At this appearance the time of the planet's
revolution on its axis was found to be somewhat greater than in
1876, in accordance with the general law exhibited in the
rotations of the sun and of Jupiter. Notwithstanding their
transient character, these two spots have afforded the only
determination of the time of revolution of Saturn which has been
made since Herschel the elder.
[Illustration with caption: THE GREAT REFRACTOR OF THE NATIONAL
OBSERVATORY AT WASHINGTON]
Of the satellites of Saturn the brightest is Titan, which can be
seen with the smallest telescope, and revolves around the planet
in fifteen days. Iapetus, the outer satellite, is remarkable for
varying greatly in brilliancy during its revolution around the
planet. Any one having the means and ability to make accurate
photometrical estimates of the light of this satellite in all
points of its orbit, can thereby render a valuable service to
astronomy.
The observations of Venus, by which the astronomers of the last
century supposed themselves to have discovered its time of
rotation on its axis, were made with telescopes much inferior to
ours. Although their observations have not been confirmed, some
astronomers are still inclined to think that their results have
not been refuted by the failure of recent observers to detect
those changes which the older ones describe on the surface of the
planet.
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