44 
MAGAZINE OF SCIENCE AND AST. 
the timo which elapses from the commencement of 
this angular motion till the return of the point to 
its original position, being the time of one rotation. 
A body when acted upon by external force may 
change tho Axis of ltotation during its motion. A 
Billiard Ball acted upon at one time by the friction 
of tho tablo, at another by the friction of tho 
cushion, at another by the inertia of another Ball, 
will rotate in the space of a few seconds upon seve¬ 
ral Axes; but as a body when noted upon by no ex¬ 
ternal force will continue in motion in the direotion 
originally impressed upon it. so a body like the 
Moon projected into space, with certain motions of 
rotation or revolution, or both, will continue to move 
in accordance with its original impulse. 
Revolution, as contrasted with Rotation, means 
the angular motion of a body round a point or cen¬ 
tre, or other body external to it. The Earth revolves 
round the Sun, the Moon round the Earth ; a stone 
in a sling revolves round the hand which is employed 
in giving it the whirling motion. The difference, 
then, between rotation and revolution lies in tho 
position of tho point or axis round which the angu¬ 
lar motion takes place. In the one case this point 
or axis iB within the circumference of the body; in 
the other it is external to it. In neither case, how¬ 
ever, would the inhabitants of a body, rotating or re¬ 
volving, ho sensible of those angular motions unless 
by reference to some other body or bodies external 
to it. How nro persons residing on the earth made 
cognizant of this angular motion by which they make 
one rotation in about 21 hours f As all parts of 
the globe partake of the same motion,—had wo 
hothing td servo as an index—nothing to mark tho 
point of departure and that of return we should be 
altogether ignorant of tho fact; We have, however, 
two special indications by which we are enabled to 
deoide upon the fact of our angular movement, and 
the timo which is required to complete one rotation 
—these indicators are the fixed stars and tho Siln. 
Tho former, from their immeasurable distance, are 
the surest indicators. A line drawn from the Earth 
in December to a fixed star may, on account of this 
distance, bo considered to bo parallel to the line 
drawn to the same star in June, although the dis¬ 
tance between the points occupied by the Earth in 
December and June is not loss than 100 millions of 
miles. A star then is a fixed mark to which refe¬ 
rence can always be made; and if we find that an 
imaginary lino drawn from the Earth to a star makes 
a certain angle with a given plane passing through 
tho axis of the earth at midnight, and that, passing 
through all the various angles marked upon the cir¬ 
cumference of a circle, it marks tho same angle at 
Or near midnight on tho following night, wo say that 
tho period elapsed between the appearance of the 
star and its return to the same spot mark tho time 
of ono rotation of the Earth on its axis. I have said 
that a star is the best indicator of the movement of 
rotation of the Earth:—tho Sun, however, is the 
most marked object of reference ; but in the case of 
tho Sun tho question becomes complicated with the 
movement of revolution round the Sun, or with tho 
angular motion of tho Earth in its orbit. Tho time 
of rotation as measured by the return of tho Sun to 
the meridian day after day, differs from the time of 
rotation as indicated by the return of a star to the 
meridian, inasmuch as during the 24 hours Which 
have elapsed between the two passages ot the Sun 
over the meridian, the Earth has moved over a cer¬ 
tain portion of its orbit. In point of fact, tbo num¬ 
ber of actual rotations of the Earth in 385 days, is, 
as near as possible, 366, as measured by the stars. 
Having thus explained and illustrated the meaning 
of the terms employed, and the mode of determining 
the angular movement of rotation of bodies. I wil 
proceed to consider the phenomena connected with 
the motion of tho Moon, and to prove that these 
phenomena are only to be explained by the angular 
movement of the Moon round an axis within its 
body, or, whioh is the same thing, a movement of 
rotation, the period of which, when referred to tho 
Sun, coincides precisely with that of its revo ution 
round the Earth. The peculiar phenomenon to 
which reference is made is the presentation of the 
same portion of the surface of the Moon to the E.rih 
during the whole of its revolution, which is as if the 
Moon were fixed at. the end of a loDg arm extending 
from tho centre of the Earth, by which connection 
its revolution round the Earth is insured. Assum¬ 
in'- this to bo a correct analogy, tho persona who 
deny tho rotation of the Moon say, that tho body 
being fixed on the arm cannot bo said to rotate oh 
an axis, its enly motion according to them being one 
of revolution. Before, however, we affirm this fact, 
let us place ourselves en the surface of tho Moon, 
and consider what our idea of our own motion would 
be. We should see a glorious object in the Earth to 
which our attention would principally be turned; 
but wo should also see the stars, and in observing 
them we should remark that they had a marked up. 
parent angular motion, that is, tho star ^rkich at 
some period of lunar timo appeared to be just above 
our heads, would some time afterwards (seven dats 
of our time) be on the horizon; in the same addi¬ 
tional period it would be under our feet-in the same 
additional period it would make its appearance on 
tho opposite horizon; and in another seven days 
it would resume its old place in the zenith, and 
tho same phenomena would be repeated from 
month to month, from year to year, from 
century to century. When wo begin to specu¬ 
late upon tho cause of this movement among tbs 
stars, wo should very possibly assume that thev 
were all made for our use—that we ” er ° 
centre of the Universe—and that the Earth, the 
Sun, and tho Stars revolve round us; and wo should, 
I have no doubt, devise very skilful combinations of 
curves to explain all these motions in accordance 
with our theory. After a time, however, more cor- 
root ideas would bo generated, and we should arms 
at tho conclusion that the apparent movement of 
the stars was caused by a real motion of our owa; 
and we should begin to measure accurately the an- 
gulnr velocity of this movement—that is, vve shouid 
observe the tune which elapsed from the period when 
the star crossed the meridian of our abode to that at 
whioh it returned to it again, nnd we should cdl 
that the time of our rotation upon osr axis, wo 
should, in point of fact, have exactly the same evi¬ 
dence of tho rotation of the Moon as we now have, 
from the stars, of tho rotation of the Earth ; and if 
this evidence bo sufficient to prove that the “ rt “ 
rotates on its axis; tbo samo evidence ought tone 
sufficient to prove that the Moon does bo hkwte. 
Were there no angular motion round the axis ol me 
Moon—that is, were the star which is ones 
on the meridian always to remain there, then 
it is evident that the face of the Moon which at 
ono time is opposite to the Earth -vy°nld after 
half a revolution in its Orbit, bo looking directly 
away from it; but we find that this is not the case 
we find that the Moon constantly turns to us the 
same face ; and we calculate therefore, and most 
accurately, that the time of its movement ot rota¬ 
tion, as referred to tho Sun, is equal to that ot ns 
revolution round the Earth : a day therefore, it 
can call it suoh, to the inhabitant of the Moon. « 
equal to our lunar month in duration, that 
garding the trilling changes caused by the WM**’ 
rit.iea of tho Moons motion : and speaking in rouna 
guraing tne irmiug . v " j 
rities of the Moon's motion ; and speaking m rouna 
numbers, the timo from Sunrise to Sunset is q 
to 168 of our hours, and the time from Sunseno 
Sunrise the same. , . „ 
There would, of course, be a difference between 
Solar and Sideral time at the Moon analogous to 
that which exists on the Earth, but to that it is un¬ 
necessary to allude at present; the. facts which are 
to be deduced from this difference being the duration 
of the revolu tion of the Earth and Moon round the Sod 
