Harrisburg telegraph. [volume] (Harrisburg, Pa.) 1879-1948, 1918-12-02

lowing, which | have many times
tried to puzzle out? They say the
shadow of the earth is the cause
of our seeing only part of the moon
at times. That is plausible enough,
for sometimes the shadow on the
moon has a convex outline, which
would correspond with the round
shape of the earth. But at other
times, tonight for instance, the
shadow is concave, and | am sure no
round body could cause this shadow.
V. w.
Your reasoning is right, V. W.,
and it does you credit. There are
] fear, few girls of your age VINO
would see anything to question III
the statement, which some incor-
rectly informed person has made TO
you, that the changing shapes, or
phases, of the moon are caused Dy
the shadow of the earth.
The fact that you are puzzled by
that assertion when you try to rec
oncile it with the actual appear
ance of the moon at different times
is as good as a diploma for you, be-
cause it shows that you use your
mind as well as your ears and your
eyes, and are not 20 be easily i'm
posed upon even by what may seem
to be authoritative statements.
1t is not the shadow of the earth
that causes the changes in the shape
of the Illuminated part of the moon
as it moves through the sky, but 1t
is the shadow of the moon itself. II
you hold up your hand before win
dow the side away from the window
&, =.
the hands, own shadow.
The moon goes round the earth,
Avpnce in about a month, travelling in
an orbit that brings it at one point
almost exactly between the earth
and the sun, and at the opposite
point almost exactly in opposition
to the sun, with the earth between
them.
The moon is a solid globe, as the
earth is, and light cannot go
through it, although it is abund-
antly reflected from its surface.
Consequently, when the moon is be-
tween the earth and the sun, or
nearly so, the side of it that is to-
ward the sun, and therefore illumi-
mated. is away from the earth, while
the side that is toward the earth is
dark because no light can pass
through the body of the moon.
When, as occurs on certain rela-
lively rare occasions, the moon gets
exactly between the earth and the
sun, as happened on June Sth, the
round black body of the moon is
seen passing over the face OF the
sun, and the sun is said TO be
eclipsed. But ordinarily the moon
does not get so precisely in line
with the sun as to move like an ,
OPAQUE screen over it. On the other
hand, the shadow of the earth never
touches the moon except when
there is a lunar eclipse, the Oppo-
site Of a solar eclipse.
Now, imagine the moon, in its
monthly journey, coming out from
between the earth and the sun,
moving toward the left hand. Only
"A little portion Of the side of the
moon that is toward the sun will,
at first, be brought into the sun
light, and that portion will be
shaped like a crescent, or bow, with
the convex side toward the sun. 1t
is the globular shape of the moon
that causes this effect.
take a ball and hold it nearly be- |
tween you and lamp several yards
away, and then, slowly moving the
ball farther out of line with the
lamp, observe how its illuminated
half grows larger, always keeping
its inner edge concave toward the
center of the ball until, when the
ball is in a position at right angles
to the direction os the lamp, the di-
vision between the light and dark
halves falls straight across the CEN
ter of the ball, and when you move
it still farther around, until you be-
gin to get yourself between the
lamp and the ball, the edge of the
Illuminated part, which now in
cludes more than half the ball,
again becomes concaye toward the
center
To make this clearer, I have
drawn a little diagram which we
viii suppose to represent the moon
viewed from the earth. At first
disregard the curves and the figures
shown upon it, and suppose it to be
in that part of its orbit where it
lies between the sun and the earth;
in that case the.whole circle would
be dark, and the sun being behind
Ct, and we could see no moon. Now,
let the moon move, in its orbit, a
the sun out from behind it toward
the right. Then that crescent por-
tion of tho moon between the right
hand edge and the curve marked
Il" would become visible by the
sunshine reflected from it, all the
rest remaining hidden in the
moons own shadow.
Next, let the moon move still far
ther TO the left bringing the sun
correspondingly farther to the
right; then an additional part of
the moon, up to curve 49," will be-
come visible. When the moon has
moved round toward the left until
the sun is 90 degrees from it to-
ward the right, just one-half the
moon, viewed from the earth, will
be in the sunlight, and the line of
division "3'' will be straight. But
push the moon further around to-
ward the left, so that the position
OF the sun begins to be say, some
what over your right shoulder, and
the illumination will reach curve
a." which is again concave toward
the center, but facing just the op-
posite way from what it did before.
After that the curve "5" will be
reached. and finally. when the illu-
mination spreads to the left-hand
edge, the whole face of the moon
heinp then lighted the nhaaa of fu11