37 5 ASTRO 
Hie turns once round upon her axis, and therefore always 
prefents the fame face to our view ; and as, during her 
courfe round the Earth, the Sun enlightens fucceffively 
every part of her globe only once, ccnfequently fhe has 
but one day in all that time, and her day and night toge¬ 
ther are as long as our lunar month. As we fee only one 
fide of the Moon, we are therefore invifible to the inha¬ 
bitants on the opposite fide, unlefs they take a journey 
to that fide which is next to us, for which purpofe fome 
of them mull travel more than 1500 miles. And as the 
Earth, from one half of the Moon, is never feen; fo, from 
the middle of the other half, it is always feen over head, 
turning round almofl thirty times as quick as the Moon 
does. To her inhabitants, the Earth feems to be the largeft 
body in the uiiiverfe, about thirteen times as large to them 
as (he does to us. As the Earth turns round its axis, the 
feveral continents and ifiands appear to the Lunarians as 
fo many fpots, of different forms ; by thefe fpots, they 
may determine the time of the Earth’s annual motion ; by 
thefe fpots they may, perhaps, meafure their time : they 
cannot-have a better dial. 
The mean time of a revolution of the Moon about the 
Earth, from one new Moon to another, when file over¬ 
takes the Sun again, is 2yd. 12I1.44'. 3". ri"'. but fhe moves 
once round her own orbit in .27b. 7I1. 43'.. 8". moving about 
229.0 miles every hour. The mean diffance of the Moon 
from the Earth is 6o§ radii, or 30^ diameters, of the 
Earth ; which is about 240,000 miles. The mean excen- 
tricity.of her orbit is ^4o, or Ar nearly, of her mean dif- 
tancq, amounting to about 13,000 miles. 
The Moon’s diameter is to that of the Earth, as 20 to 
73, or nearly as 3 to 11, or x to 3J ; and therefore it is 
equal to 2180 miles: hermean apparent diameter is 31' 16-t", 
that of the Sun being 32' 12". The furface of the Moon 
is to the furface of the Earth, as 1 to 135, or as 3 to 40 ; 
fo that the Earth reflects thirteen times as much light upon 
the M0011 as fhe does upon the Earth; and the {olid con¬ 
tent to that of the Earth, as 3 to 146, or asT to 48.!. The 
denfity of the Moon’s body is to that of the Earth, as 5 to 
4; and therefore her quantity of matter to that of the 
Earth, as 1 to 39 very nearly : the foxxe of gravity on her 
furface, is to that on the Earth, as 100 to 293. The Moon 
appears in the ecliptic only when fhe is in one of her nodes; 
in all other parts of her orbit {he is either in north or fouth 
latitude, fometimes nearer to, fometimes farther removed 
from, the ecliptic, according as fhe happens to be more 
or lefs diftant from the nodes. When the place, in which 
the Moon appears to an inhabitant of the Earth, is the 
fame with the Sun’s place, fhe is faid to be in conjunElion . 
When the Moon’s place is oppofite to the Sun’s place, fhe 
is faid to be in oppofition. When fhe is a quarter of a cir¬ 
cle diftant from the Sun, fhe is faid to be in quadrature. 
Both the Conjunction and oppofition of the Moon are termed 
fyzygies. Tiie common lunar month, or the time that 
pailes between any new Moon and the next that follows, 
is called a j'ynodical month-, or a lunation. A periodic month 
is the time the Moon takes up to defcribe her orbit; or, in 
other words, the time in which the Moon performs one 
.entire revolution about the Earth, from any point in the 
.zodiac to the fame again. If the Earth had no revolution 
round the Sun, or the Sun had no apparent motion in the 
ecliptic, the periodical and fynodical month would be the 
fame ; but, as this is not the cafe, the Moon takes up a 
longer time from one conjunction to the next than to de¬ 
fcribe its whole orbit; or the time between one new Moon 
and the next is longer than the Moon’s periodical time. 
The Moon revolves round the Earth from weft to eaff, 
and the Sun apparently revolves round the Earth the fame 
way. Now at the new Moon, or when the Sun and Moon 
are in conjunction, they both fet out from the fame place, 
to move the fame way round the Earth ; but the Moon 
moves much falter than the Sun, and confequently will 
overtake it; and, when the Moon does overtake it, it will 
be a new Moon again. If the Sun had no apparent motion 
in the ecliptic, the Moon would come up to it, or be in 
N O M Y. 
conjunction again, after it had gone once round in its om 
bit ; but, as the Sun moves forward in the ecliptic whilft 
the Moon is going round, the Moon muff; moye a little 
more than once round, before it comes even with the Sun 
again, or .before it comes to conjunction. Hence it is 
that the time between one conjuntion and the next in 
fucceffion is fomething more than the time the Moon takes 
up to go once round its orbit.; or a fynodical month is 
longer than a periodical one. This may be illuftrated by 
the following figure : 
Let S be the Sun, CF a part of the Earth’s orbit, MD a 
diameter of the Moon’s orbit when the Earth is at A, and 
md another diameter pa¬ 
rallel to the former 
when the Earth is at B. 
Whilft the Earth is at 
A, if the Moon be at 
D, fhe will be in con 
junction ; and if the 
Eartli was to continue 
at A, when the Moon 
-had gone once round its 
orbit, from D through 
M, fo as to return to D 
again, it would again be 
in conjunction. There¬ 
fore, upon the fuppoft- 
tion that the Earth has 
no motion in its orbit, 
the periodical and fyno¬ 
dical months would bee- 
qual to one another. But, 
as the Earth does not continue at A, it will move forward 
in its orbit, during the revolution of the Moon from A 
to B ; and, as the Moon’s orbit moves with it, the dia¬ 
meter MD will then be in the pofition md ; therefore, 
when the Moon has defcribed its orbit, it will be at d in 
this diameter md ; but if the Moon is at d, and the Sun 
at S, the Moon will not be in conjunction, confequently 
the periodical month is completed before the fynodical. 
The Moon, in order to come to conjunction, when the 
Earth is at B, mull be at e , in the diameter ef ; or, befides 
going once round its orbit, it muft alfo defcribe the arc de. 
The fynodical month is therefore longer than the periodi¬ 
cal, by the time the Moon takes up to defcribe the arc de. 
The motions of the Moon in general are moft of them 
irregular, and very confiderably fo. The only equable 
motion (lie has, is her l-evolution on her own axis, in the 
Tpace of a month, or time in which fhe moves round the 
Earth ; which is the reafon that file always turns the fame 
face towards us. This expofure of the fame face is nor, 
however, fo uniform, but that fire turns fometimes a little 
more of the one fide, and fometimes of the other, called 
the Moon’s libration ; and alfo fhews fometimes a little 
more towards one pole, and fometimes towards the other, 
by a motion like a kind of wavering, or vacillation. The 
former of thefe motions happens from this: the Moon’s 
rotation on her axis is equable or uniform ; while her mo¬ 
tion in her orbit is unequal, being quickeft when the Moon 
is in her perigee, and (lowed: when in the apogee, like all 
otiier planetary motions; which caufes that fometimes more 
of one fide is turned to the earth, and fometimes of the 
other. And the other irregularity arifes from this : that 
the axis of the Moon is not perpendicular, but a little in¬ 
clined to the plane of her orbit: and as this axis maintains 
its parallelifm, in the Moon’s motion round the Earth ; 
it muft neceffarily change its fituation, inrefpeCt of an oh- 
ferver oix the Eartli ; whence it happens that, fometimes 
the one and fometimes the other pole of the Moon be¬ 
comes vilible. 
The very orbit of the Moon is changeable, and does not 
always perfevere in the fame figure : for though her orbit 
be elliptical, or nearly fo, having the Earth in one focus, 
the eccentricity of tiie ellipfe is varied, being fometimes 
increafed, and fometimes diminifhed j viz. being greateft 
■when 
