Measured by the motion of spots on or near the equator, Jupiter's rotation period is about nine hours fifty minutes; measured by the motion of spots in the middle latitudes, it is about nine hours fifty-six minutes.

The red spot completes a rotation in a little less than nine hours and fifty-six minutes, but its period can not be positively given for the singular reason that it is variable.

The variation amounts to only a few seconds in the course of several years, but it is nevertheless certain.

The phenomenon of variable motion is not, however, peculiar to the red spot.
Mr.W.F.Denning, who has studied Jupiter for a quarter of a century, says: "It is well known that in different latitudes of Jupiter there are currents, forming the belts and zones, moving at various rates of speed.
In many instances the velocity changes from year to year.

And it is a singular circumstance that in the same current a uniform motion is not maintained in all parts of the circumference.