of the moon.  The secret imparted in confidence to the knot of astronomers at Juvisy came from a countryman of Galileo’s, Signor G. V. Schiaparelli, the Director of the Observatory of Milan, and its purport was that the planet Mercury always keeps the same face directed toward the sun.  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.  That, of course, means that one side of Mercury always faces toward the sun while the opposite side always faces away from it, and neither side experiences the alternation of day and night, one having perpetual day and the other perpetual night.  The older observations, from which had been deduced the long accepted opinion that Mercury rotated, like the earth, once in about twenty-four hours, had also been made upon the markings on the planet’s disk, but these are not easily seen, and their appearances had evidently been misinterpreted.

The very fact of the difficulty of seeing any details on Mercury tended to prevent or delay corroboration of Schiaparelli’s discovery.  But there were two circumstances that contributed to the final acceptance of his results.  One of these was his well-known experience as an observer and the high reputation that he enjoyed among astronomers, and the other was the development by Prof.  George Darwin of the theory of tidal friction in its application to planetary evolution, for this furnished a satisfactory explanation of the manner in which a body, situated as near the sun as Mercury is, could have its axial rotation gradually reduced by the tidal attraction of the sun until it coincided in period with its orbital revolution.