Omaha daily bee., 1899-12-10

fixed by reference to certain lunar eclipses
Thus, for example, our present year 1899 is
not the 1S99th year since the birth of Christ,
but the 1S95th, because an error OF four
years has been detected by astronomers CEN
turtles after this Christian manner OF reck-
oning years had been in use. And this is
proved by the fact mentioned by the Jewish
historian, Josephus, that upon the night that
Herod died there occurred an eclipse of the
moon. With their present accurate knowl-
edge of the moons motion it was not diff
cult for astronomers to fix the date of this
eclipse and thus detect the error in our era.
Secondly, it happens very often during a
total lunar eclipse that the moon does not
entirely disappear from view, but remains
visible with dull copper color. This is
owing to the fact that the earth's atmosphere
acts like huge lens and bends the suns
rays into the sarth's shadow that they
fall upon the moon. These rays Of the sun,
therefore. traverse a great thickness Of our
atmosphere and returning from the moon
PASS through it again. Under these circum-
stances the air exercises much greater alb
sorptive power than usual, and the spectre
scope is thus enabled to identify with Ger
tainty those lines which are termed telluric,
that is, those which are due to our own at
mosphere and which must therefore be elm
mated from the spectra of celestial bodies.
Variations of Heat.
Thirdly, the invention of the bolometer, an
instrument that detects almost infnitesimal
variations of heat, enables astronomers to
study the heat-radiating power OF the
eclipsed moon and of parts OF its surface
at time when the suns light and heat are
suddenly taken away from them after they
had been exposed to them for periods vary
ing from few minutes to nearly two weeks.
Observations of this kind are extremely del
icate and great additions to our knowledge
are expected from them.
Fourthly, during an eclipse when not only
its intense glare, but at times even all of
it's light is taken away from the moon, we
are enabled to observe the moons passage
over many small stars, which could other
wise never be seen close to the lunar disk.
These occultations of stars when observed
from stations very far apart furnish the best
means for determining the moons position,
motion, size and distance.
Fifthly Barnard has photographed the
moon when it was entirely within the earths
shadow, but very near its border-that is,
in other words, few minutes after totality
began and few minutes before 1t
ended in order to see whether our moon had
a satellite revolving about it, which under
those circumstances might be caught In the
suns rays while the moon Itself was in
darkness. While his results were negative,
the experiment is worth repeating, although
1t is not likely that any such moon of our
moon" exists.
Lastly, a lunar eclipse is picture hung
up in the sky and simultaneously visible to
half the earth. The contacts OF the lunar
disk with the earth's shadow occur at the
same absolute instant Of time for all the
observers OF the world, and hence these con-
facts would furnish an excellent means OF
determining longitudes II they could be ac-
curately noted. Owing, however to the in
distnctness of the edge OF the earth's
shadow caused by our enveloping atmos-
phere refractng the suns rays, the times
OF the contacts can scarcely be observed |
within a minute, or half a minute at most. |
larger telescope makes matters worse, as |
II rather increases the indistnctness OF cut !
line. The contacts with the earth's penum- |
bra are altogether impossible to see.
Practically Total.
The eclipse next Saturday evening misses |
being a total one by hairs breadth one |
might say. The magnitude OF an eclipse Is |
the fraction OF the suns or moons diameter |
obscured at the middle OF the eclipse. In |
lOldl eclipse WneH the hOoH tHleIS tue |
shadow completely and remains in it for an
hour or more the magnitude is greater than
is, when the magnitude is, for
example Ia; II the moon were IA larger in
diameter it would just be totally eclipsed
In this eclipse the magnitude is O996, want
ing but O.OO4 OF totality, so that only
/ -125th Of the moons diameter will not dis
appear from view in the earths shadow. AS
it is doubtful whether the naked eye can see
such small fractIcn of the moons diameter,
this eclipse is practically total one.
The accompanying diagram will enable
one to observe the eclipse intelligently. The
large circle is the earth's penumbra and the
smaller one concentric with IT, the earth's
Imbra or shadow at the place where the
moon crosses them, The earth being a
sphere casts conical shadow and con-
cal penumbra, the sections of which by
planes parallel to the background OF The sky |
are the circles on our diagram. The letters
n Il w at the ends OF the vertical and |