MOON. 
Ifast, there will be the least difference of 
the times of her rising. Now, that angle is 
the least, when the first point of Aries rises, 
at which time, in the latitude of London, 
there is only about seventeen minutes dif- 
ference of the moon’s rising on two succes- 
sive nights. Now, about the 22d of Sep- 
tember, the first point of Aries rises at the 
time the niton rises, if the moon be then at 
the full, because it will then be at the be- 
ginning of Aries. In this case, therefore, 
the moon will rise about the full for several 
nights, with but a small ditt’erence of the 
times of her rising. This happening in the 
time of harvest, it is called the harvest 
moon. As the full moon may not happen 
on the 2 2d of September, that which hap- 
pens nearest to it is called the harvest 
moon. The same small difference of the 
times of rising of the moon happens every 
month, but it not happening at the full 
moon, and at that time of the year, it is 
not taken notice of. The greatest differ- 
ence of the times of the moon’s rising at 
London on two successive nights, is about 
one hour and seventeen minutes ; and this 
happens when the moon is in the first point 
of Libra, and therefore it happens at the 
vernal full moons. 
There is a phenomenon called the hori- 
zontal moon, which is this, that it appears 
larger in the horizon than in the meridian ; 
whereas, from its being further from us in 
the former case than in the latter, it sub- 
tends a less angle when in the horizon. It 
is perhaps not easy to give a satisfactory 
answer to this deception. Gassendus 
thought, that as the moon was less bright 
in the horizon than in tlie meridian, we 
looked at it, in the founer situation, with 
a greater pupil of the eye, and therefore it 
appeared la^er. But this is not agreeable 
to the principles of optics, since the mag- 
nitude of tlie image upon the retina of the 
eye does not depend upon the size of the 
pupil. Des Cartes thought that the moon 
appeared largest in the horizon, because 
when comparing its distance with the inter- 
mediate objects it appeared then furthest 
off; and as we Judge its distance greater 
in that situation, we, of course, think it 
larger, supposing that it subtends the same 
angle. Dr. Berkeley accounts for it thus : 
faintness suggests the idea of greater dis- 
tance ; the moon appearing faintest in the 
horizon, suggests tlie idea of greater dis- 
tance ; and, supposing the angle the same, 
that must suggest the idea of a greater 
tangible object. He does not suppose the 
visible extension to be greater, but that 
tlie idea of a greater tangible extension is 
suggested, by the alteration of the visible 
extension. He says, 1. That which sug- 
gests the idea of greater magnitude, must 
be something perceived ; for that which is 
not perceived can produce no effect. 2. It 
must be something which is variable, be- 
cause the moon does not always appear of 
the same magnitude in the herizon. 3. It 
cannot lie in the intermediate objects, they 
remaining the same ; also, when these ob- 
jects are excluded from sight, it makes no 
alteration. 4. It cannot be the visible 
magnitude, because that is least in the ho- 
rizon. The cause, therefore, must lie in the 
visible appearance, which proceeds from 
the greater paucity of rays coming to the 
eye, producing faintness. Mr. Rowning sup- 
poses, that the moon appears furthest from 
us in the horizon, because the portion of the 
sky which we see, appears not an entire he- 
misphere, but only a portion of one ; and 
hence, we judge the moon to be further 
from us in the horizon, and therefore larger. 
Dr. Smith, in his optics, gives the same rea- 
son. The same circumstances take place 
in the sun. Also, if we take two stars 
near each other in the horizon, and two 
other stars near the zenith at the same an- 
gular distance, the two former will appear 
at a much greater distance from each other 
than the two latter. On this account, peo- 
ple are, in general, much deceived in esti- 
mating the altitudes of the heavenly bodies 
above the horizon, judging them to be 
much greater than they are. The lower 
part of a rainbow also appears much wider 
than the upper part ; and this may be con- 
sidered as an argument that the phenome- 
non cannot depend entirely upon the great- 
er degree of faintness of the object when in 
the horizon, because the lower part of the 
bow frequently appears brighter than the 
upper part, at the same time that it ap- 
pears broader. Also, faintness can have 
no effect upon the angular distance of the 
stars ; and as the difference of the apparent 
distance of the two stars, whose angular 
distance is the same in . the horizon and the 
zenith, seems to be fully sufficient to ac- 
count for the apparent variation of the 
moon’s diameter in these situations, it may 
he doubtful whether the faintness of the 
object enters into any part of the cause. 
The mean distance of the moon from the 
earth is about two hundred and thirty-nine 
thousand miles ; and her semidiameter is 
nearly three elevenths of the radius of tlie 
earth, or about one thousand and eighty- 
one miles. And as tlie magnitudes of sphe- 
