Page:Astronomy for Everybody.djvu/193

The question of the rotation of Venus has interested astronomers and the public ever since the time of Galileo. But the difficulty of learning anything certain on the subject is very great, owing to the peculiar glare of the planet. When seen through a telescope no sharp and well-defined markings are visible. Instead of this there is a glare on the surface, varying by gentle gradations from one region to another, as if we were looking upon a globe of polished but slightly tarnished metal. Nevertheless, various observers have supposed that they could distinguish bright or dark spots. As far back as 1667 Cassini concluded from these seeming spots that the planet revolved on its axis in a little less than twenty-four hours. During the next century Blanchini, an Italian observer, published an extensive treatise on the subject, illustrated with many drawings of the planet. His conclusion was that Venus required more than twenty-four days to revolve on its axis. Cassini, the son, defended his father's conclusion by claiming that the planet had always made one revolution and a little more between the times of Blanchini's observations on successive evenings. Thus the Italian astronomer would naturally see the spots on successive evenings a little farther advanced, and estimated the motion by this advance, not being aware that a whole revolution had been made during the interval. At the end of twenty-four days the same hemisphere of the planet would be presented to the earth as before, the number of revolutions in the meantime being twenty-five.