NA TURE 



[May lo, 1894 



THE PLANET SATURN. 



IX these days, when the telescope is in more or less 

 common use, and so many have opportunities of 

 observing the heavenly bodies, it is interesting to look 

 back on the pist and survey in a general manner the 

 thoughts and ideas of those who in the earlier period of 

 observational astronomy were not so well equipped. To 

 take the case of the great Florentian astronomer, who 

 practically had the whole Cosmos, so to speak, at the end 

 of his telescope, since he was the first who surveyed the 

 objects in the sky with something in addition to the 

 naked eye — one can picture him sweeping with his 

 " optik tube" or small telescope the starry heavens, 

 and suddenly coming across the planet which we have 

 under consideration. Here he had an object which was 

 quite unique, and which, with his small power of magnifi- 

 cation, must have puzzled him considerably. 



In a letter to the Grand Duke of Tuscany, he refers to 

 Saturn as appearmg triple (tergeminus). Later, in a com- 

 munication to the Austrian Ambassador (November 13, 

 1610), he makes the interesting statement : " When I 

 observe Saturn with a telescope magnifying more than 

 30 times, the largest star appears in the middle ; of the 

 others, one lies to the west and the other to the east 

 in a line which does not coincide with the direction of the 

 equator, and seems to touch the central star. They ap- 

 pear to me as two servants, who wait upon the aged 

 Saturn, travelling with him and not departing from his 

 side. With a telescope of smaller magnifying power the 

 star appears elongated and of the form of an olive." 



Such, then, is the earliest telescopic observation of this 

 planet that we have on record, and it might be interesting 

 to pursue Galileo's inquiries a little further, and follow his 

 state of mind when these " two servants " disappeared, 

 as was the case in his later observations, causing him to 

 look upon his earlier observations as phantoms or 

 illusions. 



With us to-day the case is different, and what we see 

 in place of the " two servants '' is the beautiful series of 

 rings which girdle the planet in the region of his equator. 

 Huyghens it was who first announced this ring system, 

 and since then observations have shown many details of 

 great interest, both in the ring itself and on the planet's 

 surface. 



Many are the objects of in(|uiry which lead observers 

 to make a stuly of the appearance of this planet. The 

 ring system and its varied shades, the belts girdling the 

 planet's surface, the dark and light spots on the belts, the 

 period of rotation &c., are only a few that might be 

 mentioned. 



Recent oppositions have enabled much work to be done 

 in these lines, and the one just passed (April 1 1) has, we 

 hope, still more increased our stock of knowledge. About 

 the present time the planet and the brightest star in the 

 constellation of the \'irgin (Spica) make a fine pair in the 

 sky. Both are fairly bright objects, and Saturn is known 

 by the more golden hue with which he shines. At the 

 present moment .Saturn is retrograding, i.e. moving in 

 the westward direction, and his position about the present 

 time is to the north of Spica. The next stationary point 

 in his orbit will be reached on June 21, conjunction 

 occurring on October 23. 



With reference to the general brightness which the 

 planet and his ring system exhibit at different limes, 

 iJr. G. Miiller' has recently made some interesting obser- 

 vations. The light-conditions, on account of the rings, are 

 referred to as very complicated. If sufficient observations 

 be considered, he has found that distinct changes in 

 brightness are apparent depending on ihc phase of the 

 planet, while much more apparent and naturally greater 

 variations are noticeable, depending on the change 



■ " Publikaiion dei AiirophyiikiliKhcn OIn. ni Poudam," 8, No. 30, 

 Stock 4. 



NO. t 280, VOL. 50] 



of the plane of the ring from the line joining Saturn and 

 the earth. When the rings are broadest, the planet in 

 mean opposition shines a little brighter than Arcturus, 

 and when they are invisible Aklebaran may be taken as 

 their equal in brightness. In referring to some of the larger 

 light changes, such as those which occur at difi'erent times 

 with the planets Mars and Jupiter, the proportions here 

 developed do not, we are told, tell us anything. In 

 1S83-S5, for instance, the reduced magnitude at 

 opposition (o'S5) was a little brighter than that 

 for 18S0 (0-90), 18S0-S3 (08S), and 1SS6-8S (ogo). 

 Other magnitudes at opposition, eight in number, 

 have been derived by Sildel : thus in 1852 three values 

 gave the mean opposition magnitude as ri6±oo7, 

 the remainder (5), made from 1857-58, gave eg" ± 002. 

 At a later date (1862-65) Zollner from fourteen observa- 

 tions suggested a magnitude of 095 ; while Muller, in this 

 paper, after a formula of his own, obtains o'SS as the mean 

 oppositional magnitude. 



With regard now to the period of rotation of the planet, 

 Herschel, in 1793, was the first who studied this question, 

 giving its length as loh. i6m. 04s., a value, accurate as 

 he stated, " to much less than two minutes either way." 



Since that time several more minute discussions have 

 arisen, from which have resulted different values, among 

 which may be mentioned Prof, .'\saph Hall's period of 

 loh. 14m. 23'8s. ± 2'3s. 



The latest important results on this question are due to 

 Mr. Stanley Williams,' who has taken every pains for the 

 determination of an accurate value, and to free the 

 results from any possibility of their being inlluenced by 

 preconceived ideas. With regard 10 the method of 

 observation employed, and the details of the observations 

 themselves, we must refer the reader to the publication 

 mentioned below, but a brief summary of the results may 

 not be out of place. 



The observations were made in 1S93, and two kinds of 

 spots were observed: (i) dark spots upon a conspicuous 

 double belt in the northern hemisphere ; and (2) bright 

 spots in the equatorial zone. 



In the case of the former, the period was obtained 

 from numerous spots, but eleven of them have been used 

 as giving well-ascertained values, a table of these figured- 

 showing that they can be arranged into two classes, the 

 means of which are loh. I4ni. 2907 and loh. 15m. 0745. 

 Between these values there is a difference of over half .\ 

 minute, a quantity too large, judging from the way sets 

 of observations agree inter se, to be due to errors of 

 observation. 



With the bright spots a similar result is noticed, only 

 here the dil'ference is not the same. Out of the five 

 series of deduced values, four may be coupled well 

 together giving a mean value of loh. 12m. 59'36s. The 

 fifth or outstanding value is 13 seconds shorter than this. 

 These different values for the periods of rotation point 

 out pretty distinctly that the spots that have been ob- 

 served are by no means fixed relatively to the ])lanel's 

 surface, but are endowed with a proper motion of their 

 own. In the case of the dark spots, the surface material 

 must have rotated over half a minute more i|uickh 

 in the same latitude upon one side of Saturn than upon 

 the other. Mr. Stanley Williams summarises the results 

 of his discussion in the following words : — 



" Between N. Kronometric latitudes 17'' and 37' the 

 surface material of Saturn rotated in 1S93 at the rate of 

 loh. 14m. 2907s. + 027s. between longitudes 45^ and 

 140 , and at the rate of loh. 15111.0745. + o 56s. between 

 longitudes 175" and 340", whilst between longitudes 340° 

 and 45 there was a region in which the surface material 

 rotated at a rale intermediate between the above values. 

 " Between .N. Kronocentric latitude 6 and about 2' S. 

 latitude, the surface material of Saturn rotated in 1893 

 at the rateof loh. 12m. 59'36s. ± 0-275. between latitudes 



i Monthly Soticei of R.A. Society, vol. hv. No.-5, M.irch 1:94, p 297. 



