238 



KNOWLEDGE & SCIENTIFIC NEWS. 



[October, 1905. 



pelled by Jupiter to travel in ellipses — not, however, 

 around the planet, but around the sun. Until, there- 

 fore, some plausible suggfestion has been made of a 

 body that could have captured these satellites, not for 

 itself, but for Jupiter, the capture hypothesis can hardly 

 be resjarded as tenable. 



Si.u .) M.U ( M.,„ 



r f tht',thrcc outcrino5t 



^atelMtrs of Juplttr. 



Though the two orbits interlock, yet owing to their 

 large mutual inclinations the satellites cannot approach 

 each other within half a million miles or thereabouts, at 

 which distance such tiny bodies could not perturb each 

 other appreciably. ,As the nodes and pcrijoves are 

 moving fairly rapidlf, it is 

 possible that after some 



centuries the orbiis may in- »i p„, , r _ 



tersect. The prospect of an q,^ X yE^f^H^NPOLE 

 actual collision is, however, 

 very slender. 



Dr. Ross calculates that 

 the node of VII. retro- 

 grades i''.i5 per annum, 

 while the perijove advances 

 i''.45. He finds the co- 

 efficients of the annual equa 

 tion, e\ection, variation, and 

 principal solar perturbation 

 in latitude to be 0^.42, o''.38, 

 o*^. 12, and o^.^o respectively. 

 The corresponding fjuan- 

 tities for V I. are considerably 

 larger than these, owing 

 to its greater eccentricit). 

 When these and other 

 f)erturbations have been 

 accurately determined, the 

 two new satellites will give 

 a determination of the mass 

 of Jupiter, which will be 

 entitled to great weight. 



The diagrams of the poles 

 of the orbits, >)cc.,are given 



as the simplest way of illustrating- their situation rela- 

 tively to the primary's equator, near which the orbits 

 of the five inner satellites lie. Owing to the proximity 

 of the pole of \"I. to our North Pole, the satellite is 

 nearly due east or west of Jupiter at elongation. The 

 pules are prob:ibly moving round the pole of Jupiter's 

 orbit as in the case of our own moon, Init the time of a 

 revolution is probably at least two centuries instead of 

 iSi years. 



It will be seen that the orbits of both VI. and \'II. 

 are smaller than that of Phoebe, but their angular 

 distances at elongation are much greater, reaching to 

 1;}°. Their high inclinations produce most remarkable 

 twists and curves in their apparent motions seen from 

 the earth. A diagram is given showing their apparent 

 places at various dates extending from December 23, 

 1904, to November 13, 1905. The orbit of VII. is now 

 almost edgewise, and it must have nearly, if not quite, 

 passed across Jupiter's disc on July 18, but so faint an 

 object could not be seen when near its primary. 



The direct motion of these satellites is unfavourable 

 to the hypothesis suggested by Prof. W. H. Pickering 

 to account for Phoebe's retrograde motion. According 

 to this the planets originally rotated backwards, and 

 very distant satellites should retain this primitive 

 motion, while solar tides were supposed to have re- 

 versed the direction of the planet's rotation before the 

 later satellites were born. 



It c;m scarcely be accidental that retrograde motion 

 exists in the families of the three outer pl.Tucts, and in 

 these only. It must be confessed, however, that Prof. 

 Pickering's key to the enigma, which seemed so pro- 

 mising at first, can no longer be accepted with great 

 confidence, though it may be possible to modify it so 

 as to cover the new facts. 



The numeration of Jupiter's f.imily is now in a state 

 of confusion, the order reckoning outwards from the 

 primary being V., I., II., 111., IV., VI., VII. 



NPOL^ of XE'^'-""'''^ 



N.POL£ Of ^ JijP.'i ilai'troA 



2. if. L s 'O '1. 



,i JO 



5tiit o j Dtc 



OP "a. 



X 



Dlajcram shiiwin^ the f'o<iition.s of the Poles of the Orbits of VI. and \'ll., with 

 reference to Jupiter's bquator and licliptic. 



