SCIENCE. 



387 





Latitude. 



Longi- 

 tude. 



Interval Be- 

 tween 

 Extreme Ob- 

 servations. 



Rotation. 



White Spot 



White " 



White '« 



Black " 



Black " 



Mean of all 



+ io".46 

 — 11". 62 



— Il".62 



+ 10". 40 

 + o"-70 



3" 57 m 

 4" 2 6 m 



O 1 ' OC m 

 2 h 22 m 



2 months. 

 2 '* 

 2 14 

 2 tl 



9 h S5 m 39-3" 



3 I.O a 



33. 6 8 

 31. o» 

 4°-5" 









9 1 ' S5 m 35.1 s 



The latitude is simply the measured distance north or 

 south of the Jovian equator, reduced to the mean dis- 

 tance of the planet from the earth. The zero of longitude 

 is the center of the great red spot. 



The white spots were egg-shaped, about 1" of arc iu 

 length, and were only visible under favorable atmo- 

 spheric conditions. 



The rotation period derived from the small spots indi- 

 cates an average displacement during two months of 

 2" of arc, or 4,600 miles, or an average drift in longitude 

 of nearly 3 miles per hour. 



ROTATION FROM EQUATORIAL SPOTS. 



From July 8 to October 1, 1880, comprising a period of 

 85 days, the longitude of a white spot, between the 

 equatorial belts, in latitude 2 ".3, was observed on 10 

 nights. The rotation, as deduced from this spot, was 

 9 h 50" 1 00.56% representing all the observations within 

 0.3 of arc, showing that the motion, so far as we know, 

 was absolutely uniform. From October 28, 1880, to 

 January 30, 1881, during a period of 94 days, another 

 white spot, in latitude 2".8, and differing 20 deg. in 

 longitude from the first, was observed on 8 nights. 



The rotation was 9 h 50" 1 o9 3 .8, with uniform motion. 



If the great red spot is supposed fixed, then the mean 

 drift of the equatorial spots would be about 270 miles 

 per hour in the direction of the planet's rotation, or the 

 spot made a complete revolution around the planet in 

 about 42 days. 



The approximate diameter of the equatorial white 

 spots was 1 ".2 of arc, or 2,800 miles. 



These observations leave the true period of the rota- 

 tion of Jupiter in a very unsettled condition. The great 

 red spot was frequently measured to ascertain whether 

 it was subject to any marked change, in position, size or 

 shape. 



The following are the mean results for the two opposi- 

 tions of 1879 and 1 880, reduced to the mean distance 

 of the planet from the earth : 





1879. 



No. of Obs. 



:88o. 



No. of Obs. 



Length 





12'/. 25* 



9 



""•55 



20 



Breadth 





3".4fi 



8 



3"-54 



10 







6". 95 



8 



7". 14 



12 * 



* Re-computed with the constants of 188 . 



The position of the major axis of Ihe spot was 

 measured as follows, the number indicating the inclina- 

 tion of the axis of Jupiter's equator as compared with 

 Marth's ephemeris. 



1880, July 27 + 20.3 

 " Aug. 6 + 2°. 5 

 " Sept. 4 4. 2 . 9 

 " Dec. 3 4- 2 . 2 



1881. Jan. 17 — o°.8. Definition poor. 



These numbers indicate a remarkable degree of per- 

 manency with regard to the size, shape and position of 

 the spot, during the two oppositions. Our observations 

 do not warrant the assumption of any considerable 

 change since September 25, 1879. 



The actual size of the object, as seen with our teles- 

 cope, was as follows : 



Length, 29,600 miles. 

 Breadth, 8,300 " 

 The smaller telescopes make the approximate length 

 considerably less than the real value. 



POLAR BELTS. 



During the opposition of 1880 the polar belts were not 

 as sharply defined as during 1879, with the exception of 

 Nos. 2 and 3, the latter of which became very conspicu- 

 ous. During the month of June, when the planet was at 

 about mean distance, no trace of polar markings could 

 be seen. And it was not until July 4, when the distance 

 was 0.948, that the belts 2 and 3 were barely visible. 

 Markings on the southern hemisphere were first seen on 

 July 24, when the distance of the planet from the earth 

 was 0.888. 



The latitude of 2 and 3 was as follows : 

 1879. 1880. 

 No. 2, + 9". 78 + 9". 75 



No. 3, + 5".g8 + 5". 89 



EQUATORIAL BELT. 



The great equatorial belt remained without any 

 material change in size or position, as the following 

 measurements will show : 



1879. 1880. 

 Latitude N. Edge, + 2". 59 4- 2". 35 



Width 6" .77 7" .04 



During both years the position of the north-edge was 

 parallel to Jupiter's equator, as given in Marth's ephe- 

 meris. 



PHENOMENA. 



When a satellite crosses the disc of the planet it usually 

 disappears in our telescope, when one-fourth to one-third 

 across the disk, and reappears at an equal distance from 

 the preceding limb, proving that the center of the disc is 

 more luminous than the satellite. 



In the case of the first satellite, it is sometimes seen to 

 transit as a grayish spot, and remains visible when on the 

 middle of the disc ; such a phenomenon was observed on 

 December 10, 1880. 



On July 3, 1880, the second satellite during transit 

 passed almost directly over the center of the great red 

 spot, when it appeared sensibly as bright as when off the 

 disc. 



On November 1, 1880, I had the good fortuue to wit- 

 ness the transit of the shadow of the second satellite over 

 the center of the red spot, and, at the same time, the 

 transit ot the shadow of the first satellite over the disc 

 of the planet. 



The shadow of the satellite, when fully projected on 

 the red spot, was distinctly visible, but not quite as black 

 as the shadow on the disc, proving that the red spot, 

 although much less luminous than the disc, was yet 

 much more luminous than the shadow. 



THEORY OF JUPITER. 



The generally accepted theory is, that the planet Jupi- 

 ter is surrounded by a dense atmosphere, that the belts 

 are the solid portions of the planet, and that the minor 

 spots are clouds floating in the atmosphere. It is diffi- 

 cult, if not impossible, to reconcile the known phenomena 

 with any theory yet proposed. But whether there are 

 a sufficient number of well determined facts to form a 

 better one, is doubtful. 



Accurate observations are needed on the markings 

 seen at different times on his disc ; not sketches and gen- 

 eral statements, but suitable micrometer measurements, 

 from which may be deduced the motions and changes 

 taking place on the surface. And until this method is 

 pursued there is but little hope of solving the problem of 

 his physical constitution. 



It has occurred to me, however, that the known phe- 

 nomena might be explained in the following hypothesis, 

 viz : the surface of the planet is covered with a liquid 



