The Oskaloosa herald., 1917-04-12
HEAVENS IN APRIL.
ANOTHER OPPORTUNITY TO SEE
THE PLANET MERCURY.
Many New Consultations Will Be
Awaiting the Observers This
Month and Will Be Studied.

c. S. Brainin, of the Columbia uni
versity observatory staff has the fol-
lowing:
The planets Neptune and Saturn
find places on this months map. Sat-
urn is still in the constellation Gem
mi. where he has been all winter, but
has now taken up his direct easterly
motion through the zodiacal groups,
which will slowly but markedly
change the confagration of that in
teresting section of the heavens. Nep-
tune is to the east of Saturn, but is
so dim that his light is less than that
of star of the seventh magnitude
and is therefore hopeless for our style
of observation.
April will also afford us another
opportunity to view the small planet
Mercury which is the nearest of all
the planets to the sun. 1t is this
very nearness to the bright, all
eclipsing disk of the sun that makes
the observation of this planet diff
cult enough so that the most favor
able occasions have to be utilized.
These occur when the orbital motion
of Mercury has brought It as far as
possible from the sun as seen from
the earth.
On April "a the planet will be at
its greatest eastern elongation. so
that for a very short time it will be
visible when the sun is below the
horizon. The word elongation is
the technical astronomical term for
angular distance from the sun as
seen from the earth; therefore, this
phenomenon means that Mercury
will be as far to the east of the sun as
it can get, and will be an evening
star.
On the 26th the angle between
planet and sun will be about 201a
degrees; with an unobstructed and
cloudless horizon we should look
near the spot where the sun has just
sunk below the horizon and about
| fifth of the way up to the zenith. We
do not, Of course, need to contine our
observations to the 4th alone, but
should take advantage of the first
clear day within a week preceding
or following the oath.
The orbit of Mercury is the inner
most Of the planetary orbits the
mean solar distansce being only about
80 million miles; the next one in or
i'der, Venus, is twice as far away from
| the sun In accordance with the
fundamental Jaws of planetary mo-
| ton Mercury therefore, has the
shortest period of revolution around
the sun and completes this in q quar-
ter of a year. For this reason also,
| Mercury receives nearly seven times
as much neat per square yard as the
| earth, and this alone would make
conditions on his surface impossible
for man. Furthermore, we know
that the albedo or rejecting power
of the planet is very low. showing the
probable absence of any atmosphere
or at most, an extremely slight one.
Again at times when Mercury passes
between the sun and our planet the
disk of Mercury is seen with good
definition and not surrounded by a
halo of light, such as an extensive
atmosphere would bring about by
refraCtion.
II appears from the best obserVa-
| lions so far that the rotation time of
| Mercury on it's axis is the same as its
period in the orbit around the suns
that the same fact, approximately, of
| the planet is always turned to the
| sun. OF course, observations of
| Mercury are so difficult to make be
cause of its nearness to the sun that
alter. observers ma" come to a diff
ferent conclusion.
An interesting thing to be noted
about Mercury is that this planet
shows phases analogous to those of
the moon. When between us and
1 the sun the unillumined side of the
planet is turned toward us, and we
have what might be termed new
Mercury; astronomically this config-
uration of earth, planet and sun is
called inferior CONNECTION of Mercury
The analogous configuration, superior
conjunction of Mercury, shows the
| Oll Tie OluOl Quo W wv GUN
from us; this would give us the face
of Mercury fully illuminated by sun
light and corresponds in way to
full moon Between these two posi-
lions we can see different gradations
Of phase.
The moon on the monthly round OF
it's orbital path about the earth con-
tinually interposes its bulk between
us and the stars which occupy the
projection of it's path, on the celestial
| sphere. For short time, then, the
star is eclipsed from our view. How
ever, the number of bright stars
which are thus subject to lunar oc-
cultationi or eclipse is very small and
not extrameiy often edo We have a
chance tooobserve this happening to
a star which is itself easily observ-
able. This month two stars of good
magnitude will be occulted and pro-
vide spectacle for us which is well
worth observation.
On April at 1927 p. m. eastern
standard time. the moon will oblit-
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