Telescopic Work for Starlight Evenings

by William F. Denning

No planetary marking in modern times has enlisted half the amount of attention that has been devoted to this object. It has endured amid all the turmoils of the Jovian atmosphere for twelve years, and has preserved an integrity of form and size which prove it to have been singularly capable of withstanding disruption. But its tint has varied greatly; so that at times the oval outline of the spot could hardly be discerned amongst the contiguous belts. In the winter of 1881 the interior of the ellipse began to lose tone, and in 1882 it faded rapidly, so that the central region of the spot assumed nearly the same light tint as the outlying bright belts. Apparently the spot had either been filled up with luminous cloudy material or had been partially obscured by the interposition of matter situated higher in the Jovian atmosphere. The elliptical contour of the object was still intact, however, though it had quite lost its bold and prominent character. Only the skeleton of its former self remained, and its entire disappearance seemed imminent. But further decadence was fortunately averted by influences unknown to us, and the spot has continued visible to this day, though shorn of the attributes which roused so much enthusiasm amongst observers more than ten years ago.

From measures at Chicago, in the years from 1879 to 1884, Prof. Hough found the mean dimensions of the spot to be:—Length 11″·75, breadth 3″·71. These figures represent a real length of 25,900 miles and a diameter of 8200 miles. The latitude of the spot was 6″·97 S.

This object has served an important end in attracting wide-spread observation, not only to itself, but to the general phenomena occurring on the surface of Jupiter. Observers, in studying the red spot, were also led to study the bright equatoreal spots and other features so plentifully distributed over the disk. It was most important this should be done; for since the time of Herschel and Schröter not much progress had been made in elucidating the proper motions of the spots and finding an accurate rotation-period for the planet. Dawes, Lassell, and many others had, it is true, secured some interesting observations and drawings, but not of the special kind required, and thus no fresh light had been thrown upon the vagaries in the behaviour of the spots, as described by the old observers. But a mass of new facts were now to be realized. Schmidt at Athens, Prof. Hough at Chicago, A. Stanley Williams at Brighton, and many others, including myself at Bristol, began systematic observations of Jupiter, with a view to learn something more of the periods, changes, and general characteristics of the spots and other features. The results were of an interesting nature, though too extensive for more than bare mention here. In 1879 the red spot gave a rotation-period of 9^h 55^m 34^s·2, but this increased to 9^h 55^m 35^s·6 in 1880-1 and to 9^h 55^m 38^s·2 in 1881-2. During the ensuing three years the period was almost stationary at 9^h 55^m 39^s·1, but in 1885-6 it further augmented to 9^h 55^m 41^s·1, since which year it has ranged between 9^h 55^m 40^s and 41^s. From ten years’ observations, the mean period of the red spot is as nearly as possible 9^h 55^m 39^s.

Bright Equatoreal Spots.—The bright spots near the equator rotated in 9^h 50^m 6^s in 1880; but in subsequent years the time slightly increased, for in 1882 I found it 9^h 50^m 8^s·8, and in 1883 9^h 50^m 11^s·4. The bright spots therefore perform a rotation in 5½ minutes less time than the red spot. The former move so much more swiftly than the latter that they pass it at the rate of 260 miles per hour, and in 44½ days have effected a complete circuit of Jupiter relatively to it. Thus a brilliant white spot, if noticed in the same longitude as the red spot on one night, will, on subsequent nights, be observed to the W. of it, and, after an interval of about 44½ days, the same objects will again occupy coincident longitude.

Dark Spots in N. hemisphere.—In the autumn of 1880 there was a confluent outbreak of dark spots from a belt in about 25° N. latitude, and these exhibited a rotation-period of only 9^h 48^m, so that they travelled more rapidly than the white spots on the equator. Some short dusky belts were also remarked slightly S. of the latitude of the red spot, and these indicated a period of 9^h 55^m 18^s. It is clear from these various results that the motion of the Jovian markings does not decrease according to their distance from the equator.

Rotation-Period.—Below are given the times of rotation ascertained by some previous observers:—

		h m s
1665.	J. D. Cassini	9 55 58
1672.	”	9 55 50
1692.	”	(A)9 50
1708.	J. P. Maraldi	9 56 48
1713.	”	9 56
1773.	Sylvabelle	9 56
1779.	(B)W. Herschel	9 54 53 to 9 55 40
1779.	”	(A)9 50 48 to 9 51 45·6
1786.	(C)J. H. Schröter	9 55 33·6
”	”	9 55 17·6
”	”	(A)9 50 27
1835.	J. H. Mädler	9 55 26·5
”	G. B. Airy	9 55 21·3
1836.	J. H. Mädler	9 55 23·5
1862.	J. F. J. Schmidt	9 55 25·7
1866.	”	9 55 46·3
1873.	O. Lohse	9 55 19·6
1880.	J. F. J. Schmidt	9 55 34·4
”	”	(A)9 50
1881.	W. F. Denning	9 48
”	”	9 55 17·9
1883.	”	(A)9 50 8·7
1885.	G. W. Hough	9 55 37·4
”	”	(A)9 50 9
1886.	A. Marth	9 55 40·6
1887.	A. S. Williams:—
	Spots in 12° N. lat.	9 55 36·5
	” 4° N. lat.	(A)9 50 40·1
	” 8° S. lat.	(A)9 50 22·4
	”30° S. lat.	9 55 17·1
1890.	(D)W. F. Denning	9 55 39

A: Bright spots near the equator of Jupiter.

B: Herschel’s observations embraced few rotations, and the periods he derived differed considerably.

C: Schröter also alleges he saw spots return to the same part of the disk in 7^h 7^m, 7^h 36^m, and 8^h 1^m!

D: From ten years’ observations of the red spot.

The foregoing list is by no means complete, for, owing to the large number of recent determinations, I have thought it advisable to omit some of them.

It should be mentioned here that the above times of rotation are derived from atmospheric features more or less volatile in nature, and that therefore the actual sphere of Jupiter rotates in a period which we have not precisely discovered. No doubt the motion of the real surface is not very different from that of the atmospheric markings above it. There is reason to think that, whatever the character of the planet’s crust may be, we have never yet obtained a glimpse of it. A dense veil of impenetrable vapours appears to surround the globe on all sides, and this is subject to violent derangement from the evolution of heated material or gaseous fluids from the surface below. These disturbances seem to be very durable in some instances as to their observed effects. The atmosphere would, in fact, appear to possess a singular capacity for retaining the impressions of its changes. The permanency of certain spots can hardly be due to continued action from those parts of the disk immediately underlying them; for their variable motions soon transport them far from the places at which they were first seen, and prove their existence to be quite independent of their longitude.