I recall my father reading from a letter to Nature,
May 15, 1884, by Mr. W.F. Denning, discussing "The Rotation Period of
Mars." From my note-book I find the passage literally transcribed:
It read--"Notwithstanding his comparatively small diameter and its slow
axial motion, the planet Mars affords especial facilities for the exact
determination of the rotation period. Indeed, no other planet appears to
be so favorably circumstanced in this respect, for the chief markings on
Mars have been perceptible with the same definiteness of outline and
characteristics of form through many succeeding generations, whereas the
features, such as we discern on the other planets, are either temporary,
atmospheric phenomena, or rendered so indistinct by unfavorable
conditions as to defy measurement and observation. Moreover, it may be
taken for granted that the features of Mars are permanent objects on the
actual surface of the planet, whereas the markings displayed by our
telescopes on some of the other planetary members of our system are mere
effects of atmospheric changes, which, though visible for several years
and showing well defined periods of rotation cannot be accepted as
affording the true periods. The behavior of the red spot on Jupiter may
closely intimate the actual motion of the sphere of that planet, but
markings of such variable, unstable character can hardly exhibit an
exact conformity of motion with the surface upon which they are seen to
be projected.
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