Retrograde rotation of the upper atmosphere of Venus.

From June through September 1970, ultraviolet photographs of Venus were obtained by the International Planetary Patrol Program. A study of these photographs, together with some from other observatories, confirms previous reports that there is a retrograde circulation of the upper atmosphere of Venus. The sense of the rotation and its approximate period are determined from the motions of well-defined cloud features on five occasions over intervals of several hours. A distinctive feature similar to the 'Y' described by Boyer and Guerin (1969, 1971) is seen many times over a forty-day interval (June 21 to August 1) and allows a good determination of the rotation period: 4.41 plus or minus 0.02 days retrograde (synodic) or 4.50 days (sidereal). This value differs significantly from other estimates, but applies only to this particular forty-day interval.