Rotation.—In 1664 Robert Hooke observed a dark spot in the southern hemisphere, and from its motion J. D. Cassini found a rotation period of 9 h. 56 m. Some few years later Cassini made the important discovery that a spot situated near the equator rotated in a period of about 9 h. 5o m. or in about 6 m. less than the dark spot discovered by Hooke farther south. It was there fore evident that the visible globe of the planet does not rotate like a solid body, and subsequent observations have revealed the fact that in general the rotation periods shown by the markings depend on the latitude in which they are situated. There is, however, no simple relationship between rotation and latitude such as is shown by the sun, although the shortest period is normally that of objects within about o° or so of the equator. Outside this swiftly rotating region, relatively slow and quick rotations are found in irregular order from the equator polewards, and the arrangement in the northern differs greatly from that in the southern hemisphere.
Special attention has been given to the drift of the surface material in different latitudes by A. Stanley Williams, W. F. Denning and others, and in recent years this department of Jovian study has been followed up assiduously by members of the Jupiter section of the British Astronomical Association. Mean values for the rotation periods are given diagrammatically in fig. 1; an examination of which will show how irregular the arrangement of the "surface currents," as they are commonly called, really is. In general these currents are sharply bounded, though occasion ally spots are observed between two of them, especially in the rifts dividing the north and south components of the equatorial belts, which show intermediate periods. The ellipse in the diagram represents the great red spot which will be described presently. Although the figures given may be regarded as fairly reliable average values, the periods deduced each year from the observa tions are found to exhibit variations, and even in the same latitude those of spots in one longitude will differ from those elsewhere. Individual spots, too, often show marked irregularities of motion, while from time to time objects appear with quite abnormal periods. For instance in 188o a number of black spots in north latitude 23° were found to be rotating in the very short time of about 9 h. 48 m., and in recent years exceptional periods, both long and short, have been observed in other parts of the disc. In 1928 a series of small black spots on the south component of the south equatorial belt rotated in rather more than 9 h. 59 m., the longest period yet recorded. ' The Red Spot.—Of all planetary markings it is safe to say that none has aroused so much general interest or received so much attention from observers as Jupiter's great red spot. It came into special prominence in 1878, but was seen actually several years earlier, viz., by W. R. Dawes in 1857, and at various times during
the next two decades, especially by Sir W. Huggins, J. Baxendell, Lord Rosse, R. Copeland and H. C. Russell. Since 1878 it has been under constant observation and has been found to vary greatly in colour, form, intensity and motion. It is situated in the southern hemisphere in latitude — 20° approximately and com monly appears as an elliptical shading or an elliptical ring in contact with the northern edge of the south temperate belt, and at various times it has presented curiously pointed ends. It is usually about 30,00o miles in length and about 7,000 or 8,000 miles in breadth. In colour it has varied from an intense brick red to the faintest grey; indeed in some years it has been quite invisible though, fortunately, its position has still been indicated by the "bay" or "hollow" it has made for itself in the southern part of the south equatorial belt in which it is usually seen lying. In recent years the spot itself has as a rule been very faint, but in 1919 and 1927 when the hollow disappeared the spot regained something of its former intensity. In 1927 it also recovered some thing of its redness. The early history of the red spot has been carefully investigated by W. F. Denning, who, by means of pub lished drawings and records of observations, either of the spot itself or its hollow, has succeeded in carrying our knowledge of the object back as far as Schwabe's drawing of the hollow on Sept. 5th, 1831. He has also suggested the identity of the present red spot with the object observed by Hooke in 1664 to which reference has already been made. But although it seems quite probable that the two objects are one and the same long gaps in the observations make it impossible to regard identity as more than a very plausible speculation. In any case, however, the well established identification with the feature shown by Schwabe shows the red spot to be remarkably long-lived in view of the well-known transitoriness of the Jovian markings in general. It might be imagined from its long enduring nature that the red spot is some sort of projection above the visible superficial va pours or clouds from a more or less solid globe beneath them ; but its motion, especially in longitude, shows that this cannot be the case. There is, of course, nothing of a fixed nature visible on Jupiter from which its longitude can be measured. Observers, therefore, make use of a mathematical meridian, rotating uni formly in a period of 9 h. 55 m. 40.6 s., whose relative position to the centre of the planet's disc at any time can be calculated, and which some years ago represented the rotation of the spot very closely. As illustrating the wanderings of the spot Table I. is appended which gives the longitude measured from this meridian at successive oppositions since 1894 when the meridian coincided with the centre of the spot.