Schiaparelli had satisfied himself, by a careful series of
observations, of the truth of his strange announcement, but before
giving it to the world he determined to make doubly sure. Early in 1890
he withdrew the pledge of secrecy from his friends and published his
discovery.
No one can wonder that the statement was generally received with
incredulity, for it was in direct contradiction to the conclusions of
other astronomers, who had long believed that Mercury rotated on its
axis in a period closely corresponding with that of the earth's
rotation--that is to say, once every twenty-four hours. Schiaparelli's
discovery, if it were received as correct, would put Mercury, as a
planet, in a class by itself, and would distinguish it by a peculiarity
which had always been recognized as a special feature of the moon, viz.,
that of rotating on its axis in the same period of time required to
perform a revolution in its orbit, and, while this seemed natural enough
for a satellite, almost nobody was prepared for the ascription of such
eccentric conduct to a planet.
The Italian astronomer based his discovery upon the observation that
certain markings visible on the disk of Mercury remained in such a
position with reference to the direction of the sun as to prove that the
planet's rotation was extremely slow, and he finally satisfied himself
that there was but one rotation in the course of a revolution about the
sun.
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