Ground-Based Observations of Saturn's North Polar Spot and Hexagon.

Ground-based observations of two conspicuous features near the north pole of Saturn, the polar vortex and the hexagonal wave structure, were made from July 1990 to October 1991, 10 years after their discovery. During this period the polar spot drifted in longitude, relative to system III, by -0.0353 degrees per day on average. Superimposed on this mean motion, the spot also underwent short-term rapid excursions in longitude of up to approximately 14 degrees at rates of up to approximately 1 degrees per day. The spot also exhibited irregular variations in its latitude location. A combination of these data together with those obtained by Voyager 1 and 2 in 1980 and 1981 shows that the spot drifted -0.0577 degrees per day for the 11-year interval from 1980 to 1991. The large lifetime of both features indicates that they are insensitive to the strong variations in the seasonal heating of the cloud layers in the upper polar atmosphere.