ASTRONOMY. 
and will be the opposite pole; the line 
P R will be the axis of the sphere. 
To obtain precise views of the motions of 
the heavenly bodies, it is necessary to be able 
to assign precisely the place in which they arc. 
This is done by means of several imaginary 
lines or circles, supposed to be described 
upon the surface of the sphere. These cir- 
cles are divided into degrees, minutes, and 
seconds. The great circle of the sphere, 
Q E, which is perpendicular to the axis of 
the world, and of course 90° distant from 
either pole, is called the equator. The 
smaller circles which the stars describe in 
consequence of their apparent diurnal mo- 
tions, are called parallels, because they are 
parallel to the equator. The equator divides 
the heavenly sphere into two equal parts, 
the north and south ; but, to be able to as- 
sign the position of the stars, it is necessary 
to have another circle passing through the 
poles, and cutting the equator perpendicu- 
larly ; this is called the meridian, which is 
supposed to pass through the poles, and al- 
so directly over the head of the observer M, 
and the point N exactly opposite to that. 
The first of these points is called the zenith, 
and the second is denominated the nadir. 
The meridian divides the circles described 
by the stars into two equal parts, and when 
they reach it they are either at their greatest 
height above the horizon, or they are at 
their least height. The situation of the 
pole is readily found, it being precisely half 
way between the greatest and least height 
of those stars that never set. Since HMO, 
the visible part of the heavens, contains 
180°, and it is 90° between the pole P, 
and the equator EQ; if, therefore, we 
take away P E from the semicircle HMO, 
there remains 90° for the other two arcs 
P H and E O, that is, the elevation of the 
pole and the equator, are together equal to 
90", so that the one being known, and sub- 
tracted from 90°, the other also is found. 
Hence it is known, that “ the elevation of 
the pole at any place, is the complement of 
the elevation of the equator or what that 
elevation wants of 90°. Hence also the 
“ elevation of the equator is equal to the 
distance from the pole P to the zenith M 
for the elevation of the equator is the dif- 
ference between that of the pole and 90°. 
When we travel towards the north, we 
perceive that the north pole does not re-- 
main stationary, but rises towards the ze- 
nith, in proportion to the space that we pass 
over. On the contrary, it sinks just as 
much when we travel towards the south, 
VOL. I. 
from which we infer, that the surface of the 
earth is not plane, as would appear to a su- 
perficial observer, but curved. 
The heavenly bodies appear to describe 
a complete circle round the earth every 24 
hours ; but besides these motions which are 
common to them all, there are several 
which possess motions peculiar to them- 
selves. The sun is farther towards the 
south during winter than during summer ; 
he does not therefore keep the same sta- 
tion in the heavens, nor describe the same 
circle every day. The moon not only 
changes her form, diminishes and increases, 
but, if she is observed in relation to cer- 
tain fixed stars, it will be found that 
she proceeds to the eastward, making 
progress every day, till in about a month 
she makes a complete tour of the heavens. 
There are eight other stars which are con- 
tinually changing their place; sometimes 
they seem to be moving to the westward, 
sometimes to the eastward, and sometimes 
they appear stationary for a considerable 
time: these are called planets. There are 
other bodies which appear only occasion- 
ally, move for some time with very great 
velocity, and afterwards advance beyond 
the regions visible to us : these are comets, 
of which one is now (November, 1807), ap- 
parent. The greater number of the hea- 
venly bodies always retain the same, or 
nearly the same relative distance from each 
other, and are, on that account, called fixed 
stars. 
OF THE FIGURE AND MOTION OF THE 
EARTH. 
. The earth, as we have observed, was long 
considered as a large circular plane, spread- 
ing out on all sides to an indefinite distance ; 
but it is now ascertained that it is of a sphe- 
rical figure, nearly resembling that of a 
globe. The evidence for this fact is deci- 
sive, without having recourse to scientific 
principles, by considering that the cele- 
brated navigators Magellan, Sir Francis 
Drake, Lord Anson, and captain Cooke, 
have all at different times sailed round the 
earth. They set out from European ports, 
and, by steering their course westward, ar- 
rived at length at the very place from 
whence they departed, which could not 
have happened, had the earth been of any 
other than a spherical or a globular figure. 
This form is also apparent, from the circu- 
lar appearance of the sea itself, and the cir- 
cumstances which attend large objects 
when seen at a distance on its surface. For 
D d 
