THE PLANET JUPITER IN OCT. AND NOV. 1880. 
3 
either of decay or further development be presented, they will be 
quickly noticed by the host of observers who are engaged in 
watching what is certainly one of the most singular and attrac- 
tive of the astronomical phenomena of our time. 
Some important purposes will be served by the long endu- 
rance of this unique object. It will give us the means 
of computing, with greater facility and accuracy than was ever 
attempted before, the period of rotation of Jupiter. This has 
been already attempted with some success, and the resulting 
values in two instances are 9 h 55 m 34 s 2 and 9 h 55 m 33 s, 9. 
These were derived from observations during the opposition of 
1879, and they show that the period of 9 h 55 m 26 s , which has 
been commonly adopted hitherto, is fully 8 seconds short of the 
true interval. But though the new determination of 9 h 55 m 34 s 
above referred to may be relied on as extremely exact, it should be 
suggested that the work has been undertaken prematurely. It 
is advisable not to be too sanguine of results depending on short 
periods, however carefully the details have been worked out. 
The final discussion of the question must be left to a future 
time, for it will be obviously preferable to rely on a period 
extending over several years than on a period extending over a 
few months only. The planet, as observed on two dates, say 
four months apart, has performed less than 300 rotations, 
whereas, if the interval is extended to four years, the number 
of rotations will be more than 3500 ; and seeing that the large 
spot now visible is likely to endure for a period at least equal 
to this, we can well postpone the ultimate analysis to the time 
immediately preceding the end of its visible existence. We shall 
then be enabled, by a comparison of a few of our best observa- 
tions, to deduce the period with more precision than could result 
from a mere partial investigation. 
But just as astronomers had been congratulating themselves 
upon such excellent means of finding the length of the J ovian 
day, we had the announcement that the markings upon the disc 
of the planet are influenced by a variety of different motions. 
The great red spot, which has created so much stir amongst ob- 
servers of planetary phenomena, cannot keep pace with the 
white spots which alternate with the darker areas around the 
shaded equatorial girdle of the planet, nor do the black spots of 
smaller type (some varieties of which have lately been spread 
along a faint belt in the north hemisphere) maintain permanent 
positions, either with respect to each other, or to the other 
markings on the surface. In fact, as observed on different 
nights, the appearance of the planet is vastly altered ; a series 
of rapid currents, probably atmospheric, has displaced the 
relative positions of the chief obj ects. While the great red spot 
in the southern hemisphere makes its regular apparitions at 
