288 



KNOWLEDGE & SCIENTIFIC NEWS. 



[Dec, 1904. 



point of the orbit to Saturn, in the different years ; 

 (2), by lea\ing the eccentricity out of account and 

 simply considering the change of tilt of the apparent 

 orbit, treated as a circle. Prof. Pickering deduced it 

 by method (i), and I got the same result from method 

 (2), being obliged to treat the orbit as circular since I 

 had not enough material to determine the eccentricity. 



The fact that (i) and (2) are in complete accord 

 establishes the retrograde motion beyond reasonable 

 doubt, and renders highly improbable the suggestion 

 made by Mr. Monck and others that the satellite ob- 

 served in 1904 is a different body from that observed 

 in 1898 and 1900, both moving in direct orbits. For it 

 would be a most astonishing coincidence that two 

 independent satellites moving in direct orbits should be 

 so related that a single retrograde orbit of large 

 eccentricity should be capable of exactly simulating 

 the movements of both. 



The retrograde motion is still further confirmed by 

 the fact that the observations indicate that the node 

 of the orbit advances (about 3° per annum), since the 

 node moves in the opposite direction to the revolving 

 body. 



The sidereal period of Phoebe is 547 days, or exactly 

 i^ years ; the period from " New " to " New " is 

 about 26 days shorter, or 521 days ; this is also the 

 average period between successive inferior conjunctions 

 as seen from the earth. Thus is it alternately east and 

 west of Saturn, for about 260 days in each position ; 

 while its maximum elongation considerably exceeds 

 half a degree, as compared with 10 '.4 for Japetus, 

 II '.3 for Jupiter's satellite IV., the two greatest 

 elongations of satellites previously known. The mean 

 distance of Phoebe from Saturn is exactly eight millions 

 of miles, while the greatest and least distances are 

 9j and 6] millions. The inclination of its orbit to that 

 of Saturn is about 5°. 



Even as seen from Saturn, Phoebe would only ap- 

 pear like a tiny star of the fifth or sixth magnitude ; 

 so that it might remain undiscovered for ages by 

 imaginary .Saturnians, just as Uranus, although faintly 

 visible to the naked eye, was not discovered till 1781. 



Its diameter is estimated to be somewhere about 

 150 miles, slightly greater than that of Jupiter's 

 satellite V. The two satellites, though similar in size, 

 present a startling contrast in their motions; Y. is 

 remarkable for its proximity to Jupiter and its short 

 period of 12 hours; also for the very rapid motion of 

 the perijove, which makes two entire revolutions in a 

 year; this arises from the action of Jupiter's equatorial 

 protuberance. In Phoebe's case the perturbations pro- 

 duced by Saturn's oblateness and by the other satellites 

 must be insignificant. The solar perturbations, how- 

 ever, assume an importance which they do not possess 

 in the case of any other satellite except our moon. 

 Prof._ N'ewcomb estimates that the coefficient of the 

 cvection is about 40, three times the amount for our 

 moon; this will shift Phoebe some 2\ as seen from the 

 earth,^ .ind will, therefore, be a readily measurable 

 quantity, '{'he apse moves round Saturn about f" 

 annually in the srmie direction as Phoebe. It is 

 thought that the effect of Jupiter's action on Phoebe's 

 motion will also be appreciable; there is a good deal 

 of matter here awaiting mathematical treatment, and 

 it is not impossible that some further light may in- 

 cidentally be thrown on the theory of our own moon. 



Prof. Pickering gives some speculations re the bear- 

 ing of Phoebe's retrograde motion on the nebular 

 hypothesis. lie supposes the planets to have once 

 formed rings of matter revolving round the sun; then 



since the inner portion of the ring would revolve the 

 quickest, when the ring coalesced into a planet the 

 part next the sun would be moving quickest, i.e., the 

 planet would be rotating in a retrograde direction. 

 Thus he supposes that all the planets originally had 

 retrograde rotations, and next asserts that this state 

 of things was unstable owing to the action of solar 

 tides, which tended to turn the planet over so as to 

 make the direction of rotation the same as that of 

 revolution. This point will need careful examination 

 by our leading mathematicians, but if we assume it 

 provisionally it will explain a good many things about 

 the solar system. Phoebe is supposed to have been 

 born in very remote ages, when Saturn rotated back- 

 wards; while Saturn was turned over before the birth 

 of Japetus and the inner satellites. It also appears 

 that distant satellites, like our moon, Phoebe, and 

 Japetus, are compelled by the sun to move in planes 

 near the primary's orbit ; while near* satellites, such as 

 those of Mars, Jupiter, Saturn (7 inner), and Uranus 

 (presumably) are in the equatorial plane of their 

 primary. 



The theory explains the retrograde motion of the 

 Uranian and Neptunian svstems by supposing that the 

 solar tides have been too weak at such great distances 

 to turn the planets over, though Uranus would seem 

 to have been turned about half way, and Neptune one- 

 quarter of the way. Thus the new theory tends to 

 bring these outer planets into line with the others, and 

 to remove a difficulty which had always been felt with 

 regard to the application of the nebular hypothesis to 

 them. Going to the other extreme, the slow rotations 

 of Mercury ar^d Venus, which are now accepted by 

 many astronomers, would likewise find an explanation 

 in solar tides, so that these would seem to have left 

 their traces on the system from one end to the other. 

 They were doubtless much more powerful in distant 

 ages, when the planets were larger and more diffused 

 than they are at the present day. 



* The word " near " is to be understood relatively to the size of 

 the primary, compared with which Jupiter's satellites are all much 

 nearer to him than our moon to the earth. 



The Inner Nebvilace of 

 the Pleiacdes. 



Ry Dr. Max Wolf, F.R.A.S. 

 The original plate of the accompanying photograph 

 was taken by the writer with the " A " lens of the 

 16-inch Brashcar twin telescope, on December 22, 1902, 

 with five hours' exposure. The plate was carefully 

 backed with a black coating, so that the well-known 

 halation circles do not appear round the bright stars. 

 The accomp.'inying photograph has been enlarged three 

 times from the original negative. In order to bring 

 out the inner nebula; it was necessary to greatly pro- 

 long the process of copying, so that the fainter outer 

 nebulae have become over-darkened. The extremely 

 curious straight lines of nebulous stream are very well 

 seen. The inclined and partly doubled stream near 

 Electra* is a defect and not a true nebula. We see 

 that all the brighter stars of the Pleiades are 

 systematicallv connected by streams of nebulous matter 

 beautifully fine in structure. 



•Electra is the nebulous star 28 inches from the west side of the 

 plate, and 37 inches from the south'side. 



