Another effect of the libratory motion of the sun as seen from Mercury
is represented in the next figure, where we have a view of the planet
showing both the day and the night hemisphere, and where we see that
between the two there is a region upon which the sun rises and sets once
every eighty-eight days. There are, in reality, two of these lune-shaped
regions, one at the east and the other at the west, each between 1,200
and 1,300 miles broad at the equator. At the sunward edge of these
regions, once in eighty-eight days, or once in a Mercurial year, the sun
rises to an elevation of forty-seven degrees, and then descends again
straight to the horizon from which it rose; at the nightward edge, once
in eighty-eight days, the sun peeps above the horizon and quickly sinks
from sight again. The result is that, neglecting the effects of
atmospheric refraction, which would tend to expand the borders of the
domain of sunlight, about one quarter of the entire surface of Mercury
is, with regard to day and night, in a condition resembling that of our
polar regions, where there is but one day and one night in the course of
a year--and on Mercury a year is eighty-eight days. One half of the
remaining three quarters of the planet's surface is bathed in perpetual
sunshine and the other half is a region of eternal night.

And now again, what of life in such a world as that? On the night side,
where no sunshine ever penetrates, the temperature must be extremely
low, hardly greater than the fearful cold of open space, unless
modifying influences beyond our ken exist. It is certain that if life
flourishes there, it must be in such forms as can endure continual
darkness and excessive cold. Some heat would be carried around by
atmospheric circulation from the sunward side, but not enough, it would
seem, to keep water from being perpetually frozen, or the ground from
being baked with unrelaxing frost. It is for the imagination to picture
underground dwellings, artificial sources of heat, and living forms