MOON. 
some, been lliouglit seas ; ami by others, to 
he only a great number of caverns and pits, 
the dark sides of which next to the sun, 
would cause those places to appear darker 
than the rest. The great irregularity of 
the line bounding the light and dark parts, 
on every part of the surface, proves that 
there can be no very large tracts of water, 
as such a regular surface would necessarily 
produce a line, terminating the bright part, 
perfectly free from all irregularity. Also, 
if there was much water upon its surface, 
and an atmosphere, as conjectured by some 
astronomers, the clouds and vapours might 
easily be discovered by our telescopes ; 
but no such phenomena have ever been 
observed. 
On April 9, 1787, Dr. Herschel disco- 
vered three volcanoes in the dark part of 
the moon; two of them seemed to be al- 
most extinct, b\it the third showed an ac- 
tual eruption of fire, or luminous matter, 
resembling a small piece of burning char- 
coal covered by a thin coat of white ashes ; 
it had a degree of brightness about it, as 
strong as that with which such a cogl would 
be seen to glow in faint daylight. The 
adjacent parts of tlie volcanic mountain 
seemed faintly illuminated by the eruption. 
A simdar eruption appeared on May 4, 
1783. On March 7, 1794, a few minutes 
before eight o’clock in the evening, Mr. 
Wilkins, of Norwich, an eminent archi- 
tect, observed, with the naked eye, a very 
bright spot upon tlie dark part of the moon; 
it was there when he first looked at the 
moon, and the whole time he saw it, which 
was about five minutes; it was a fixed 
steady light, except the moment before-it 
disappeared, when its brightness increased. 
The same phenomenon was also observed 
by Mr. T. Stretton, in St. John’s Square, 
Clerkenwell, ^London. On April 13, 1793, 
M. Piazza, astronomer royal, at Palermo, 
obsei ved a bright spot on the dark part of 
the moon; and several other astronomers 
have observed the same phenomenon. 
It has been a doubt amongst astronomers, 
whether the moon l)as any atmosphere; 
some suspecting that at an occultation 
of a fixed star by the moon, the star did 
not vanish suddenly, but lost its light gra- 
dually, and thence concluded that the moon 
has an atmosphere. M. Schvoeter, of Lili- 
antban, in the Duchy of Bremen, has en- 
deavoured to establish the existence of an 
atmosphere, ti'om tlie following observa- 
tions. 1. He observed the moon when 
two days and a half old, in the evening 
soon after sunset, before the dark part 
was visible, and continued to o.bserve it till 
it became visible. Two cusps appeared 
tapering in a very sharp, faint, prolonga- 
tion, each exhibiting its furthest extremity 
faintly illuminated by the solar rays, before 
any part of the dark hemisphere was visi- 
ble ; soon after, the whole dark limb ap- 
peared illuminated. This prolongation of 
the cusps beyond the semicircle, he thinks 
must arise from the sun’s rays being re- 
fracted by the moon’s atmosphere. He 
computes also the height of the atmosphere, 
which refracts light enough into the dark 
hemisphere to produce a twilight, more lumi- 
nous than the light reflected from the earth 
when the 010011*18 about 32“ from the new, 
to be 1356 Paris feet, and that the great- 
est height capable of refracting the solar 
rays, is 5376 feet. 2. At an occultation of 
Jupiter’s satellites, the third disappeared, 
after having been l" or 2" of time indis- 
tinct; the fourth became indiscernible 
near the limb ; this was not observed of 
the other two. See the Philosophical Trans- 
actions, 1792. 
Many astronomers have given maps of 
the moon ; but the most celebrated are 
those of Hevelius in his Selenographia ; in 
which he has represented the appearance of 
the moon in its different stales from the 
new to the full, and from the full to the 
new; these figures Mayer prefers. Lan- 
grenus and Ricciolus denoted the spots 
upon the surface, by the names of philoso- 
phers, mathematicians, and other celebrated 
men; giving the names of the most cele- 
brated characters to the largest spots. 
Hevelius marked them with the geographi- 
cal names of places upon the earth. The 
former distinction is now generally used. 
Very nearly the same face of the moon is 
always turned towards the earth. It being 
subject to only a small change within cer- 
tain limits, those spots which lie near the 
edge appearing and disappearing by turns ; 
this is called its libration. The moon turns 
about its axis in the same direction in which 
it revolves in its orbit. Now the angular 
velocity about its axis is uniform, and it 
turns about its axis in the same time in 
which it makes a complete revolution in its 
orbit ; if therefore the angular motion about 
the earth were also uniform, the same face 
of the moon would always be turned to- 
wards the earth. For if the moon had no 
rotation on her axis, when she is on oppo- 
site sides of the earth, she would shew dif- 
ferent faces ; but if, after she has made 
half a revolution in her orbit, she has also 
turned half round her axis, then the face, 
