Herald and news. [volume] (West Randolph, Vt.) 1878-1943, 1893-04-05
A LESSON IN ASTRONOMY.
Whv the hadow on the Moon IL Not Al
WAvs Convex.

wavs Convex.
/ was talking one evening with /,
bright, intelligent boy of fifteen. about
some of the more familiar astronom.
zeal subjects. when he surprised me
greatly by a question that he asked.
Ny surprise arose from the fact that ]
knew the boy to be a good student,
who stood well 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 misrepresentation
Of the subject, not only among boys
and girls, but among grown people.
and it is the purpose of this short ar
ticle to give a simple explanation of it.
Here is the question asked by my
young friend: 1--WE are told that one
proof of the earth's spherical form is
the round shadow that it throws upon
the moon. bnt when the moon is half
.fulr the edge of the shadows is
straight, and when ii is three-quarters
full the edge of the shadow is con-
cave. Now, why is the shadow not al
ways convex, as it 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 J
can not describe the phenomenon SC
that you may draw a mental picture
Of it.
You are standing, we win say, on 8
big ball ont in space. Away off yondei
is another big ball. glowing with light
Between you and the glowing ball is .
smaller, but non-luminous one. As ths
latter emits no light of its own, the side
that IS toward you is dark and of course
you can not see il.
This is the phase called the ''nev
noon" The ball on which you stand Il
the earth. the glowing ball is the u
and the dark ball between you and tn,
sun IS the moon. The other side OF th'
moon. the side toward the sun. is bright
for the sun is shining on it. At thi
time the three balls are almost in
straight line with each other.
In a few days the moon. which 4
perpetually revolving around the earti
from west to east, moves above tn
straight line enough for You to catcl
the first glimpse Of its illuminates
side. and you see IT as a thin crescent
Every day it moves higher and you sc,
more and more Of its bright side.
When it reaches a point directly over
head You see one-half Of that side
which IS on%e-fourth Of the sphere. ans
when it begins 10 go down on the othe
side Of the earth from the sun. IN,
bright part becomes convex and til,
lark part eoncave. Of course. When j
yets down far enough OF the side Of th'
Earth away from the sun to be in a line
with those two bodies. you see all it,
Illuminated side, and then it is full
noon.
As it continues in its course arouno
The earth. the same phases are passec
through. but in reverse order.
1t Is very plain. then, that the shad
Nv Of the earth does not cause tbs
Moons phases. Sometimes. however
he moon. in its monthly revolution
moves in the same plane as the eartla
orbit. and gets into Us shadow. Their
xe have a lunar eclipse At no othei
lime does the earth's shadow touch the
noon.
To make this still palmer. let me re
mind Won that when The SIN Is IN The
vest. the earth's shadow must neces
ariiv be thrown out into space toward
he east But the crescent moon ~
seen IN the west. You know-how, then
could it be caused by : shadow that Is
,wav OF in the east. on the other side
Il the earth2-Northington's Mega
,inc.