at
Flagstaff, Arizona, and which has also been corroborated by others, we
see at once how important is its bearing on the habitability of the
planet. It adds another difficulty to that offered by the remarkable
changes of distance from the sun, and consequent variations of heat,
which we have already discussed. In order to bring the situation home to
our experience, let us, for a moment, imagine the earth fallen into
Mercury's dilemma. There would then be no succession of day and night,
such as we at present enjoy, and upon which not alone our comfort but
perhaps our very existence depends, but, instead, one side of our
globe--it might be the Asiatic or the American half--would be
continually in the sunlight, and the other side would lie buried in
endless night. And this condition, so suggestive of the play of pure
imagination, this plight of being a two-faced world, like the god
Janus, one face light and the other face dark, must be the actual state
of things on Mercury.

There is one interesting qualification. In the case just imagined for
the earth, supposing it to retain the present inclination of its axis
while parting with its differential rotation, there would be an
interchange of day and night once a year in the polar regions. On
Mercury, whose axis appears to be perpendicular, a similar phenomenon,
affecting not the polar regions but the eastern and western sides of the
planet, is produced by the extraordinary eccentricity of its orbit. As
the planet alternately approaches and recedes from the sun its orbital
velocity, as we have already remarked, varies between the limits of
twenty-three and thirty-five miles per second, being most rapid at the
point nearest the sun. But this variation in the speed of its revolution
about the sun does not, in any manner, affect the rate of rotation on
its axis. The latter is perfectly uniform and just fast enough to
complete one axial turn in the course of a single revolution about the
sun. The accompanying figure may assist the explanation.

[Illustration: DIAGRAM SHOWING THAT, OWING TO THE ECCENTRICITY OF ITS
ORBIT, AND ITS VARYING VELOCITY, MERCURY, ALTHOUGH MAKING BUT ONE TURN
ON ITS AXIS IN THE COURSE OF A REVOLUTION ABOUT THE SUN, NEVERTHELESS
EXPERIENCES ON PARTS OF ITS SURFACE THE ALTERNATION OF DAY AND NIGHT.]

Let us start with Mercury in perihelion at the point _A_. The little
cross on the planet stands exactly under the sun and in the center of
the illuminated