Nature of the Red Spot.—There is much in connection with the red spot that remains in mystery. Its dimensions, form, and motion have severally been ascertained within small limits of error, and the alterations in its tint and degree of visibility have been recorded with every care. But we can only conjecture as to the origin, character, and end of this remarkable formation. What agency produced it, and moulded the definite elliptical outline it has always preserved—what forces control its oscillations of speed, and keep it suspended so long in the aerial envelope of Jupiter—are matters of pure theory. When, in July 1878, it first came under notice it was a well-developed object, and though Russell in 1876, Lord Rosse and others in 1873, and Gledhill and Mayer in 1869-70 had delineated forms suspiciously like the red spot and situated in the same latitude, yet the several features may not have been absolutely identical, for nothing was seen of the spot in 1877 or in some other years. But there is a strong probability that the red ellipse of 1869-70 must have been the red spot in an incipient stage of its formation. The object may have undergone temporary obscuration, similarly to Cassini’s spot two centuries ago.
I. 1857, Nov. 27. (Dawes.) II. 1859, Dec. 29. (Huggins.)
III. 1858, Mar. 2. (Huggins.)IV. 1870, Jan. 23. (Gledhill.)
V. 1872, Feb. 2. (Gledhill.)VI. 1885, Feb. 25. (Denning.)
My own opinion of the spot is that it represents an opening in the atmosphere of Jupiter, through which, in 1878-82, we saw the dense red vapours of his lower strata, if not his actual surface itself. Its lighter tint in recent years is probably due to the filling-in of the cavity by the encroachment of durable clouds in the vicinity. Parts of some of the more prominent belts display an intense red hue like that formerly shown by the red spot, and they may be due to the same causes. Extensive fissures are probably formed in the atmosphere, and quickly distended in longitude by the natural effect of the planet’s tremendous velocity of rotation. It is curious, however, that these rents, after a certain distention, assume a durable outline until they lose their colouring and are temporarily if not finally obliterated.
When the red spot was visible under its best conditions I frequently examined it, hoping to detect some mark well in its interior which might serve as a clue to the true rotation-period of the sphere of Jupiter. For if the spot consisted of a clear patch in the planet’s atmosphere, I thought it possible some real object on the surface might be discerned through it, in which case the difference in its motion and that of the red spot would enable the rate of motion of the globe to be found. If the spot moves more slowly than the planet, then a surface-marking must appear to pass from the E. to the W. side of the spot; but no such evidence could be obtained, owing to the absence of suitable markings. The red tint of the great spot seemed very general over the entire area of the ellipse until its central regions paled in 1882. There were two dark specks, one at the E. and another at the W. extremity of the spot; but these were unchangeable as regards position.
Jupiter, 1886, April 9d 10h 12m. (10-inch reflector; power 252.)
The spot, though placed very near the border of the great S. belt, has never been connected with it, though in Jan.-Feb. 1884, May 1885, and March-April 1886 the spot became temporarily attached to a belt on its S. side. There was some controversy as to this feature, Prof. Hough, from observations with the 18½-inch refractor at Chicago, alleging that at no time had the spot coalesced with or been joined to any belt in its vicinity. But in 1886 many observers succeeded in detecting the junction of the markings alluded to, and Prof. Young gave a drawing of the appearance as seen with power 790 on the 23-inch objective at Princeton (see ‘Sidereal Messenger,’ vol. v. p. 292). The spot and belt were probably at different heights in the Jovian atmosphere, so that there was no commingling of material, one object being simply projected on the other, for the elliptical form of the red spot remained visible all the time. The latter moves more slowly than the connecting belts, and, when clear of them, is often seen with a white aureola fringing its environs.