In the first place, they are of singular permanence for clouds. The fleeting forms we know as such assume in the Jovian air a stability worthy of Jove himself. In their general outlines, they remain the same for years at a time. “Constant as cloud” would be the proper poetic simile there. But while remaining true to themselves, they prove to be in slow, unequal shift with one another. Thus Jupiter’s official day differs according to the watch of the particular belt that times it. Spots in different latitudes drift round lazily in appearance, swiftly in fact, those near the equator as a rule the fastest. Nor is there any hard and fast latitudinal law; it is a go-as-they-please race in which one belt passes its neighbor at a rate sometimes of four hundred miles an hour. The mean day is 9ʰ 55ᵐ long.
Jupiter and its “great red spot”— a drawing
by Dr. Lowell, April 12, 7ʰ 0ᵐ-5ᵐ, 1907.
Jupiter and its “great red spot”— a drawing
by Dr. Lowell, April 12, 7ʰ 28ᵐ-42ᵐ, 1907.
A side-light is cast upon the Jovian state of things by the “great red spot,” which has been more or less visible for thirty years, and which takes five minutes longer than the equatorial band to travel round. Its tint bespoke interest in what might be its atmospheric horizon. Yet it betrayed no sign of being either depressed or exalted with regard to the rest of the surface. “In 1891,” as Miss Clerke puts it, “an opportunity was offered of determining its altitude relative to a small dark spot on the same parallel, by which, after months of pursuit, it was finally overtaken. An occultation appeared to be the only alternative from a transit; yet neither occurred. The dark spot chose a third. It coasted round the obstacle in its way, and got damaged beyond recognition in the process.” It thus astutely refused to testify.
Sun spots—after Bond.