468 



NATURE 



\_March 15, 1888 



ON THE APPEARANCES PRESENTED BY 

 THE SATELLITES OF JUPITER DURING 

 TRANSIT. 



A PAPER was read by Mr. Edmund J. Spitta, at the 

 November meeting of the Royal Astronomical 

 Society, of especial interest to those who have devoted 

 their attention to Jovian phenomena. As the paper itself 

 is a long one, being the result of over four years' work, we 

 must refer our readers for details to the paper itself; but, 

 speaking briefly, the author observes that since the 

 discovery of the satellites by Galileo in 1610, astronomers 

 have been puzzled by their discordant appearances during 

 transit, but more especially by the fact that these pheno- 

 mena do not apply equally to all the satellites, or even 

 in some instances to the same satellite in two successive 

 revolutions. It appears that notably the fourth— the 

 farthest from its primary — as it approaches the disk of 

 Jupiter, becomes rapidly and increasingly fainter until it 

 arrives at contact. When once on the limb it shines with 

 a moderate brilliancy for about ten or fifteen minutes, then 

 becomes suddenly lost to view for another period of about 

 the same duration, and lastly reappears, but as a dark 

 spot which grows darker and darker until it equals the 

 blackness of its own shadow on the planet. The appear- 

 ance presented by the second satellite, however, is entirely 

 different, for it seems never to have been seen otherwise 

 than pure white during transit ; whereas the first and 

 third differ yet again from the preceding two. The former 

 is sometimes a steel-gray, and at others a little darker, 

 whereas the latter has been seen perfectly white, and yet 

 so black as to be mistaken for the fourth ; both appear- 

 ances having been witnessed by Maraldi as far back as 

 1707, and that too in successive revolutions. 



The author seems to have spent some years in examining 

 these phenomena on all possible occasions, and under 

 different conditions, such as before, during, and after 

 opposition ; and to have collected all published and un- 

 published observations ; and also to have devised an 

 occulting eye-piece — movable shutters in the focus of a 

 Ramsden eye-piece — for the express purpose of shutting 

 off the light of Jupiter ; but, to use his own words, 

 "without adding to the pre-existing knowledge of the 

 subject." 



The fact of having witnessed, when on the banks of the 

 Rhine in 1886, the transit of a brilliantly illuminated ship's 

 lantern as a dark spot on the disk of the rising full moon, 

 suggested the carrying out of a series of experiments to 

 ascertain the proportions of light which two bodies must 

 possess, so that the smaller should appear gray or black 

 when superimposed on the larger ; and it was hoped that 

 if the facts and figures thus experimentally obtained 

 corresponded with the albedos of the satellites themselves 

 as compared with Jupiter, it would not be unreasonable 

 to suspect that the abnormal appearances presented by 

 the satellites depended on functional idiosyncrasies of the 

 eye itself, rather than upon physical peculiarities of the 

 Jovian system. 



Space will not allow a description of the experiments, 

 which were somewhat numerous, the photometer employed 

 being an adaptation of that arranged by Prof. Pritchard, 

 of Oxford; but, speaking in short, small disks of different 

 tintings of Indian ink, representing the satellites, were 

 superimposed on larger ones of various sizes of pure white 

 cardboard, and it was found that, with certain restrictions, 

 the difference of albedo (a term expressing " the relative 

 capacity for reflection of diffused light from equal areas ") 

 between the smaller and the larger caused the gray and 

 black appearances, and that they were not due to any 

 difference in the quantity of light reflected from either. 

 For a moon to appear gray or black, a difference of albedo 

 was required of 042 in the first case, and of o'87 in the 

 second, whilst moons of a superior albedo remained white 

 during transit. 



Further, the effect of one moon approaching another. 



was gone into, and the fading of the smaller was 

 likewise found to be in direct proportion to the rela- 

 tion its albedo bore to that of the greater, and was in 

 no way connected with the amount of light reflected by 

 either. The effects in the appearance of the same Httle 

 moons when in transit over different portions of a sphere 

 were also studied, and, strange as it may seem, t/ie whole 

 flf the phenomena of the dark transit were thus accidejitally 

 reproduced^ and this caused much surprise, seeing it was 

 brought about by such simple means. The concluding 

 experiments consisted in photometrically ascertaining, for 

 the first time, the reflective ability of different portions of 

 an unpolished sphere ; and the results obtained are set 

 forth in the following abridged table ; column .1 giving 

 the exact angle of the observation, and column 2 the 

 resulting albedo. 



30 

 40 

 50 

 60 



65 

 70 



75 

 80 



83 

 86 30' 



735 

 •500 



•367 

 •323 

 •261 

 •172 



•133 

 •080 

 •049 

 •027 



A large number of facts and figures having been ascer- 

 tained, attention was then directed to obtaining the relative 

 albedos of the real satellites themselves as compared with 

 Jupiter. The reduction of the observations was attended 

 with several difficulties, each of which had to be dealt with ; 

 but one of them especially deserves a passing mention, 

 and it is this, viz. that the eye does not seem to be im- 

 pressed in the photometer with the light coming from an 

 object of sensible area, such as Jupiter, to the same 

 extent as it is from a point of light such as is shown by 

 the satellites. A suggestion from Capt. Abney, however, 

 relieved the difficulty, and, this systematic error removed, 

 the results came out in an extremely satisfactory manner, 

 for it was then found that the albedos of the satellites corre- 

 sponded very approximately with the requirements of the 

 experiments, as the following abridged table shows ; in 

 column I is shown the number of the satellite, in column 

 2 its difference in magnitude with that of Jupiter, and in 

 column 3 the resulting albedo. 



Thus is it shown to be more than probable that the 

 reason the fourth satellite is uniformly black during 

 transit, when it has passed its period of disappear- 

 ance, is owing to its albedo being so low as to grant 

 the difference between it and the background necessary 

 for a body to appear black when superimposed on another 

 as ascertained by the experiments. Its preliminary 

 whiteness and disappearance are also shown to be a 

 question of relative albedo, for they are due to the fact that 

 a sphere at its limb and edges loses so much in reflective 

 ability, that up to that moment, the satellite possesses 

 sufficient albedo (as compared with the background in 

 that situation) to maintain its whiteness. So too with the 

 second satellite : its albedo proves to be so high that it is 

 capable of preserving its brilliancy throughout the entire 

 transit. The third and first satellites evidently possess 

 sides of differing albedo, one high enough to maintain a 

 brighter aspect than the other, or even, as in the case of 

 the third, to make it appear white when one side is 

 presented to the earth, and dark when the other. In 

 conclusion, to quote from the original paper, "it is not un- 

 reasonable to conclude that these anomalous phenomena 

 are due to functional idiosyncrasies in the eye itself, rather 

 than to physical peculiarities of the Jovian system." 



