December, 1905. 



KNOWLEDGE & SCIENTIFIC NEWS. 



291 



The R.ed Spot OLnd 



SoutK Tropical Spot on 



Jupiter. 



The red spot has exhibited a very fluctuating rate of 

 motion during the past five years. In 1877 the rotation 

 period of this marking was gh. 55m. 33"4S., and it showed 

 an uninterrupted, though slightly variable, slackening of 

 speed until igoo, when its period conformed with that 

 adopted for System II. of Marth-Crommelin's Ephemeris, 

 viz., gh. 55m. 40-633. Since igoo, however, the rate has 

 oscillated between gh. 55m. 38s. and gh. 55m. 42s. In 

 the present year, as in igoo, the object has been running 

 nearly level with the rate of System II., and its longitude 

 has not differed materially from 26' since August, 1904. 

 The following are some observations selected from a 

 large number of transits obtained at Bristol in recent 

 years. 



During the period under review the variable motion of 

 this marking has originated differences in longitude 

 amounting to about 23°. Marked accelerations occurred 

 in 1902 and 1904, while the motion was retarded in the 

 last half of 1903, and during the present autumn another 

 decided slackening appears to be in evidence. 



To what cause these irregularities have been due 

 appears somewhat doubtful, but it has been conjectured 

 that a large dusky marking, first seen in the spring of 

 igoi, in about the same latitude (south tropical zone), and 

 moving at a more rapid rate than the red spot, has been 

 the means of quickening the speed of the latter. This 

 south tropical disturbance has a rotation period of nearly 

 gh. 55m. igs., and, as it overtakes and passes the red 

 spot, may well exercise some accelerating influence upon 

 it. There is indeed strong evidence to support this con- 



clusion, but more observations are required. The two 



markings were in conjunction in igo2 (July) and igo4 

 (June), and the phenomenon will be repeated in igo6 

 (May) if the rate of motion of the red spot remains 

 approximately the same as during the past twelve months. 

 Taking the mean rotation period of the latter marking in 

 recent years as gh. 55:11. 40s., and that of the south 

 tropical spot as gh. 55m. igs., there is a difference of 

 21 seconds in their times, and this indicates that conjunc- 

 tions must occur at intervals of about 700 days. 



Between the present time and next April (Jupiter will 

 be very near the sun in May and June, igo6) it will be 

 important to secure a large number of accurately observed 

 transits of the red spot for the purpose of determining 

 whether there is any decided increase in its velocity. 

 The south tropical spot is now some 1 10° east of it (about 

 3 hours following) and the distance is rapidly closing up. 

 If, therefore, this south tropical disturbance is responsible 

 tor the longitudinal displacements of the red spot, the 



Variations in motion of the Red 5pot during^ last eight years. 



The diagram exhibits the variable motion in longitude of the red 

 spot in and since the year i8g8. The conjunctions of this marking 

 with the south tropical spot are also indicated. An inspection of 

 the diagram will show that the motion of the red spot appears to 

 have been accelerated at the time the south tropical spot was first 

 seen, and that both before and after the conjunctions of 1902 and 

 1904 great accelerations occurred. 



fact will shortly become apparent in a marked increase 

 of the normal rate of the latter, and we shall find its 

 longitude becoming less. 



In recent months the S. tropical spot has presented a 

 remarkable development, and its length has been aug- 

 mented from 43°'5 to about 60". My observations of the 

 p. and f. ends of this object during the past four months 

 have been as under : — 



The increase of length between August 7 and October 19 

 was 15 ', which corresponds to a real distension of about 

 11,000 miles. This enlargement must have occurred 



