 
_ 
23 
23 
47 
66 
_ 
4 
4 
12 
16 
18 
21 
27 
66 
44 
43 
51 
141 
25 
25 
38 
85 
23 
15 
29 
83, 
35 
C 30 
49 
95 
• 
in 
21 
43 
32 
33 
50 
110 
17 
10 
20 
51 
24 
10 
20 
44 
31 
14 
22 
69 
28 
28 
29 
51 
45 
41 
47 
108 
m 
38 
36 
39 
11:5 
m 
22 
21 
45 
97 
38 
42 
62 
73 
„ ‘ 
34 
10 
27 
84 
12 
13 
16 
19 
29 
13 
21 
31 
2 
2 
13 
14 
45 
3 
4 
64 
27 
19 
31 
60 
7 
3 
10 
31 
7 
4 
0 
9 
37 
O 
0 
35 
19 
0 
0 
24 
7 
0 
0 
9 
13 
0 
0 
12 
18 
0 
0 
24 
i 
