Displacement of the clouds of Venus.

THE apparent four day rotation of the cytherean atmosphere was first pointed out by Boyer (private communication) in 1957 and the retrograde sense of the rotation was discovered by Boyer and Camichel1 from the cloud markings often visible on ultraviolet photographs. The presence of this rotation has been reported several times since (refs. 2–8 and unpublished work of Kuiper et al.). The periodicity shows itself in two ways: (a) any particular formation, if observed over a period of hours, demonstrates a retrograde rotation of period approximately four days, although the periods obtained in this way scatter about a mean value; (b) large scale markings with easily recognizable shapes reappear at intervals of four days or multiples of this value, and the appearance of particular formations can be predicted for long periods in advance. Fig. 1 shows the short term evolution for a type of cloud pattern often observed.