178 



KNOWLEDGE. 



June, 1915. 



eccentricity, 0-163 ; period, 3-125 years. The motion is 

 retrograde, as in the case of the eighth satellite. These 

 elements are only approximate, both because the arc of 

 observation is short, and also because the orbit dilTers 

 markedly from an cUipse, owing to tlic great perturbations 

 lirinlucod by the Sun. The late Dr. Hill showed that a 

 direct sateHite with five lunations iu its primary's year 

 would have its Syzygy Axis less than the Quadrature Axis 

 in the ratio 10/11. Mr. J. Jackson (Monthly Notices, 

 December, 1913) has investigated the case of retrograde 

 motion, finding that the ratio of axes would be 16/17 in 

 an orbit nearly the same as that of IX. Stable motion is 

 possible at a greater distance from the primary when the 

 motion is retrograde. The extreme distances of IX from 



pair. A similar relation holds between the sixth and seventh 

 satellites, whose periods arc about two hundred and fifty 

 and two hiuiflrcd and sixty days respectively. 



ti:mi'i:i.'s comet and the planet uranus. 



— Professor Samjison has made an examination of 

 Lcverrier's suggestion that this comet and the Leonid 

 meteor swarm were captured by Uranus and their period 

 changed to thirty-three years in the year a.d. 126. So 

 great is the weight justly attached to all Leverrier's cal- 

 culated conclusions that even his unproved suggestions 

 (like the present one) are treated in most textbooks as though 

 they were undoubted facts. But a moment's consideration 

 will show that such a conclusion cannot be accepted as 



D 





EO 



EAST 



WEST 



^ »- ^ 



B 



O 



C<3-^ 



SOUTH 



Figure 156. 

 Tracing from the plates on which Jupiter IX was discovered, 

 ABC are the positions of IX on 1914, July 22, 23. 24. 

 D E F are the positions of VIII on the same days. 

 The exposure on each day was about 2J hours, so that the star-trails are about one-tenth as long as the daily motion of 

 the satellites. It will be seen that the rates of motion of VIII and IX among the stars are very nearly the same. Con- 

 sequently the images of IX are points. The rings round the satellites are merely drawn to facilitate identification. 

 The distance from A to C is about 12'. Jupiter was iV east of IX and 13' south of it. 



Jupiter are twenty-three and fourteen million miles, which 

 arc comparable to the distances between the orbits of the 

 terrestrial planets. 



VIII and IX will ultimately give a very reliable value 

 of the mass of Jupiter, but observations covering many years 

 will be required for this purpose. 



MINOR PLANETS. — A note on these tiny planets fits 

 in appropriately after the consideration of the small outer 

 satellites of Jupiter. The work of search, now almost en- 

 tirelj- done by photography, and of observing known planets, 

 has been definitely organised and distributed. There is as 

 yet no evidence that we are approacliing the exhaustion of 

 the zone. Last year more than sixty discoveries were 

 announced, but a certain percentage — possibly one-third — 

 of these will prove to have been observed before. Nearly 

 eight hundred planets have now received permanent 

 numbers, some fifty more have had approximate orbits 

 calculated, while some two hundred more are known to 

 exist, but have not been observed sufficiently for the cal- 

 culation of orbits. Two long-missing planets have lately 

 been recovered. One — 99 Dike — had been missing since 

 1868, the year of its discovery. It was the only missing 

 planet in the first hundred, so its recovery is a matter for 

 congratulation. 353 Ruperto-Carola, missing since 1893, has 

 also been rcobserved. 



The Trojan group, which forms approximate equilateral 

 triangles with the Sun and Jupiter, are being kept under 

 observation. The theory of their motion is very interesting. 



JUPITER'S NINTH SATELLITE.— A later determina- 

 tion of the orbit of this satellite has been published, which 

 makes the period two years two months — -very nearly the 

 same as that of the eighth satellite. These two satellites 

 had probably a common origin, and form a connected 



certain unless we know with absolute accuracy the history 

 of the swarm for every moment of time from a.d. 126 to 

 the present. Such accuracy is altogether unattainable in 

 meteoric orbits. The probable error of the Leonid radiant 

 is about a degree. Different members of the swarm have 

 sensibly different tracks. The perturbations calculated 

 by Professor Adams were only mean perturbations, and 

 might differ very considerably from the true perturbations 

 in a period of finite length. Moreover, Professor Sampson 

 has examined Leverrier's papers, and finds that all he did 

 was to see that the node on Uranus' orbit was in the right 

 position in 126 ; he never examined the position of the 

 major axis of the orbit, so that he was not acting with his 

 wonted caution and accuracy in making his suggestion of 

 capture. Professor Sampson shows that the date a.d. 126 

 for the capture is most improbable, and that a.d. 885 

 satisfies the data much better. However, for the reasons 

 given above, I think that any date for the occurrence should 

 be treated merely as a more or less probable suggestion. 



The late Mr. R. A. Proctor suggested that the comet 

 families of the giant planets (with their attendant meteor 

 swarms) were not captured by them, but were expelled from 

 their interiors in mighty eruptions. It appears to me that 

 this theory has been too hastily rejected by astronomers. 

 Jupiter, the giant planet that we know the best, exhibits 

 at times on his surface clear evidence of tremendous activity ; 

 and there is no necessity to consider the expulsion of 

 comets as a frequent occurrence. About two births per 

 century in the case of Jupiter, and still less in the case of 

 the other giant planets, would suffice to balance the wastage. 

 The objection has been made that, if the eruption lasted 

 many hours, the rotation of the planet would cause a great 

 dispersion of the resulting orbits. However, the total mass 

 of comets or meteor swarms, cosmically speaking, is small ; 



