The rotation time of Venus, the determination, that is, of the planet’s day, is one of the fundamental astronomical acquisitions of recent years. For upon it turns our whole knowledge of the planet’s physical condition. More than this, it adds something which must be reckoned with in the framing of any cosmogony. It is not a question of academic accuracy merely, of a little more or a little less in actual duration, but one which carries in its train a completely new outlook on Venus and sheds a valuable side-light upon the history of our whole planetary system. Unconsciously influenced, one is inclined to think, by terrestrial analogies, astronomers for more than a couple of centuries, ever since the time of the first Cassini in 1666, deemed the day of Venus to be just under twenty-four hours in length. So well attested was its determination, and so precisely figured to the minute, that it imposed itself upon text-books which stated it as an acquired fact down to the last second. Nevertheless, Schiaparelli was not so sure, and proceeded to look into the matter. He first looked for himself, and then looked up all the old observations. His chief observational departure was observing by day as near to noon as possible; because then the planet was highest, to say nothing of the taking off from its glare by the more brilliant sky. From certain dark markings around two bright spots near the southern cusp, of one of which spots the detection dates from the time of Schroeter, and from a long, dark streak stretching thence well down the disk, he convinced himself that no such period as twenty-four hours could possibly be correct, inasmuch as whenever he looked, the markings were always there. His notes read, “Same appearance as yesterday,” day after day, until he would really have saved ink and penmanship had he had the phrase cut into a die and stamped. He concluded that the rotation was at least six months long, and was probably synchronous with the planet’s time of revolution. This was in 1889. In 1895 he became still more sure, and showed how the older observations were really compatible with what he had found.