If Venus were as much flattened at
the poles as the earth is, it would seem that the fact could not escape
detection, yet the necessary observations are very difficult, and Venus
is so brilliant that her light increases the difficulty, while her
transits across the sun, when she can be seen as a round black disk, are
very rare phenomena, the latest having occurred in 1874 and 1882, and
the next not being due until 2004.
Upon the whole, probably the best method of settling the question of
Venus's rotation is the spectroscopic method, and that, as we saw, has
already given evidence for the short period.
Even if it were established that Venus keeps always the same face to the
sun, it might not be necessary to abandon altogether the belief that she
is habitable, although, of course, the obstacles to that belief would be
increased. Venus's orbit being so nearly circular, and her orbital
motion so nearly invariable, she has but a very slight libration with
reference to the sun, and the east and west lunes on her surface, where
day and night would alternate once in her year of 225 days, would be so
narrow as to be practically negligible.
But, owing to her extensive atmosphere, there would be a very broad band
of twilight on Venus, running entirely around the planet at the inner
edge of the light hemisphere. What the meteorological conditions within
this zone would be is purely a matter of conjecture. As in the case of
Mercury, we should expect an interchange of atmospheric currents between
the light and dark sides of the planet, the heated air rising under the
influence of the unsetting sun in one hemisphere, and being replaced by
an indraught of cold air from the other.
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