Knoxville journal. [volume] (Knoxville, Iowa) 1874-1986, 1876-05-25
Interesting Facts About the Moon.
| Extracts from q Recent Lecture by Prof c. A.
Young Of Dartmouth College.l

The most convenient way oi ueermih'
ing the distance of the moon from the
earth is from two distant stations, whose
positions on the eartb's surface have been
accurately ascertained One OF these sta
lions is usually at the Cape of Good Hope
and the other either at Greenwich, Paris or
Berlin, etc The distance between the two
stations, measured on the same meridian,
forms the base line, and the observed di-
rection of the moon, when it crosses the
meridian, win give US the angles at the
base, from which the distance can be
calculated. This distance is, in round
numbers, o35, 900 miles, or about ten
times the circumference Of the earth. A
good pedestrian could travel that distance
in twenty-three or twenty -four years. The
determinations Of the moons distance are
so accurate that the probable error does
not exceed fifteen or twenty miles. This
distance is not, however, constant, because
the moons path is not circle but an
oval, the eccentricity Of which amounts to
about one-eighteenth.
The size of the moons diameter is de-
termined by measuring its apparent di-
ameter in the telescope, the difficulty Of
the operation consisting In the fact that
the brightness of the disk causes it 10
present circumference which is not de-
fined with perfect sharpness. Having
measured the apparent diameter of the
moon, and knowing the value OF the
earth's diameter, as seen from the moon,
simple proportion will give us the
moons real diameter, 2,159-6 miles, or
about the 1-12Gtb part of the distance be-
tween the earth and the moon, that is to
say, 100 moons placed in line would fill
up the distance. The determinations Of
the value of the moons diameter are cor
rect 10 within two or three miles.
Then, as the volumes of spheres
are TO each other as the cubes of their
diameters, the volume of the moon is
1908 divided by 21G0.3, or about LAG
that Of the earth, that is, 40 moons rolled
up together would make a ball as large
as the earth. The determination of the
density, and consequentlyof the weight,
OF the moon is more difficult than that Of
the most remote Of the planets. One meth
cd of accomplishing it consists in study
ing the effect on the tides when the at
fractions of the sun and moon conspire
to raise them, and when they act in oppo-
site directions. In this way relation is
established between the masses of the sun
and moon. If the sun and moon were at
equal distances from the earth, their at
fractions would be in direct proportion to
their masses, but the sun is about 400
times further off; hence the law that the
attraction is inversely as the square of the
distance must be also applied. This
method, however, is not very accurate. A
better one depends on the fact that the
earth and the moon revolve about their
common center Of gravity, and that the
position Of that center must necessarily
depend on the relative masses of the two
bodies. The earth describes a much
smaller orbit about that center than the
moon, and would be displaced from the
position which it would have if it trav-
eled alone around the sun. This displace-
ment will appear in the observed position
Of the sun, and can be calculated. 1t has
been found TO be 81 seconds of arc; and
from this it results that the earth's mass is
81 times that of the moon. Hence the
moons density is 3-5 that Of the earth.
The force of gravity on the moon is
only l-6 Of that on the earth, that is, a
man able TO jump up feet on the earth
would be able to jump up Is feet on the
moons surface.
The moons path around the earth
would always be an oval OF exactly the
same dimensions if the earth alone acted
upon ii; but owing 10 the attraction of
the sun, the moon is sometimes in adj
vance and sometimes behind the place
she ought to occupy according to the laws
governing motion in an elliptical orbit.
These attractions are called perturbations
and necessltate as many as sixty TO seventy
five different corrections in calculating the
position which the moon Is to occupy at
any required moment. Up 10 about 1870
the calculated position of the moon was
only about two miles out of the way;
but since that time some error has
crept into the nautical almanac, and
the difference is now i've to seven miles.
Prof Airy thinks some perturbation must
have been overlooked If mariner had
a watch that kept perfectly Greenwich
time, he could always ascertain his posi-
ton by consulting the nautical almanac.
The moon is indeed a perfect timekeeper
in its passage across the heavens; but its
motion is so slow that ii would take very
accurate observations 10 obtain the time
from its position.
According to Zollner the light of the
moon is only one six hundred and eight
cen thousandth of that Of the sun. If the
sky were packed full of moons, IL would
not give US quite as much light as the
sun. 1t has been found that. when the
moon is half full, it does not give half
as much light as when it is full, because
the mountains then cast shadows, while
there are no shadows at all on the full
moon. From study of these shadows,
Zollner has found that the average slope
OF the hills and mountains on the moon
must be about 80 deg, without reference
to their height.
Zolner has ascertained, by experiment
and calculation, that the moon reflects
only about one-sixth of the light it re-
ceiyes, in other words that its reflective
capacity is the same as that Of sandstone
rock. snow reflects seventy ght per
cent, granite ten per cent, and marble
fifty per cent. Sir John Herschel has
come to the same conclusion. II have
frequently he stated, compared the
moon setting behind the gray perpendicu-
lar facade of the Table Mountains illu-
minated by the sun just risen in the OPP
positc quarter of the horizon, when ii has
been scarcely distinguishable in bright
ness from the rock in contact with it,"
Until quite recently, ii was supposed
that no heat could be detected in the rays
of the moon. They were collected in the
focus OF large mirror, and directed upon
a very delicate thermopile connected with
a galvanometer. The lecturer had this
Al. ri