Rotation Period of the Planet Venus

Abstract

ACCORDING to my observations, this planet seems to have a very slow rotation, round an axis almost perpendicular to the plane of the orbit. Thus, a dusky spot remained fixed with regard to the terminator for three or four hours on the same dates, as well as on the following days. The behaviour of other spots was analogous ; and the interposition of Cytheræan cloud would occasionally render them invisible. The southern regions appeared persistently dusky, while a similar, though smaller, dusky area would, now and then, be seen near the north limb.