86 



SCIENCE. 



[N. S. Vol. XVII. No. 420. 



probable error on the rotation period, as 

 determined in this way, varies between 

 ± 0.02 sec. and ± 0.07 see. The curve is 

 perfectly smooth for the first six years, 

 showing that the motion of the spot was 

 very regular. Since that period the curve 

 is not absolutely smooth, which may be due 

 to the faintness of the object, and the 

 shifting of the center from which the meas- 

 urements were made, when the measures 

 were referred to the bay in the equatorial 

 belt. My measures, when the spot was 

 very indistinct, have been referred to the 

 center of the bay, and that may account for 

 the small irregularities in the curve during 

 the later years. From the diagram it is 

 seen that the rotation period of the planet 

 reached its maximum between 1898 and 

 1899, being 41.7 seconds. Previous to 1898 

 the spot had an apparent retrograde mo- 

 tion on the disk of the planet, and since 

 that time the spot apparently has come to 

 rest, and now has a direct drift around the 

 planet. The rotation period for the last 

 400-day interval is 39.75 seconds, but the 

 actual period at the present time is about 

 three seconds less than it was in 1898. 

 From the inspection of this curve, taken in 

 connection with the rotation period which I 

 found for 1870, it would seem to require a 

 long cycle to make the rotation period the 

 same as it was in 1879. The dotted curve 

 indicates the 'mean' rotation period at 

 any instant, counting from September 25, 

 1879. The 'mean' period for the interval 

 1879 to 1902 is 91^ 55°' 39.93^. 



In 1880, when the red spot was most 

 conspicuous, it was seen, when brought 

 on the disk by rotation, at 87 degrees of 

 longitude, or 2^ 35™ in time from the cen- 

 tral meridian, when its length was only 

 second of arc. "When the spot is wholly on 

 the disk its longitude is 71.4 degrees and 

 the apparent length 3.7". It is possible that 

 the rotation period may be connected with 



its visibility, viz., when the spot comes 

 back to the same rotation period it had in 

 1879 it may become more conspicuous and 

 reddish in color. This object has drifted in 

 longitude about three and one fourth times 

 around the planet since 1879, assuming the 

 rotation period at that time to be the true 

 rotation period of the planet. It seems to 

 me, however, more probable that the time 

 of rotation of the planet is longer than any 

 period hitherto determined, in which case 

 all objects would drift in the same direc- 

 tion. The object also has a motion in lati- 

 tude, and the total displacement in twenty- 

 three years has been 1.7", or about 4,000 

 miles drift in latitude. The rate of drift 

 in longitude and the visibility may possibly 

 be due to the greater or less submergence 

 of the spot in the material which composes 

 the surface of the planet. 



The diagram shows the mean latitude 

 of the red spot at each opposition cor- 

 rected for the elevation of the earth above 

 Jupiter's equator. It seems that during 

 these twenty-three years the spot has ap- 

 proached nearly 1" nearer the equator 

 than it was in 1879. The short time scale, 

 the vertical lines being intervals of 400 

 days, makes the displacement appear more 

 abrupt than it really is. The Jovicentric 

 latitude is given on the right hand of the 

 diagram. At the present time this is 

 about eighteen degrees. We might add 

 that this displacement in latitude of the 

 red spot is very much less than the dis- 

 placement of the great equatorial belt. 



The most conspicuous marking on the 

 surface of the planet is the great equatorial 

 belt, which is always visible. This belt 

 may appear as one belt, but usually is com- 

 posed of two portions lying on either side 

 of the equator of the planet. In 1880 it 

 was practically one belt extending without 

 break for a short time across the surface 

 of the equator. From the study of the 



