1(38 
go behind it. That their orbits are within 
that of the Earth is evident, because 
they are never seen in opposition to 
the sun, that is, appearing to rise from the 
horizon when the sun is setting. On the 
contrary, the orbits of all the other planets 
surround that of the earth ; for they some- 
times are seen in opposition to the sun, and 
they never appear to be horned, but always 
nearly or quite full, though sometimes they 
appear a little gibbous, or somewhat defici- 
ent from full. 
We mentioned above, that all the planets 
move round the sun in elliptical orbits. The 
sun is situated in one of the foci of each of 
them. That focus is called the lower focus. 
If we suppose the plane of the earth’s orbit, 
which passes through the centre of the sun, 
to be extended in every direction, as far as 
the fixed stars, it will mark out among them 
*• great circle, which is the ecliptic ; and with 
this the situations of the orbits of all the other 
planets are compared. 
The planes of the orbits of all the other 
planets must necessarily pass through the 
centre of the sun ; but if extended as far as 
the fixed stars, they form circles different 
from one another, as also from the ecliptic ; 
one part of each orbit being on the north, 
.and the other on the south side of the eclip- 
tic. The orbit therefore of each planet cuts 
the ecliptic in two opposite points, which are 
called the nodes of that particular planet, and 
different from the nodes of another planet. 
A line passing from one node of a planet to 
the opposite node, or the line in which the 
plane of the oi'bit cuts the ecliptic, is called 
the line of nodes. That node, where the 
planet passes from the south to the north 
side of the ecliptic, is called the ascending 
node, and the other is the descending node. 
The angle which the plane of a planet’s orbit 
.makes with the plane of the ecliptic, is call- 
ed the inclination of that planet’s orbit. 
Thus (Astronomy, Plate U. fig. 2.) where F 
represents the sun, -the points A and B re- 
present the nodes, and the line A B. the line 
of nodes formed by the intersection of the 
planes of the orbits C and D. The angle 
E F G is the angle of inclination of the planes 
of the two orbits to eaeh other. The dis- 
tance of either focus from the centre of the 
orbit, is called its eccentricity. 
The two points in a planet’s orbit which 
are farthest and nearest to the body round 
which it moves, are called the apsides ; the for- 
mer of which is called the higher apsis, or 
aphelion ; the latter is called the lower apsis, 
or perihelion. The diameter which joins 
these two points, is called the line of the ap- 
sides. "When the sun and moon are nearest 
to the earth, they are said to be in perigee. 
When at their greatest distance from the 
earth, they are said to be in apogee. 
When a planet is situated so as to be be\ 
tween the sun and the earth, or so that the . 
sun is between the earth and the planet, 
then that planet is said to be in conjunction 
frith the sun. When the earth is between 
the sun and any planet, then that planet is 
said to be in opposition. It is evident that 
the two inferior planets must have two con- 
junctions with the sun ; and the superior pla- 
nets can have only one, because they can 
never come between the earth and the sun. 
\yi)en a planet comes 'directly between us 
ASTRONOMY. 
and the sun, it appears to pass over the sun’s 
disc, or surface, and this is called the transit 
of the planet. When a planet moves 
from west to east, viz. according to the 
order of the signs, it is said to have di- 
rect motion, or to be in consequents. Its 
retrograde motion, or motion in anteceden- 
ts, is when it appears to move from east to 
west, viz. contrary to the order of the signs. 
The place that any planet appears to oc- 
cupy in the celestial hemisphere, when seen 
by an observer supposed to be placed in the 
sun, is called its heliocentric place. The 
place it occupies when seen from the earth, 
is called its geocentric place. 
The planets do not move with equal velo- 
city in every part of their orbits, but they 
move faster when they are nearest to the 
sun, and slower in the remotest part of their 
orbits ; and they all observe this remark- 
able law, that if a straight line is drawn from 
(he planet to the sun, and this line is sup- 
posed to .be carried along by the periodical 
motion of the planet, then the areas which 
are described by this right line and the path 
of the planet, are proportional to the times 
of the planet’s motion. That is, the area de- 
scribed in two days, is double that which is 
described in one day, and a third part of 
that which is described in six days ; though 
the arcs, or portions of the orbit described, 
are not in that ratio. 
The planets being at different distances 
from the sun, perform their periodical revo- 
lutions in different times; but it has been 
found that the cubes of their mean distances 
are constantly as the squares of their peri- 
odical times, viz. of the times of their per- 
forming their periodical revolutions. 
These two last propositions were discovered 
by Kepler, by observations on the planets ; 
but sir Isaac Newton demonstrated, that it 
must have been so on the principle of gravi- 
tation, which formed the basis of his theory. 
This law of universal attraction, or gravita- 
tion, discovered by Newton, completely 
confirms the system ,of Copernicus, and 
accounts for all the phenomena which were 
inexplicable on any other theory. 
The sun, as the largest body in our sys- 
tem, forms the centre of attraction, round 
which all the planets move; but it must not 
be considered as the only body endued with 
attractive power, for all the planets also have 
the property of attraction, and act upon each 
other, as well as upon the sun. The actual 
point, therefore, about which they move, 
will be the common centre of gravity of all 
the bodies which are included in our system ; 
that is, the sun, with the primary and secon- 
dary planets. But because the bulk of the 
sun greatly excels that of all the planets 
put together, this point is in the body of the 
sun. The attraction of the planets on each 
other, also disturbs their motions, and 
causes some irregularities. 
It is this mutual attraction between them 
and the sun, that prevents them from living 
off from their orbits by the centrifugal force 
which is generated by their revolving in a 
curves while die centrifugal force keeps them 
from falling into the sun by tjie force of 
gravity, as they would do if it were not for 
this motion impressed upon them. Thus 
these two powers balance each other, and 
preserve order and regularity in the system. 
It is an established maxim in philosophy, 
that if, when a body is projected in a straight 
line, it !b acted upon by another force, draw- 
ing it towards a centre, it will be made to 
describe a curve, which will be either a circle 
or an ellipsis, according to the proportion 
between the projectile and centripetal force. 
If a planet at B (fig. 3.) gravitates or is at- 
tracted towards the sun S, so as to fall from B 
to y in the time that the projectile force 
would have carried it from B to X, it will 
describe the curve B Y, by the combined 
action of these two forces, in the same time 
that the projectile force singly would have 
carried it from B to X, or the gravitating 
power singly have caused it to descend from 
B to y ; and these two forces being duly 
proportioned, the planet, obeying them both, 
will move in the circle B Y 1’ V. 
But if, whilst the projectile force would 
carry the planet from B to b, the sun’s at- 
traction should bring it down from B to 1, 
the gravitating power would then be too 
strong for the projectile force, and would 
cause the planet to describe the curve B C. 
When the planet comes to C, the gravitating 
power (which always increases as the square 
of the distance from the sun S diminishes) 
will be yet stronger for the projectile force, 
and by conspiring in some degree with it, 
will accelerate the planet’s motion all the 
way from C to K, causing it to describe the 
arcs B C, C I), D E, E F, &c, all in equal 
times. 
Having its motion thus accelerated, it 
thereby acquires so much centrifugal force, 
or tendency to lly off at K, in the line K k, 
as overcomes the sun’s attraction ; and the 
centrifugal fore£ being too great to allow the 
planet to be brought nearer to the sun, or 
even to move round him in the circle k m n, 
&c. it goes off, and ascends in the curve K 
L M N, &r. its motion decreasing as gradu- 
ally from K to B, as it increased from B to 
K ; because the sun’s attraction now acts 
against the planet’s projectile motion, just as 
much as it acted with it before. 
When the planet has got round to B, its 
projectile force is as much diminished from 
its mean slate as it was augmented at K ; and 
so the sun’s attraction being more than suf- 
ficient to keep the planet from going off at 
B, it describes the same orbit over again, by- 
virtue of the same forces or powers. 
A double projectile force will always ba- 
lance a quadruple pow er of gravity. Let the 
planet at B, have twice as great an impulse 
thence towards X, as it had before ; that is, 
in the same length of time that it was pro- 
jected from B to b, as in the last example, 
let it now be projected from B to c; and it 
will require four times as much gravity to 
retain it in its orbit ; that is, it must fall as 
far from B to 4, in the time that the projec- 
tile force would carry it from B to c, other- 
wise it would not describe the curve B D, 
as is evident from the figure. But inas- 
much time as the planet moves from B to C, 
in the higher part of its orbit, it moves from 
I to K, or from K to L, in the low er part erf 
it ; because from the joint action of these tw o 
forces, it must always describe equal areas 
in equal times throughout its annual course. 
These areas are represented by the triangles 
B S C, C S 13, L) S E, E S F, &c. whose 
contents are equal to one another from the 
properties of the ellipsis. 
We have now given a general idea of the 
