Published online by Cambridge University Press: 12 April 2016
From the start of this century until the mid-1970’s the rotation rate of Uranus was reported to be 10.8 h in a retrograde sense (see, for example, Allen, 1955), but a cursory examination of the origin of this datum reveals that little confidence should be placed in it.
Three independent techniques for measuring the rotation rate are available, each very difficult and not including the most direct method of observing the motion of features across the disc. Visual observers have reported features from time to time (see Alexander, 1965, for a full historical account), but the mean diameter of Uranus is only 3.6 arcsec and large high contrast features are rare in the visible spectrum, if they exist at all.