1909] on Recent Results of Astronomical Research. 563 



while to consider this system a Httle in order to appreciate the 

 importance of the latest addition. [A diagram of the system was 

 shown.] Of the first four satellites not much need be said : their 

 discovery by Galileo in 1610 was one of the first-fruits of the inven- 

 tion of the telescope. It was to the study of their eclipses that we 

 owe one of the most epoch-makins; of all scientific discoveries, the 

 discovery that light moves with a finite speed and not instantaneously ; 

 and lest it should be supposed that they are now only of historic 

 importance, I may add that it has recently been suggested that it 

 may be possible ultimately to determine by their aid that extraor- 

 dinarily elusive quantity the motion of the aether relative to the 

 sun.* In actual size these four satellites are quite considerable, 

 being, in fact, larger than our moon. For 380 years these were all 

 that were known, and the remaining four that have been found in 

 quite recent times are very faint and minute objects. One hesitates 

 to give an estimate of their actual sizes, but, at a rough guess, pro- 

 bably Satellites YII and VIII are globes about twenty-five to thirty 

 miles in diameter. No. Y was discovered by Barnard in 1892, and 

 is remarkable for its closeness to the planet. It takes barely twelve 

 hours to go round, and as Jupiter rotates once in about ten hours 

 the satellite nearly keeps up with the planet. VI and VII are a 

 curious pair. The order and regularity which seemed to characterise 

 Jupiter's system is now quite disobeyed. There is a great gap 

 separating them from the inner satellites. Their orbits are very 

 nearly equal in size, in fact. No. VI takes about 250 days to revolve, 

 and No. VII only seven days longer. The orbits are both inclined 

 at 30° to the plane of the ecliptic, but in different directions, so that 

 they do not intersect, and there is no fear of a collision. The orbits 

 are actually interlocked. 



The natural place where one might expect to find new satellites of 

 Jupiter, and where, perhaps, new satellites may yet be discovered, would 

 be in the great gap which intervenes between the inner satellites and 

 these two ; but the eighth satellite was actually far outside them all. 

 Its orbit also is inclined 30^ to the ecliptic. No. VIII is altogether 

 a record-breaking satellite. When at the farthest point of its orbit 

 it is 21,000,000 miles from Jupiter, a distance suggesting rather the 

 interval between one planet and another than between an ordinary 

 satellite and its primary. Venus sometimes approaches the Earth 

 within a distance not much greater than this, and the minor planet 

 Eros occasionally comes very much nearer to us. The satellite will 

 take more than two years to go round the orbit, f 



* It would seem, however, to be impossible to separate this quantity 

 mathematically from the eccentricities of the orbits of the satellites, so that 

 it is unlikely that a determination can be made in this way. 



t A recent paper by Cowell, Crommelin, and Davidson shows that the 

 orbit is so far from being a closed curve that it is almost meaningless to 

 speak of a definite " period " of the satellite. 



