Other Worlds | Page 9

northern hemisphere) and midwinter our planet

draws 3,000,000 miles nearer the sun, but the change occupies six
months, and, at the earth's great average distance, the effect of this
change is too slight to be ordinarily observable, and only the
astronomer is aware of the consequent increase in the apparent size of
the sun. It is not to this variation of the sun's distance, but rather to the
changes of the seasons, depending on the inclination of the earth's axis,
that we owe the differences of temperature that we experience. In other
words, the total supply of heat from the sun is not far from uniform at
all times of the year, and the variations of temperature depend upon the
distribution of that supply between the northern and southern
hemispheres, which are alternately inclined sunward.
But on Mercury the supply of solar heat is itself variable to an
enormous extent. In six weeks, as we have seen, Mercury diminishes its
distance from the sun about one third, which is proportionally ten times
as great a change of distance as the earth experiences in six months.
The inhabitants of Mercury in those six pregnant weeks see the sun
expand in the sky to more than two and a half times its former
magnitude, while the solar heat poured upon them swiftly augments
from something more than four and a half times to above eleven times
the amount received upon the earth! Then, immediately, the retreat of
the planet begins, the sun visibly shrinks, as a receding balloon
becomes smaller in the eyes of its watchers, the heat falls off as rapidly
as it had previously increased, until, the aphelion point being reached,
the process is again reversed. And thus it goes on unceasingly, the sun
growing and diminishing in the sky, and the heat increasing and
decreasing by enormous amounts with astonishing rapidity. It is
difficult to imagine any way in which atmospheric influences could
equalize the effects of such violent changes, or any adjustments in the
physical organization of living beings that could make such changes
endurable.
But we have only just begun the story of Mercury's peculiarities. We
come next to an even more remarkable contrast between that planet and
our own. During the Paris Exposition of 1889 a little company of
astronomers was assembled at the Juvisy observatory of M.
Flammarion, near the French capital, listening to one of the most

surprising disclosures of a secret of nature that any savant ever
confided to a few trustworthy friends while awaiting a suitable time to
make it public. It was a secret as full of significance as that which
Galileo concealed for a time in his celebrated anagram, which, when at
length he furnished the key, still remained a riddle, for then it read:
"The Mother of the Loves imitates the Shapes of Cynthia," meaning
that the planet Venus, when viewed with a telescope, shows phases like
those of the moon. The secret imparted in confidence to the knot of
astronomers at Juvisy came from a countryman of Galileo's, Signor G.
V. Schiaparelli, the Director of the Observatory of Milan, and its
purport was that the planet Mercury always keeps the same face
directed toward the sun. Schiaparelli had satisfied himself, by a careful
series of observations, of the truth of his strange announcement, but
before giving it to the world he determined to make doubly sure. Early
in 1890 he withdrew the pledge of secrecy from his friends and
published his discovery.
No one can wonder that the statement was generally received with
incredulity, for it was in direct contradiction to the conclusions of other
astronomers, who had long believed that Mercury rotated on its axis in
a period closely corresponding with that of the earth's rotation--that is
to say, once every twenty-four hours. Schiaparelli's discovery, if it were
received as correct, would put Mercury, as a planet, in a class by itself,
and would distinguish it by a peculiarity which had always been
recognized as a special feature of the moon, viz., that of rotating on its
axis in the same period of time required to perform a revolution in its
orbit, and, while this seemed natural enough for a satellite, almost
nobody was prepared for the ascription of such eccentric conduct to a
planet.
The Italian astronomer based his discovery upon the observation that
certain markings visible on the disk of Mercury remained in such a
position with reference to the direction of the sun as to prove that the
planet's rotation was extremely slow, and he finally satisfied himself
that there was but one rotation in the course of a revolution about the
sun. That, of course, means that one side of Mercury always faces
toward the sun while the opposite side always faces away from it,