  We investigate the long-term motion of Saturn's North-Pole Hexagon and the
structure of its associated eastward jet, using Cassini ISS and ground-based
images from 2008 to 2014. We show that both are persistent features that have
survived the long polar night, the jet profile remaining essentially unchanged.
During those years the hexagon vertices showed a steady rotation period of 10
hr 39 min 23.01 $\pm$ 0.01 s. Analysis of Voyager 1 and 2 (1980-1981) and HST
and ground-based (1990-91) images shows a period shorter by 3.5s, due to the
presence at the time of a large anticyclone. We interpret the hexagon as a
manifestation of a vertically trapped Rossby wave on the polar jet and, because
of their survival and unchanged properties under the strong seasonal variations
in insolation, we propose that both hexagon and jet are deep-rooted atmospheric
features that could reveal the true rotation of the planet Saturn.
