m
brightness of the planet's disc to be marbled with a filmy streak, which
returned periodically to the same position in about twenty-three hours
twenty-eight minutes. This approximate estimate was corrected by the
application of a more definite criterion. On December 28, 1789, the
southern horn of the crescent Venus was seen truncated, an outlying
lucid point interrupting the darkness beyond. Precisely the same
appearance recurred two years later, giving for the planet's rotation a
period of 23h. 21m.[837] To this only twenty-two seconds were added by
De Vico, as the result of over 10,000 observations made with the
Cauchoix refractor of the Collegio Romano, 1839-41.[838] The axis of
rotation was found to be much more bowed towards the orbital plane than
that of the earth, the equator making with it an angle of 53 deg. 11'.

These conclusions inspired, it is true, much distrust, consequently
there were no received ideas on the subject to be subverted.
Nevertheless, a shock of surprise was felt at Schiaparelli's
announcement, early in 1890,[839] that Venus most probably rotates after
the fashion just previously ascribed to Mercury. A continuous series of
observations, from November, 1877, to February, 1878, with their records
in above a hundred drawings, supplied the chief part of the data upon
which he rested his conclusions. They certainly appeared exceptionally
well-grounded; and the doubts at first qualifying them were removed by a
fresh set of determinations in July, 1895.[840] Most observers had
depended, in their attempts to ascertain the rotation-period of Venus,
upon evanescent shadings, most likely of atmospheric origin, and
scarcely recognisable from day to day. Schiaparelli fixed his attention
upon round, defined, lustrously white spots, the presence of which near
the cusps of the illuminated crescent has been attested for close upon
two centuries. His steady watch over them showed the invariability of
their position with regard to the terminator; and this is as much as to
say that the regions of day and night do not shift on the surface of the
planet. In other words, she keeps the same face always turned towards
the sun. Moreover, since her orbit is nearly circular, libratory effects
are very small. They amount in fact to only just one-thirtieth of those
serving to modify the severe contrasts of climate in Mercury.

Confirmatory evidence of Schiaparelli's result for Venus is not wanting.
Thus, observations