The equatoreal spots also deserve continued vigilance on the part of observers. It has already been stated that the bright spots vary rapidly; their motions are not uniform in rate, and what is now wanted is a large number of new observations. Does accelerated velocity occur with increased brilliancy of these objects? Are their alternating disappearances and revivals uniform in period? and are they really due to transitory obscurations of the same durable forms? Are the dark spots which frequently border the white spots implicated in effacing the latter? Many other questions like these are suggested by the curious behaviour of the markings, and the discriminating observer will know how to gather the materials likely to aid in answering them. The rotation-period has been already found in regard to many features; but this element may be re-investigated with profit, for the velocity of the spots offers a very complex problem for solution. Do the markings generally exhibit a retardation of speed as long as they subsist? Abnormal spots, such as those which made their apparition in the autumn of 1880, should be traced through any vagaries they may present; and peculiar shape or direction in the belts will also merit study, as possibly supplying facts of consequence. It will be important to learn whether objects in a certain latitude have a common rotation-period, or whether different forms give different times. The rate of motion shown by certain features may depend upon their character, and not so much upon their position in latitude.
The altitudes of the various markings affords another promising line of research. The appearances and changes of closely contiguous features may be expected to furnish useful data in this connection. Owing to their proper motions they apparently overlap each other at times, and in their alterations of aspect the observer may discover the clue to their relative heights. The subject is discussed in a practical and interesting way by Mr. Green (Memoirs R. A. S. vol. xlix. p. 264) and by Mr. Stanley Williams (‘Zenographic Fragments,’ i. p. 112), and these works should be consulted by everyone engaged in the study of Jovian phenomena.
It is unfortunate that the observer, in delineating this object, must perforce adopt an extremely hurried method of representing what he sees at the telescope. The planet turns so quickly upon his axis that forms near the central meridian become sensibly displaced in a few minutes; indeed, it has been stated that an interval of two minutes only is sufficient to introduce a change obvious to simple eye-estimation. In order, therefore, to complete a sketch, the utmost dispatch is requisite; for this object cannot be depicted from the combined outcome of several evenings of observation. The proper motions of the different features prevent this. With Mars, or any orb exhibiting markings relatively constant, collective results are extremely valuable, and more trustworthy than pictures depending upon an isolated observation.
Amateurs, in entering upon these observations, should be prepared for rapid changes in the apparent aspect of Jupiter caused by his rotation, and not hastily infer them to be real. They should also hesitate before placing confidence in any anomalous results obtained under indifferent seeing; for bad images have been directly responsible for many misleading announcements.
Occultation of Jupiter, Aug. 7, 1889.
Occultations of Jupiter by the Moon.—Phenomena of this kind are always awaited with keen interest by the possessors of telescopes; but it is rarely that all the circumstances are favourable. The first recorded instance appears to have been in A.D. 847. In 1792, on April 7, Schröter observed an occultation of this planet, with a desire to verify his suspicion of a lunar atmosphere. He saw that “some of the satellites became indistinct at the limb of the Moon, while others did not suffer any change of colour. The belts and spots of Jupiter appeared perfectly distinct when close to the limb of the Moon.” On Jan. 2, 1857, an occultation took place under conditions which rendered it visible to many observers in this country, and the most interesting fact elicited was that at emersion a dark border was seen attached to the arc of the Moon projected on the planet. Mr. Lassell described this dark border as “a shadowy line, in character, magnitude, and intensity extremely like Saturn’s obscure ring projected on the ball.” During the thirty years following 1859 only two occultations visible in England occurred, and the last of these, on August 7, 1889, was widely observed. On this occasion Capt. Noble and others redetected the shadowed edge of the Moon seen by Lassell in 1857. “It was a strongly marked shading, following the outline of the Moon’s limb.” At Bristol I recorded that, at the disappearance, the outer margin of our satellite was fringed with light where it crossed the planet; but at the reappearance this effect had vanished, and the appearance was perfectly normal. The disk of Jupiter, where it met the edge of the lunar disk, looked dusky by the effects of contrast; but I saw no marked shading with a sharply terminating boundary, such as appears to have been remarked elsewhere. As the planet emerged definition was superb, the belts were lividly distinct, and the spectacle was one of the prettiest that could be imagined. The red spot was going off the W. limb, and the disk was covered with belts; many of them near the poles were extremely narrow, like fine lines drawn with a sharp lead pencil. I used a 4-inch refractor, powers 65 and 145, with this instrument the foregoing sketch was made. The exceptional distinctness of the Jovian markings on this occasion shows that the proximity of the Moon has certainly no tendency to efface planetary details, but rather to intensify them[40].
On Sept. 3, 1889, an occultation of Jupiter was visible in America, and observed by Mr. Brooks at Geneva, N.Y., with a 10-1/8-inch equatoreal. His drawing, made from a photograph and eye-observations, shows nothing of a dark fringe bordering the Moon’s limb.