August 4, 1898] 



NATURE 



Zl-^ 



T 



March 22 



April 15 



M 17 



I 



THE RED SPOT ON JUPITER, AND I/S 

 SUSPECTED IDENTITY WITH PREVIOUS 

 MARKINGS. 



HE outlines of the red spot are still faintly distinguishable j 

 on a night of good definition. With a lo-inch reflector | 

 and power of 312, I have obtained the following estimated 

 transits : — 



Spot on 

 Date central meridian Longitude 



1808 h. m. 



10 43 ... 236 



10 26 ... 226 



12 6 ... 236 



„ 18 ... 8 o ... 25-2 



,, 22 ... II 16 ... 24*9 



May 14 ... 9 25 ... 24 I 



June 7 ... 9 20 .. 25 9 



At the present time the spot follows the zero meridian 

 (System II.) of Mr. Crommelin's ephemerides in Monthly 

 Notices by 26 degrees, which is equivalent to 43 minutes. 



During recent observations the spot has not appeared to be 

 quite centrally placed within the concavity in the great southern 

 belt. Its position is slightly on the following side. 



Now that this singular marking has been watched for a 

 l^eriod of twenty years, the time may be opportune for referring 

 to the question whether it can be physically identified with the 

 large spot seen at intervals by Cassini, 

 Hooke and Maraldi about two centuries 

 ago, and with more modern observations 

 of somewhat similar formations by Key in 

 June 1843, by Dawes in 1857, by Lassell 

 and Muggins in 1858 and 1859, by Gledhill 

 and Mayer in 1869, 1870 and 1871, by 

 Ro.sse and Copeland in 1873, and by 

 Russell and Bredichin in 1876. In some 

 instances the features alluded to exhibited 

 a very suggestive resemblance to the red 

 spot, and were, moreover, situated in, or 

 nearly in, the same latitude. 



This question of identity, when the 

 details come to be considered, presents 

 so many difficulties that, though the 

 affirmative view has much in its sup- 

 port, it scarcely admits of definitive 

 settlement in respect to the more ancient 

 '>l)servations. For our knowledge of the 

 Ider spots we have to depend upon 

 Irawings of the planet ; and it is noto- 

 rious that delineations by different ob- 

 s -rvers are rarely consistent as to the form 



of an object, or accurate as to its position on the disc. Before the 

 apparition of the red spot in 1878, the great utility of taking 

 the times when the markings passed the central meridian of 

 Jupiter had not been sufficiently recognised, and such observ- 

 ations had been rarely attempted. 



Apart from the approximate character of former materials, 

 the extremely variable motion of the Jovian features presents a 

 serious impediment when we attempt to demonstrate the absolute 

 identity of any of them. Were the observed velocities equable, 

 and the spots permanent markings on the real surface, like those 

 discerned on Mars, the matter would be simplified, and we 

 should possess a well-assured base for investigation. It would 

 be easy to determine whether a modern spot occupied the same 

 longitude as one of its prototypes visible at a distant period. 

 Thu-si, the Kaiser Sea, as we see it to-day on Mars, can be un- 

 mistakably identified as one of the principal lineaments drawn 

 by Iluygens in 1659 and subsequent years. But the visible 

 markings on Jupiter appear to be quite of another character. 

 They are atmospheric details which display vagaries inducing 

 great changes of appearance and displacements in longitude, so 

 that we can only speak with confidence of individual markings 

 which have been retained continually under telescopic scrutiny. 

 It is true that a break of a few months in such observations 

 need not, in particular cases, be fatal to the identification of 

 markings. There must necessarily occur such breaks during 

 the interval when Jupiter is near conjunction with the sun ; but 

 notwithstanding this, there has been no difficulty whatever in 

 recognising the red spot at every reappearance of the planet 

 since 1878. When, however, there occur breaks of two or 



three years in observations of a supposed identical feature, 

 doubts are at once introduced by the lack of connecting links to 

 bridge over the intervals. This is the case affecting the various 

 features which are suspected to have been early representations 

 of the modern red spot ; there are many links wanting in the 

 chain of evidence necessary to prove their identity. 



I have been carefully comparing the various observations of 

 apparently analogous markings in the southern hemisphere of 

 Jupiter since 1857, with the view of associating them if possible 

 and discovering what rates and changes of motion influenced 

 them. The result of the examination has tended to strengthen 

 the idea that Gledhill's ellipse of 1869-70, Lord Rosse's and 

 Dr. Copeland's red spot of 1873, ^"^ Russell's and Bredichin's 

 oval spot of 1876 were really one and the same object. I 

 believe that all these observations are to be satisfactorily ac- 

 counted for on the theory of identity. Certainly there are 

 some small differences due to the approximate character of the 

 materials available for discussion. The times of passage of the 

 objects across the central meridian have in most cases to be 

 estimated from their positions as drawn either west or east of it. 

 But it must happen that, in getting transits from such rough 

 data, our resulting values will be sometimes erroneous to the 

 extent of 15 or 20 minutes, and occasionally perhaps it will 

 amount to 30 minutes. Even the latter quantity is not, however, 

 always a very serious item, for when the rotation of a spot has 

 to be derived from, say, observations extending over two years, 

 it only introduces an error of i second in the resulting period. 



TH£ GREAT EBB SPOT on Jl/PITER 



Va rial-ion /n Rota tt on Period 1869 to /898 . 



There is little doubt that the red spot before its remarkable 

 intensification of colour, and prior to freeing itself from the 

 obscuring material which apparently veiled it in 1877, had been 

 increasing its velocity of rotation. We know that after 1878 it 

 gradually slackened. When Gledhill first observed the spot in 

 the autumn of 1869, its period of rotation appears to have been 

 about 9h. 55m. 35s. Slightly increasing in velocity, the rate up 

 to the close of 1872, when Lord Rosse and Dr. Copeland 

 redetected the spot by means of the six-foot reflector, was 

 9h. 55m. 34'5s. It had been seen in the interim by several 

 others. Mr. Gledhill saw the ellipse resting on, and actually in 

 contact with, the great .southern equatorial belt on December i, 

 1871, and on January 5, 6 and II-I2 it was seen by Messrs. 



E. B. Knobel, II. Pratt and J. Birmingham respectively (Astro- 

 noinical Register, January and February 1872, and EnglisJe 

 Mechanic, September 13, 1872). Several others, including Dr. 



F. Terby, appear to have recognised it at about this period. 

 During the interval from Rosse and Copeland's observations in 

 the winter and spring of 1873, to Russell and Bredichin's in the 

 .summer of 1876, the mean period of the spot was 9h. 55m. 34s., 

 and between June 1876 and Dennett's observation of July 27, 

 1878, it had further decreased to about 9h. 55m. 33 5s. Sub- 

 sequently to this the motion of the spot has slackened until, 

 now, twenty years after Dennett's observation, its period is 

 9h. 55m. 41 '55., or 9 seconds more. The variation of motion 

 since 1869 can perhaps be graphically represented by a diagram. 



The slackening of its motion is still evident, but it is very 

 slight as compared with that which look place in the years from 

 1879 to 1884. 



NO, 1501, VOL. 58] 



