The belts, spots and irregular markings on Jupiter have now been assiduously studied during nearly three centuries. These markings are extremely variable in their tones, tints and relative velocities, and there is little reason to doubt that they are atmospheric formations floating above the surface of the planet in a series of different currents. Certain of the markings appear to be fairly durable, though their rates of motion exhibit considerable anomalies and prove that they must be quite detached from the actual sphere of Jupiter. At various times determinations of the rotation period were made as follows:—

Date. Observer.  Period. Place of Spot.
1672J. D. Cassini9 h. 55 m. 50 s.Lat. 16° S.
1692”9 h. 50 m.Equator.
1708J. P. Maraldi9 h. 55 m. 48 s.S. tropical zone
1773J. Sylvabelle9 h. 56 m.  ”    ”
1788J. H. Schröter9 h. 55 m. 33.6 s.Lat. 12° N.
1788”9 h. 55 m. 17.6 s.Lat. 20° S.
1835J. H. Mädler9 h. 55 m. 26.5 s.Lat. 5° N.
1835G. B. Airy9 h. 55 m. 21.3 s.N. tropical zone.

A great number of Jovian features have been traced in more recent years and their rotation periods ascertained. According to the researches of Stanley Williams the rates of motion for different latitudes of the planet are approximately as under:—

 Latitude.  Rotation Period.
+85° to +28°9 h. 55 m. 37.5 s.
+28° to +24°9 h. 54½ m. to 9 h. 56½ m.
+24° to +20°9 h. 48 m. to 9 h. 49½ m.
+20° to +10°9 h. 55 m. 33.9 s.
+10° to −12°9 h. 50 m. 20 s.
−12° to −18°9 h. 55 m. 40 s.
−18° to −37°9 h. 55 m. 18.1 s.
−37° to −55°9 h. 55 m. 5 s.

W. F. Denning gives the following relative periods for the years 1898 to 1905:—

 Latitude.  Rotation Period.
N.N. temperate9 h. 55 m. 41.5 s.
N. temperate9 h. 55 m. 53.8 s.
N. tropical9 h. 55 m. 30 s.
Equatorial9 h. 50 m. 27 s.
S. temperate9 h. 55 m. 19.5 s.
S.S. temperate9 h. 55 m. 7 s.

Fig. 1.—Inverted disk of Jupiter, showing the different currents and their rates of rotation.

The above are the mean periods derived from a large number of markings. The bay or hollow in the great southern equatorial belt north of the red spot has perhaps been observed for a longer period than any other feature on Jupiter except the red spot itself. H. Schwabe saw the hollow in the belt on the 5th of September 1831 and on many subsequent dates. The rotation period of this object during the seventy years to the 5th of September 1901 was 9 h. 55 m. 36 s. from 61,813 rotations. Since 1901 the mean period has been 9 h. 55 m. 40 s., but it has fluctuated between 9 h. 55 m. 38 s. and 9 h. 55 m. 42 s. The motion of the various features is not therefore dependent upon their latitude, though at the equator the rate seems swifter as a rule than in other zones. But exceptions occur, for in 1880 some spots appeared in about 23° N. which rotated in 9 h. 48 m. though in the region immediately N. of this the spot motion is ordinarily the slowest of all and averages 9 h. 55 m. 53.8 s. (from twenty determinations). These differences of speed remind us of the sun-spots and their proper motions. The solar envelope, however, appears to show a pretty regular retardation towards the poles, for according to Gustav Spörer’s formula, while the equatorial period is 25 d. 2 h. 15 m. the latitudes 46° N. and S. give a period of 28 d. 15 h. 0 m.

The Jovian currents flow in a due east and west direction as though mainly influenced by the swift rotatory movement of the globe, and exhibit little sign of deviation either to N. or S. These currents do not blend and pass gradually into each other, but seem to be definitely bounded and controlled by separate, phenomena well capable of preserving their individuality. Occasionally, it is true, there have been slanting belts on Jupiter (a prominent example occurred in the spring of 1861), as though the materials were evolved with some force in a polar direction, but these oblique formations have usually spread out in longitude and ultimately formed bands parallel with the equator. The longitudinal currents do not individually present us with an equable rate of motion. In fact they display some curious irregularities, the spots carried along in them apparently oscillating to and fro without any reference to fixed periods or cyclical variations. Thus the equatorial current in 1880 moved at the rate of 9 h. 50 m. 6 s. whereas in 1905 it was 9 h. 50 m. 33 s. The red spot in the S. tropical zone gave 9 h. 55 m. 34 s. in 1879-1880, whereas during 1900-1908 it has varied a little on either side of 9 h. 55 m. 40.6 s. Clearly therefore no fixed period of rotation can be applied for any spot since it is subject to drifts E. or W. and these drifts sometimes come into operation suddenly, and may be either temporary or durable. Between 1878 and 1900 the red spot in the planet’s S. hemisphere showed a continuous retardation of speed.