The smallest of them, and the nearest to the sun, is
Mercury, which is regarded as uninhabitable because it has no
perceptible supply of water and air, and because, owing to the
extraordinary eccentricity of its orbit, it is subjected to excessive
and very rapid alterations in the amount of solar heat and light
poured upon its surface, such alterations being inconsistent with the
supposition that it can support living beings. Even its average
temperature is more than six and a half times that prevailing on the
earth! Another circumstance which militates against its habitability
is that, according to the results of the best telescopic studies, it
always keeps the same face toward the sun, so that one half of the
planet is perpetually exposed to the fierce solar rays, and the other
half faces the unmitigated cold of open space. Venus, the next in
distance from the sun, is almost the exact twin of the earth in size,
and many arguments may be urged in favor of its habitability, although
it is suspected of possessing the same peculiarity as Mercury, in
always keeping the same side sunward. Unfortunately its atmosphere
appears to be so dense that no permanent markings on its surface are
certainly visible, and the question of its actual condition must, for
the present, be left in abeyance.
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