ASTRONOMY. 
rotation of the earth that we are unable to 
perceive it. For as the motion of a ship at 
sea, when she sails swiftly over the smooth 
surface of the water, is almost impercepti- 
ble to the passengers and company on 
board; much more so must it be with such a 
large body as the earth, which has no impedi- 
ments or obstacles of any kind to meet with 
in its way, or to disturb its motion. And in a 
manner, equally easy may another objec- 
tion be removed, which lias frequently been 
brought against this doctrine. It has been 
asserted, that if the earth moved, a stone 
dropped from the top of a tower, or any 
other high building, would not fall just at 
the bottom of it, as the building must have 
advanced considerably forward during the 
time of the fall. But this is evidently a 
mistake ; for it is well known, by repeated 
experiments, that if a body be projected 
from another body in motion, it will always 
partake of the motion of that other body. 
Thus, a Stone dropped from the top of a 
mast, while the ship is under sail, is not 
left by the vessel, but tails exactly at the 
foot of the mast. And if a bottle of wa- 
ter be hung up in the cabin, with its neck 
downwards, it will empty itself, drop by 
drop, into another bottle placed exactly 
underneath it, though the ship shall have 
run many feet whilst each drop was in the 
air. This motion of the earth round its 
axis, which, from the instances already 
given, has been sufficiently proved, is called 
its diurnal or daily motion, and is that which 
occasions the regular return of day and 
night, and all the celestial appearances be- 
fore mentioned. But there is also another 
motion of the earth, called its annual or 
yearly motion, which occasions the various 
vicissitudes of the seasons, summer, winter, 
spring, and autumn. And the proofs of 
this second motion may be easily gathered 
from celestial appearances, in nearly the 
same manner as the former. For as that 
luminary seems to move round the earth, 
from east to west, in the space of a day, 
which is really owing to the diurnal revolu- 
tion of the earth upon its axis, in a con- 
trary direction ; so likewise he seems to have 
an annual motion in the heavens, and to rise 
and set continually in different parts of 
them ; which is certainly occasioned by tire 
daily motion of the earth in its orbit, or 
path round the sun, which it completes in 
the space of a year. 
OF THE SOLAR SYSTEM. 
It is folly proved that the planets, with 
the earth which we inhabit, and also the 
moon, revolve round the sun, which is fixed 
in the centre of the system. There are two 
kinds of planets, primary and secondary. 
The first move round the sun, and respect 
him only as the centre of their motions. 
The secondary planets, called also satellites 
or moons, are smaller planets, revolving 
round the primary, while they, with the 
primary planets about which they move, are 
carried round the sun. The planets move 
round the sun at various distances, some 
being much nearer to him than our earth, and 
others being much farther off. There are 11 
primary planets, which are situated with re- 
spect to their distances from the sun as fol- 
lows : Mercury g ; Venus $ ; the Earth © ; 
Mars # ; Ceres, Pallas, Juno, Vesta, Jupi- 
ter V- ; Saturn b ; and the Herschel planet, 
or the Georgium Sidus $. (See Plate I. 
Astronomy.) Of these our earth is accom- 
panied by one moon, Jupiter has four moons, 
Saturn has seven moons, and the Herschel 
planet has six moons. None of these moons, 
except our own, can be seen without a good 
telescope. The other five planets do not 
appear to have any satellites, or moons. 
All the planets move round the sun from 
west to east, and in the same direction do 
the moons revolve round their primaries, 
excepting those of the Herschel planet,, 
which seem to move in a contrary direc- 
tion. The paths in which they move round 
the sun are called their orbits. Tiiese or- 
bits are elliptical; but the eccentricity of 
the ellipses is so small, that they approach 
very nearly to circles. They perform their 
revolutions also in very different periods of 
time. The time of performing their revolu- 
tions is called their year. The planets are 
evidently opaque bodies, and they shine 
only by reflecting the light which they re- 
ceive from the sun ; for Mercury and Venus, 
when view ed by a telescope, often appear to 
be only partly illuminated, and have the ap- 
pearance of our moon when she is cusped or 
horned, having the illumined part always 
turned towards the sun. From the appear- 
ance of the boundary of light and shadow 
upon their surfaces, we conclude that they 
are spherical; which is confirmed by some 
of them having been found to tutu periodi- 
cally on their axes. Venus and Mercury 
being nearer to the sun than our earth, are 
called inferior planets, and all the rest, 
which are without the earth’s orbit, are cal- 
led superior planets. That the first go round 
the sun is certain, because they are seen 
sometimes passing between us and the sm. 
Dd 2 
