ASTRONOMY. 
and sometimes they go behind it. That 
their orbits are within that of the earth is 
evident, because they are never seen in op- 
position to the sun, that is, appearing to rise 
from the horizon in the east when the sun is 
setting in the west, which is another proof 
that the earth is not the centre of celestial 
motions. On the contrary, the orbits of all 
the other planets surround that of the earth ; 
for they sometimes are seen in opposition to 
the sun, and they never appear to be horn- 
ed, but always nearly or quite full, though 
sometimes Mars appears a little gibbous, or 
somewhat deficient from full. 
Since all the planets move round the sun 
in elliptical orbits, the sun itself is situated 
in one of the foci of each ellipse. That fo- 
cus is called the lower focus. If we sup - 
pose 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 a 
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 ne- 
cessarily 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 ecliptic. Therefore 
the orbit of each planet cuts the ecliptic in 
two opposite 'points, which are called the 
nodes of that particular planet, and the 
nodes of one planet cut the ecliptic in planes 
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 orbit cuts the ecliptic, is called 
the line of nodes. That node where the 
planet passes from tire 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 called the inclination of that planet’s orbit. 
Thus fig. 2, Plate II. where F represents 
the sun, the points A and B represent the 
nodes, and the line A B the line of nodes 
formed by the intersection of the planes of 
the orbits C arid D. The angle EFG is 
the angle of inclination of the planes of the 
two orbits to each other. A line drawn' 
from the lower focus of a planet’s orbit (viz. 
where the sun is) to either end of the conju- 
gate axis of its orbit, (which line is equal to 
half the transverse axis) is called the mean 
distance of the planet from the sun. But 
according to some, the mean distance is a 
mean proportional between the two axes of 
that planet’s orbit. The distance of either 
focus from the centre of the orbit is called 
its eccentricity. The two points in a pla- 
net’s orbit which are farthest and nearest to 
the body round which it moves are called 
the apsides ; the former 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 apsides. 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 between 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 with 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 conjunctions with the sun, 
and the superior planets can have only one, 
because they can never come between the 
earth and the sun. When a planet comes 
directly between us and tlje 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. ac- 
cording to the order of the signs, it is said 
to have directmotion, or to be in consequen- 
tia. Its retrogade motion, or motion in an- 
tecedents, is when it appears to move from 
east to west, viz. contrary to the order of 
the signs. The place that any planet ap- 
pears to occupy 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 
then- orbits ; and they all observe this re- 
markable law, that if a straight line be 
drawn from the planet to the sun, and this 
line be supposed to be carried along by the 
periodical motion of the planet, then the 
areas which are described by this right line 
and the path ot the planet are proportional 
to the times of the planet’s motion. That 
is, the area described 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 
