But, owing to
the eccentricity of the orbit, the sun swings much faster toward the
east than toward the west, the eastward motion occupying about
thirty-seven days and the westward motion about fifty-one days.
[Illustration: THE REGIONS OF PERPETUAL DAY, PERPETUAL NIGHT, AND
ALTERNATE DAY AND NIGHT ON MERCURY. IN THE LEFT-HAND VIEW THE OBSERVER
LOOKS AT THE PLANET IN THE PLANE OF ITS EQUATOR; IN THE RIGHT-HAND VIEW
HE LOOKS DOWN ON ITS NORTH POLE.]
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.
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