MOON. 
which would otherwise have been shewn, 
will be turned behind, and the same face 
will appear. And thus, if the moon’s angu- 
lar velocity about her axis were always 
equal to her angular velocity in her orbit 
about the earth, the same side of the moon 
would be always towards the earth. But 
as the moon's angular velocity about her 
axis is uniform, and her angular velocity in 
her orbit is not uniform, their angular velo- 
cities cannot continue always equal, and 
tlierefore the moon will sometimes show a» 
little more of her eastern parts, and some- 
times a little more of her western parts; 
this is called a libration in longitude. Also, 
the moon’s axis is not perpendicular to the 
plane of her orbit, and therefore at oppo- 
site points of her orbit, her opposite poles 
are turned towards the earth ; therefore 
her poles appear, and disappear, by turns ; 
this is called a libration in latitude. Hence, 
nearly one half of the moon is never visible 
at the earth, and therefore nearly one half 
of its inhabitants (if it have any) never saw 
the earth, and nearly the other half never 
lose sight of it. Also the time of its rota- 
tion about its axis being a month, the length 
of the lunar days and nights will be about a 
fortnight each. It is a very extraordinary 
circumstance, that the time of the moon’s 
revolution about her axis should be equal to 
that in her orbit. 
Sir I. Newton, from the altitude of the 
tides upon the earth, has computed the 
altitude of the tides on the moon’s surface 
to be ninety-three feet, and therefore the 
diameter of the moon, perpendicular to a 
line joining the earth and moon, is less than 
the diameter directed to the earth, by one 
hundred and eighty-six feet. Hence, .says 
he, the same face must always be towards 
tlie earth, except a small oscillation ; for if 
the longest diameter should get a little out 
of that direction, it would be brought into 
it again by the earth’s attraction. 'The sup- 
position of D. de Mairan is, that the hemis- 
phere of the moon next the earth is more 
dense than the opposite one ; and, hence, 
the same face would be kept towards the 
earth, upon the same principle as before. 
When the moon is in conjunction with 
the sun, she is then said to be new, and 
her dark side being next to the earth, she 
is then invisible. As she recedes from the 
sun, we first discover some of her bright 
part, and she appears horned till she gets 
90° degrees from the sun, when she appears 
half enlightened, or dichotomised ; from 
thence, till she comes into opposition, she 
appears above half enlightened, or gibbous ; 
and at opposition she appears full orbed, 
the same tace being then turned towards the 
earth which is .towards the sun, and she is 
then said to be at her full. And from op- 
position to conjunction, her apparent bright 
part decreases as it before increased. When 
the moon is about three days from the new ; 
the dark part is very visible, by the light 
reflected from the earth, which is moon- 
light to the lunarians, considering our earth 
as a moon to them ; and in the most favour- 
able state, some of the spots may be then 
seen. But when the moon gets into quad- 
ratures, its great light prevents the dark 
part from being seen. According to Dr. 
Smith, the strength of moon-light at the 
full moon, is ninety thousand times less 
than the light of the sun ; but from expe- 
riments made by M. Bouguer, he con- 
cluded it to be three hundred thousand 
times less. The light of the moon, con- 
densed by the best mirrors, produces no 
sensible etfect upon the thermometer. 
Our earth, in the course of a month, 
shows the same phases to the lunarians, as 
the moon does to us ; the earth is at the 
full, at the time of the new moon, and at 
new, at the time of the full moon. The 
surface of the earth being about thirteen 
times greater tlian that of the moon, it af- 
fords thirteen times more light to the moon 
than the moon does to us. 
Dr. Herschel has measured the height of 
a great many of the lunar mountains, and 
finds, that a few excepted, they generally 
do not much exceed half a mile. Before 
he measured them, they were reckoned 
much higher, being generally over-rated. 
He observes, that .it should be examined 
whether the mountain stands on level ground, 
which is necessary that the measurement 
may be exact. As the spectator is carried 
by the earth’s rotation, his horizon will con- 
tinually change its situation, and therefore 
it will continually cut the moon’s orbit, at 
different points, till it has gone through the 
whole orbit ; and the inclination of the or- 
bit to the horizon will be continually chang- 
ed. Now the difference between the times 
of the rising of the moon on tw'o successive 
nights, will depend upon the angle which 
the moon s orbif makes with the horizon ■ 
the less the angle is; the less the moon will 
have descended below the horizon, at the 
time when the horizon is brought into the 
same situation it was twenty-four hours 
before; therefore, when the angle which the 
moon’s orbit makes with the horizon is the 
LI 2 
