It will be seen that for some years the spot drifted in the direction of increasing longitude, but in the summer of 1902 it reversed its motion and, apart from some irregularities between 1904 and 1908, it has since drifted in the direction of diminish ing longitude completely round the planet more than twice rel atively to the adopted zero meridian.
There is another object of long enduring character which calls for mention in connection with the red spot. Like the latter it is situated in the south tropical zone and is known as the south tropical disturbance, but differences in their respective rates of rotation have many times brought the two objects into conjunction. The disturbance was first seen by Major P. B. Molesworth on Feb. 28, 1901. It was then a mere dark hump at the southern edge of the south equa torial belt, but it rapidly developed, spread across the zone and became greatly extended in longitude with a bright spot at each end. At times its length has exceeded 180° and it has frequently displayed much structural detail. It disappeared together with the southern portion of the south equatorial belt and the red spot hollow—probably in consequence of overlying vapours—in 1919, reappeared in 1920, but again became invisible with the objects above mentioned in 1926. The following points relating to the association of the disturbance with the red spot seem to be of special interest : (a) During the earlier part of its history the disturbance was rotating much more rapidly than the red spot and overtook it at intervals of a little more than two years. But its rotation subsequently became slower and in 1925 its period actually exceeded that of the red spot.
(b) It is not known in what way the dark material of the dis turbance has got past the red spot at conjunction, whether round it or under it. At the earlier conjunctions there seems to have been an almost immediate transference of dark matter to the other side of the spot, but at later conjunctions the process has been much more protracted.
(c) The red spot has shown a very decided attraction for the disturbance, each end of the latter being accelerated on approaching the spot and retarded after passing it.
(d) Both the red spot and the disturbance have been accelerated during the transference of the dark material of the latter from one side of the spot to the other.
It is very difficult to find any satisfactory explanation of the red spot. The special facts which have to be taken into con sideration are the following: its long enduring character; its form and colour and the changes it undergoes; its motion, especially in longitude but also to a slight extent in latitude ; its attraction for the south tropical disturbance ; its seeming repulsion of the material of the south equatorial belt in which it has formed a bay or hollow ; its concomitant association with the disturbance in changes of rotation period. Possibly the vortex theory fits the
facts best, but the pointed ends and dark spots at the tips, some-' times seen, would seem to render this hypothesis untenable. More accurate knowledge of the nature and relative levels of the Jovian markings appears to be required before any theory can be put forward with confidence.
All observers have re marked on the very striking analogy between Jupiter and the sun, and the following points of similarity are specially noteworthy: both bodies are of low density, that of Jupiter being 1.3 times and that of the sun 1.4 times that of water ; both show a darken ing at the limb due to atmospheric absorption : and on both spots near the equator rotate in less time than those in higher latitudes. A resemblance may also be traced between Jupiter's belts and the spot zones on the sun, while, occasionally, bright objects have been observed at or close to the planet's limb which became either inconspicuous or quite invisible near the central meridian, suggesting that like the solar faculae their visibility was due to their elevation. Some of these objects were seen during the apparition of 1919, and E. M. Antoniadi, using the great 32.7 in. refractor at Meudon near Paris, has observed white objects near the limb resembling the solar faculae in appearance as well as in behaviour. It is accordingly tempting to regard Jupiter as a sort of semi-sun, not hot enough to be self luminous to any appreciable extent—for were this the case the satellites would be visible when eclipsed in the planet's shadow—but possessing con siderable stores of energy which are the prime cause of the ex tensive and violent disturbances observed on the visible surface. It seems almost impossible that these disturbances can be the effect of mere solar radiation which can have only part of the
intensity it has at the distance of the earth. Reference to Table II., in the article PLANET shows that, at Jupiter's distance, a black, rotating globe without an atmosphere, would have a surface tem perature of —152° C. As a matter of fact radiometric measure ments by Coblentz and Lampland with the thermocouple and water cell transmission (as described in the article above men tioned) indicate temperature of approximately — oo° C. which, though considerably higher than such as could be derived from solar radiation alone, is very much lower than that hitherto thought probable from a consideration of the disturbances in the planet's surface features.