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Nature volume 13, page 513 | Download Citation



THE ROTATION OF VENUS.—It was Jean Dominique Cassini (Cassini I. as he has frequently been designated) who during his residence in Italy, made the first serious attempt to ascertain the time of rotation of this planet and the position of the axis. His observations with one of Campani's long telescopes appear to have been commenced about the middle of the seventeenth century, as related in the Journal des Savans, 1667, Dec. 12, but it was not until the evening of Oct. 14, 1666, that he perceived any spot of sufficiently definite aspect to be of service for the purpose in view. It is described as “Une partie claire située proche de la section, et fort éloignée du centre de cette planèce vers le septentrion.” At the same time several dusky spots were noted. These observations were continued till June 1667, but Cassini expressed himself very cautiously with regard to the inferences to be drawn from them. They appeared to indicate a return of the bright spots to the same position upon the disc at intervals of about twenty-three hours, but from the short time that the spots could be followed Cassini was unable to decide whether the appearances were to be attributed to an axial rotation or to a libration. “De dire maintenant,” he says, “supposé que ce soit toujours la même partie luisante, si ce mouvement se fait par une libration, c'est ce que je n'oserais encore assurer, parce que je n'ai pas pu voir la continuité de ce mouvement dans une grande partie de l'arc, comme dans les autres planètes, et par cette même raison cela sera toujours très-difficile à déterminer.”

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