Oct. 1 8, 1883] 



NATURE 



59i 



of them may be again observed during the ensuing winter, 

 although the apparent diameter of the planet is very 

 small and opposed to a critical investigation. Schiapar- 

 clli has, however, pointed out that " on January 1, 1884, 

 the position of Mars with respect to its solstice will be 

 identical with that which it had on February 13, 1882, 

 and its apparent diameter will be 13", that is to say, about 

 equal to the apparent diameters which the planet had 

 during the discovery of the parallel canals. Every tele- 

 scope able to distinguish a dark line o""2 in breadth on a 

 bright ground, and to separate the one from the other 

 two such lines when the interval between axis and axis is 

 o" - 5, could be employed for these observations and to 

 study the duplications if these should be reproduced." 



It is to be hoped that some of our best instruments 

 will be devoted to this work during the ensuing winter, 

 and that something of these curious features will be ob- 

 served before the favourable opposition of 1892, when 

 they will certainly be seen if ever. But there is the 

 alternative that they are possibly variable and subject to 

 periodical disappearances, though we can hardly consider 

 this probable in view of the permanent character of the 

 chief markings on the planet. 



Jupiter has always been an object of great attention to 

 the possessors of telescopes, not only on account of the 

 readiness with which the satellites may be perceived, but 

 also on account of the prominence and variety of his 

 belt scenery and the marked changes which it undergoes 

 from year to year. The well known red spot which ap- 

 peared in the planet's southern hemisphere, and which 

 first came prominently into notice in July, 1878, has 

 proved a great stimulus to observation of late years. 

 The large dimension of this spot, its intensely red colour, 

 its definiteness of outline, and its durableness have com- 

 bined to render it an object of extreme interest, and the 

 phenomena of this planet, whenever it is described in 

 future years, will never be complete without a reference 

 to this marvellous feature. Persistently visible through- 

 out a period exceeding five years, during which it has 

 co npleted more than 4500 rotations, it would be imagined 

 that it must afford an excellent means of fixing the rota- 

 tion period of Jupiter with a degree of exactness far sur- 

 passing all previous efforts. But the spot has shown a 

 retarded motion which constantly causes it to lag behind 

 its predicted place. In other words, the rotation period 

 has been lengthening. Mr. Marth found the average 

 time to be 9h. 55m 34 47s. from the observations between 

 1 878—1881, but the spot now crosses the central meridian 

 of Jupiter about two hours after its predicted times based 

 on the period just referred to. In fact its time of rota- 

 tion has lengthened fully three seconds during the past 

 two years. But even had this spot been influenced by a 

 perfectly accordant motion during the whole period that 

 it has been watched, we could not regard its time as 

 showing the true length of the Jovian day, for the disk of 

 this planet exhibits a variety of spots which generally 

 move much swifter than the red spot. On the equator the 

 markings rotate in 9h. 50m. 7s. Both the dark and bright 

 spots which alternate with each other on and near the 

 equator, and are included in the two principal dark belts, 

 participate in this rapid movement, and they show a fairly 

 regular period, so that it is impossible to decide at present 

 as to the true rotation of the planet's sphere. The brightest 

 spot of all has been attentively followed since November, 

 1880, and it is found that relatively to the red spot it has, 

 owing to its greater velocity, completed twenty-three 

 revolutions of Jupiter. In other words, during 1026 days 

 it'- swift proper motion has enabled it to sweep round, no 

 less than twenty-three times, the vast circumference of the 

 Jovian sphere ! 'What phenomena can possibly have 

 given rise to such a remarkable difference of motion in 

 objects of fairly permanent character ? The explanation 

 is involved in mystery, and may never be forthcoming, 

 but there can be no question as to the importance of 



following up these observations. The red spot now seems 

 unfortunately to be in a state bordering on extinction. 

 It has become so faint that it is only distinguished with 

 care under circumstances of favourable definition. 



Apart from the spots, which evidently offer a large field 

 of very interesting work, there are the belts, which are 

 constantly varying in colour, position, number, and in- 

 tensity. At every opposition of the planet a series of 

 sketches should be made of the aspect of the disk, for 

 some important issue may result from the comparison 

 of such drawings if existing over a long number of 

 years. Some of the leading features may be found 

 to recur at certain definite periods, for the drawings of 

 the present day show many curious forms bearing a 

 striking analogy to some observed at former times. In 

 any case a reliable set of sketches for each opposition 

 must give us an excellent basis for investigating the 

 varying features of this planet, and may afford us a clue 

 to some of the marvellous changes evidently progressing 

 upon his surface. Telescopes of very moderate size can 

 be usefully employed in these observations. I have seen 

 sketches of Jupiter made with a 36- inch reflector, iSi inch 

 refractor, iSj-inch reflector, and with many other instru- 

 ments ranging from 12 to 18 inches. I have also 

 examined a large number of similar sketches obtained 

 with refractors of from 4j inches to 6 inches, and re- 

 flectors from si inches to 6i inches, and carefully com- 

 pared them together. The large instruments appear to 

 have had no advantage whatever. Indeed, judging from 

 the amount of detail presented in the sketches, and from 

 the descriptions accompanying them, the small apertures 

 would seem to have rather the best of the comparison ! 



One explanation may be that the detail rendered 

 visible on Jupiter by large instruments is so extensive 

 that it cannot adequately be delineated during the short 

 interval available for sketching. The features change 

 very rapidly, owing to the planet's swift axial rotation, 

 and drawings made on different nights, when the longi- 

 tudes are assumed to be coincident, are not reliable, 

 because the different markings have become severally 

 displaced owing to their differences of motion. 



Saturn, though diversified with behs similarly to 

 Jupiter, is less attractive as an object for telescopic 

 study. The image of Saturn is beautiful as a picture, 

 but there is a sameness about it which observation, re- 

 newed again and again, has a tendency to make mono- 

 tonous. His belts are generally faint, and seldom show 

 spots of decided character. During the last opposi- 

 tion the planet's southern equatorial belt was very dark 

 and well defined, and exhibited differences in depth 

 of shading, but these were not sufficiently distinct to be 

 followed. On the equatorial margin of this belt the disk 

 was very bright, and immediately contiguous tc it are 

 signs of white spots, which would suggest very similar 

 phenomena to that discerned on Jupiter. Spots of suffi- 

 ciently definite outline to be observed with certainty upon 

 the globe of Saturn are extremely rare. Prof. Hall's 

 white spot of 1877 affords one notable example of this 

 kind, and doubtless the number of such observations 

 would be greatly increased were more attention directed 

 to this planet. Though sketches of Jupiter are frequent 

 enough, we rarely see attempts to delineate Saturn, and 

 thus we have comparatively few records as to the arrange- 

 ment of his belts. This is to be regretted on many 

 grounds. No doubt the rings of this planet monopolise ob- 

 servers to such a degree that they neglect other phenomena 

 which, though less attractive, may be of greater signi- 

 ficance and interest. Whenever such observations are 

 practicable, particular attention should be given to the 

 configuration of the belts, and a searching examination 

 made for any definite markings which may be sufficiently 

 obvious and permanent to be followed on successive 

 nights, and thus enable a new determination of the rotation 

 period. William Herschel found this to be loh. 16m. 044s., 



