Page:Astronomy for Everybody.djvu/194

Schröter tried to decide the question for Venus in the same way that he supposed himself to have decided it for Mercury. He directed his attention especially to the fine sharp horns of the crescent, when the planet was nearly between the earth and the sun. At certain intervals he supposed one of them to be a little blunted. Ascribing this appearance to the shadow of a high mountain, he concluded that the time of rotation was twenty-three hours twenty-one minutes.

From the time of Schröter no one professed to throw any more light on the question until 1832. Then De Vico, of Rome, announced that he had rediscovered the markings found by Blanchini. He concluded that the planet rotated in twenty-three hours twenty-one minutes, in agreement with Schröter's result.

This close agreement between the results of observations by four distinguished observers led to the very general acceptance of twenty-three hours twenty-one minutes as the time of rotation of the planet. But there was much to be said on the other side. The great Herschel, with the most powerful telescopes that had ever been made, was never able to make out any permanent markings on Venus. If anything like a spot appeared, it varied and disappeared again so rapidly that no evidence of rotation could be afforded by it. This negative result has always been reached by the large majority of observers.

But a new and surprising theory has been recently put forth by Schiaparelli, and maintained by Lowell. This is that Venus rotates on its axis in the same period that it revolves around the sun; in other words both Mercury