Grant County herald. [volume] (Lancaster, Wis.) 1850-1968, 1893-04-27
A LESSON IN ASTRONOMY.
why the shadow on the Moon Is Not AL.
WAYS Convex.

WHY the shadow on the Moon Is Not AL
wavs Convex.
was talking one evening with a
bright, intelligent boy of fifteen. about
some of the more familiar astronom-
ical subjects. when he surprised me
greatly by question that he asked.
My surprise arose from the fact that /
knew the boy to be a good student,
who stood we'll in his classes, and that
he should be ignorant of so simple a
thing as that about which he asked the
question seemed almost incredible.
Since then, however | have found
that there is a general misapprehension
of the subject, not only among boys
and girls, but among grown people,
and it is the purpose of this short ar
tiele to give simple explanation of it
Here is the question asked by my
young friend: I'VE are told that one
proof of the earth's spherical form is
the round shadow that it throws upon
the moon, but when the moon is half
fall the edge of the shadows is
straight, and when it is three-quarters
full the edge of the shadow is con-
cave. Now, why is the shadow not al
ways convex, as il is when the moon
is seen as a crescent"
The boy thought, as You see, that
the moons phases are caused by the
interposition of the earth's shadow. A
little reflection will show you that this
is simply impossible Let me see if
can not describe the phenomenon so
that you may draw a mental picture
of it.
You are standing, we will say, on
big ball out in space. Away off yonder
is another big ball, glowing with light.
Between you and the glowing ball is a
smaller, but non-luminous one. As the
latter emits no light of its own, the side
that is toward you is dark and of course
you can not see IT.
This is the phase called the new
moon" The ball on which you stand is
the earth. the glowing ball is the sun,
and the dark ball between you and the
sun is the moon. The other side of the
moon, the side toward the sun, is bright,
for the sun is shining on it. AT tins
time the three balls are almost IN q
straight line with each other.
In a few days the moon, which is
perpetually revolving around the earth
from west to east. moves above the
straight line enough for you to catch
the first glimpse of its illuminated
side, and yor see it as a thin crescent.
Every day it moves higher and you see
more and more of its bright side.
When it reaches point directly over
head you see one-haf of that side,
which is one-fourth of the sphere, and
when it begins to go down on the other
side of the earth from the sun. the
bright part becomes convex and the
dark part concave. of course. When it
gets down far enough on the side of the
earth away from the sun to be in a line
with those two bodies you see all its
illuminated side, and then il is full"
moon.
AS it continues in its course around
the earth. the same phases are passed
through, but in reverse order.
1t is very plain, then, that the shad-
ow of the earth does not cause the
moons phases. Sometimes, however,
the moon, in its monthly revolution,
moves in the same plane as the earth's
/ orbit, and gets intooits shadow. Then
we have lunar eclipse. At no7other
time does the earth's shadow touch the
moon.
To make this still palmer. let me re-
mind you that when the sun is in the
west, the earth's shadow must neces-
sarily be thrown out into space toward
the east But the crescent moon is
seen in the west, you knOw-how, then,
could il be caused by a shadow that is
away off in the east, on the other side
Of the earth?-WorthingtOn's Maya.
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