With a six-inch telescope of the best quality, and with
time to choose the most favorable moment, one will be as well
equipped to settle the question of the rotation of Venus as the
best observer. The few days near each inferior conjunction are
especially to be taken advantage of.
The questions to be settled are two: first, are there any dark
spots or other markings on the disk? second, are there any
irregularities in the form of the sharp cusps? The central
portions of the disk are much darker than the outline, and it is
probably this fact which has given rise to the impression of dark
spots. Unless this apparent darkness changes from time to time, or
shows some irregularity in its outline, it cannot indicate any
rotation of the planet. The best time to scrutinize the sharp
cusps will be when the planet is nearly on the line from the earth
to the sun. The best hour of the day is near sunset, the half-hour
following sunset being the best of all. But if Venus is near the
sun, she will after sunset be too low down to be well seen, and
must be looked at late in the afternoon.
The planet Mars must always be an object of great interest,
because of all the heavenly bodies it is that which appears to
bear the greatest resemblance to the earth.
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