ECLIPSES OF THE MOON
Of all the weird happenings of the nighttime sky, eclipses of the moon
are the most impressive. Rarely is there a year without one. What is the
cause? Simply the earth getting in between sun and moon, and thereby
shutting off the sunlight which at all other times enables us to see the
moon. As the earth is a dark body it must cast a black shadow on the
side away from the sun, and it is the moon's passing into this shadow or
some part of it that causes a lunar eclipse.
Sun and earth being so different in size, the earth's shadow must
stretch away from it into space, growing smaller and smaller, until at
length it comes to an end--the apex of a cone 857,000 miles long. If we
cut off this shadow at the moon's distance from the earth, we find it
about 6,000 miles in diameter at that point; and this accounts for the
fact that the curvature on the side of the moon, when the eclipse is
coming on and where it is dropping into the shadow, is always much less
rapid than the curvature of the moon's own disk is.
When an eclipse is approaching, the eastern limb will be duskily
darkened for half an hour or more, because the moon must first pass
through the outer penumbra, or half-shadow which everywhere surrounds
the true shadow itself. If the moon hits only the upper or lower part of
the shadow, the eclipse will be only partial, and during the progress
of the eclipse it will seem as if the uneclipsed part had swung or
twisted around in the sky, from the western limb of the moon to the
eastern. But when the moon passes through the middle regions of the
shadow, the eclipse is always total, and direct sunlight is wholly cut
off from every part of the moon's face, for a greater or less length of
time, according to the part of the shadow through which it passes. When
passing centrally through the shadow, the total eclipse will last about
two hours, as the moon's diameter is about one-third of the breadth of
the shadow; and the eclipse will be partial about two hours longer, an
hour at beginning and an hour at the end, because the moon moves over
her own breadth in about an hour.
While the moon is wholly immersed in the shadow, her body is
nevertheless visible, as a dull tarnished copper disk; and this is
caused by the reddish sunlight which grazes the earth all around and is
refracted or bent by our atmosphere into the shadow itself. If this belt
or ring of terrestrial atmosphere happens to be everywhere filled with
dense clouds, as was the case in 1886, even the familiar copper moon of
a total lunar eclipse disappears completely in the black sky.
Quite different from a solar eclipse, all the phases of a lunar eclipse
are visible at the same time on the earth wherever the moon is above the
horizon. Eclipses of the moon are therefore seen with great frequency at
any given place as compared with solar eclipses, which are restricted to
relatively narrow areas of the earth's surface. Nor are lunar eclipses
of very much significance to the astronomer, mainly because of the
slowness and indefiniteness of the phenomena. It is a good time to
observe occultations of faint stars at the moon's edge or limb, and
several such programs have been carried out by cooperation of
observatories in widely separate regions of the world: the object being
improvement in our knowledge of the distance of the moon, and in the
accuracy of the mathematical tables of her motion. Search by photography
for a possible satellite, or moon of the moon, has been made on several
occasions, though without success.
A lunar eclipse was first observed and photographed from an aeroplane,
May 2, 1920. At the request of the writer, two aviators of the United
States navy ascended to a height of 15,000 feet above Rockaway, and
secured many advantages accruing from great elevation in viewing a
celestial phenomenon of this character.