178 



KNOWLEDGE 



[August, 1902, 



of a region of 8]>aco coutaininr; two millions of stars of 

 the same magiiituJc as our sun, wo siiall find that there are 

 500,000 stars more brilliant, but 12,500,000 Ktars fainter 

 than our sun. 



From the very succinct rrioiw' that I have just j^'iven 

 of the researches on the structure of the visible universe 

 effected during the last few years— and I have said nothing 

 about the distribution of tlie nebula; so as to economise 

 space, though this is a subject very intimately connected 

 with the constitution of the stellar system— it will be 

 evident, I hope, that not only has there been much new 

 material brought together, but that it is possil)lo to draw 

 several interesting conclusions from them. 



Perhaps after some years of assiduous work it will be 

 possible to establish more firmly on this ground of truly 

 international scientific effort conclusions whii'h at present 

 are vague and doubtful ; perhaps we shall soon see a little 

 more clearly into this sublime mystery of the structure of 

 the universe. In any case, a corner of the veil which has 

 covered it through so many centuries has begun to be 

 lifted. 



JUPITER'S GREAT RED SPOT AND ITS 

 SURROUNDINGS. 



By W. F. Denning, f.b.a.s. 



During the last quarter of a century no other planetary 

 marking has incited so much interested attention and 

 study from telescopic observers, as that familiarly known 

 as " The Great Eed Spot on Jupiter." The size of the 

 ohject, the striking aspect it exhibited in the years from 

 1878 to 1882, its beautifully elliptic form and the extended 

 duration of its visibility, have all contributed to render it 

 a feature of exceptional character. It may, indeed, be 

 almost regarded as unique, and it has certainly served an 

 excellent purpose in stimulating enthusiasm and directing 

 observers to the study of the physical changes affecting 

 the leviathan planet of our system. 



The spot has undergone some curious variations. It 

 seems liable to temporary disappearance amid the sea of 

 Jupiter's atmospheric vapours. But through all the vicis- 

 situdes and diversity which have affected it in recent years, 

 it appears to have retained an oval shape and a nearly 

 uniform size, though there is evidence to show that its 

 dimensions are now somewhat smaller than formerly. It 

 displayed an intense brick red colour in 1873, and also in 

 1878 and four following years, but it then faded into a 

 lighter tint, and, subsequently, became an elliptic ring 

 ■which showed signs of further decadence. Lately, the 

 spot has almost suffered obliteration, having resolved 

 itself into a faint dusky stain, darkest at the following 

 end, and forming but a very feeble relic of the magnificent 

 object presented to us nearly twenty-five years ago. The 

 eye is directed to the exact position of the marking by a 

 remarkable irregularity in the southern side of the southern 

 equatorial belt, in which there is a large bay or hollow 

 immediately north of the spot and partly enclosing it. 



Old observations have not yet been fully investigated, 

 in order to determine the date when the red sj^ot first 

 impressed itself upon the face of the planet. Under the 

 circumstances this will be a difficult point to decide, owing 

 to the want of continuity in the observational evidence. 

 Eobert Hooke, on I66i, May 9 (O.S.), was undoubtedly 

 the first to discover a large dark sj)<>t on the southern 

 hemisphere of Jupiter ; but he does not appear to have 

 made any further observations of it. J. D. Cassini, fortu- 

 nately, re-detected the object in the summer of 16(35, and 

 watched it during subsequent years. He ascertained that 



its period of rotation was 9h. 56m., and that it had a 

 somewhat precarious existence, for it alternately appeared 

 and di»ap]>eared at irregular intervals. 



Was the spot discovered l)y Hooke in 1664, and studied 

 by Cassini in later years, identical with the feature which 

 has enlisted such widespread int^-rest in our own day ? 

 There is evidence strongly countenancing the supposition 

 of identity. The latitude of Hooke-Cassini's spot was the 

 same as that of the great red spot, and the motions of the 

 two objects ai)pear to have been approximately the same. 

 But the ancient marking was a roundish object described' 

 as one-tenth of the apparent diameter of Jupiter, while 

 the great red spot is about one-fourth In fact, while the 

 old spot was about 8000 miles in diameter, the present one 

 when most conspicuous was 26,000 miles long and about 

 9000 miles wide. These differences occasion doubts, with- 

 out, perhaps, absolutely neirativing the theory of identity. 

 Possibly, with the lap.se of time, the spot has distended 

 its material in a longitudinal direction as a result of the 

 swift axial rotation of the planet. 



In past years there are many breaks in recorded obser- 

 vations of Jupiter, and this is particularly the case between 

 the results of Maraldi (1713) and Sylvabelle (1773). 

 Thereafter W. Herschel, Schroeter, Miidler, Schwabe, and 

 others obtained a considerable number of observations. 

 But there are many missing links and not a few contra- 

 dictions. It is, however, hard to believe that the red spot 

 could have been plainly visible during the last twentj' 

 years of the eighteenth century, and yet have escaped the 

 special notice of Herschel and Schroeter. 



Schwabe's drawings in 1831 appear to be the first un- 

 mistakably showing the hollow in the belt, which in recent 

 years has contained the red spot. Our knowledge of the 

 aspect and position of the latter enables us to follow it, 

 notwithstanding its extreme faintness. Seventy years ago, 

 when Schwabe's earlier observations were made, the ap- 

 pearance may have been precisely similar to that exhibited 

 at the present time, in which case the feeble ellipse, in- 

 lying the hollow, would easily have been overlooked in 

 Schwabe's telescope. Schwabe frequently delineated the 

 hollow under conspicuous outlines between 1831 and 1856, 

 and this feature probably owes its existence to repulsive 

 action exercised by the red spot. It is, however, rather 

 curious that, while the belts north of the spot evade and 

 bend abruptly away from the latter object, the southerly 

 belts occasionally bend towards and coalesce with it. 



The red spot is a boat-shaped object (as seen from a 

 vertical position), and it floats in a wide current situated 

 from about 20° to 35° of south latitude. This current has 

 always given a very equable rate of velocity, and the 

 rotation period has been found to be 9h. 55m. 19s. Dark 

 and white spots, and various irregularities on the belts 

 marking this region, all participate in this velocity, but 

 the red spot shows a singular departure from the rule, for 

 it travels slower than the involving stream, and loses 53 

 seconds per day (nearly half a degree), which is equivalent 

 to about 370 miles. The spot is, in fact, like a great 

 barge slowly drifting eastwards in a wide current. It is 

 probable that the rate of neither the current nor the spot 

 coincides with the exact rotation period of the globe of 

 Jupiter. We have learned the period of the small planet 

 Mars to the tenth of a second (24h. 37m. 22-65s.), for 

 some of the markings displayed on the ruddy Martian 

 surface represent permanent lineaments, which are invari- 

 ably yisible, though the atmosphere of that planet is apt to 

 affect their appearance in some degree. But the condi- 

 tions are very dissimilar in regard to Jujiiter, forming, as 

 he does, an immense orb probably in a very heated state, 

 and with his real features masked by dense gaseous 

 vapours undergoing frequent changes. 



