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.
Accepting the accuracy of Schiaparelli's discovery, which was corroborated in every particular in 1896 by Percival Lowell in a special series of observations on Mercury made with his 24-inch telescope at Flagstaff, Arizona, and which has also been corroborated by others, we see at once how important is its bearing on the habitability of the planet.

It adds another difficulty to that offered by the remarkable changes of distance from the sun, and consequent variations of heat, which we have already discussed.