10 



KNOWLEDGE 



[Jakuaby 2, 1899. 



Strong reasons hav6 been adduced to show that Saturn 

 is one of the youngest, if not the very youngest planet of 

 our system. The chief of these is its low mean density. 

 Density increasing with age, a very low mean density is 

 only compatible with youth. "This noble ball," says 

 Ah V-^/rV'.^t'Webb in his fascinating style, " has so little density that it 

 ^ — T~'^Vould float like oak on water, which is actually heavier, 

 and, therefore, if any were found there, would sink to its 

 centre."* 



The changes in the mottled appearance of the planet's 

 belts, brought forward by modern discovery, is another 

 argument in favour of its youth. At the distance of Saturn 

 solar energy seems too enfeebled to produce the phenomena 

 actually seen ; so that we are obliged to have recourse, for 

 their interpretation, to the assumption that the planet 

 still retains a part of its original heat, and we have seen 

 that intrinsic heat in a given stage of a planet's life is only 

 a synonym of youth. But. inasmuch as the shadows of the 

 satelUtes in transit across the planet have been repeatedly 

 described as being quite " black," such heat is at best 

 dark heat, the oscillations producing which send through 

 the ether undulations of greater wave-length than those 

 appropriate to vision. 



The iirst feature which strikes the student when 

 observing Saturn in a powerful telescope is the rapid 

 waning of the luminosity of the globe towards the limb. 

 This appearance, which is also observable on Jupiter and 

 Uranus, is a necessary consequence of the law of irregular 

 reflection or diffusion of light, according to which the 

 intensity of the illuminated surface varies inversely with 

 the angle formed by the incident rays with the normal to 

 the surface. Such is the deportment of a white unpolished 

 globe illuminated by a powerful distant light. Con- 

 sequently, terrestrial analogies lead us to the probability 

 that the visible surface of the major planets would seem 

 to be composed of more or less uniform, dull oceans of 

 precipitated vapours — clouds — with scarcely any light- 

 scattering medium above. 



Diametrically opposed phenomena are shown by the 

 brighter limbs of the inferior planets, whose dazzling 

 atmospheres extend high above the ' cloud region.' Here, 

 as suggested by Sir William Herschel, the line of sight 

 encounters an increased number of luminous particles 

 towards the limb, whose luminosity is thereby, of course, 

 increased. 



In 1805, with a shght opening of the ring, Herschel 

 drew attention to a peculiar appearance of the globe of 

 Saturn, which presented itself as " square-shouldered." 

 Various explanations have been given of this remarkable 

 phenomenon, but none, to the writer's knowledge, that seems 

 plausible. Thus Proctor speaks of Herschel's observation 

 as a " discovery that the planet's outline occasionally 

 fluctuates in such sort that instead of the normal ellipse 

 we have abnormal peculiarities,"! and proceeds to explain 

 the phenomenon by a hypothetical variability of Saturn's 

 surroundings to the depth of sercral thousand mUfs, 

 becoming at times transparent, at others opaque. Ingenious 

 as the interpretation doubtless is, we must confess that it 

 strikes us a priori as somewhat forced and unnatural. 

 Strikingly abnormal planetary appearances are, as a rule, 

 products of the most trivial causes, and rather than 

 launch into the uncertainty of fearlessly explaining away 

 what we know next to nothing about, we ought to keep 

 our minds alertly vigilant to detect the unsuspected 

 action of some elementary physical law. Optical science 

 proves usually a valuable auxiliary in such cases, for more 



* " Celestial Objects," 4th ed., p. 172. 

 t " Old and ^'ew Astronomy," p. 627, 



Fig. 1. — Saturn's Dark 

 Polar Cap, 1895, August 9 

 (9i 0. G., power 220). 



than once innoxious contrast effects have given birth to 

 the most marvellous interpretations." Returning to the 

 question of the square- shoulder of Saturn, we see at a 

 glance that the usual darkness of his polar cap (Fig. 1) 

 must partly check irradiation, thus paving the way to an 

 optical " square-shoulder." This ought to happen, and 

 really happens, whenever the 

 opening of the ring is not very 

 considerable, and when the joint 

 action of both the sombre polar 

 caps is brought to bear in the 

 production of the optical illusion. 

 The abnormal outline of the 

 shadow cast by the planet on the 

 ring next deserves our close 

 consideration. Many eminent ob- 

 servers have seen this outline 

 assume a form scarcely compatible 

 with the theory. Proctor saw in 

 this a confirmation of his suggestion of a variable outline 

 of the ellipsoid, while Herr Wonaszek concluded that 



"the plane form of the system of rings 



assumes a conical curve on which the shadow of a 

 sphere can appear as a concave curve." I Somewhat 

 difi'erent are the writer's impressions. The shadow 

 has presented to him, during the last twelve years, 

 nothing truly abnormal ; nothing that could not be most 

 thoroughly accounted for by the known laws of optics. 

 The convexity towards the globe (Fig. 2) is only what we 

 ought to expect (u) from the effect of irradiation and 

 vagaries of vision in blunting the bright angles of the ring 

 along the shadow, and (A) from the varying luminosity of 

 the ring, and the consequent exaggerated effect of irradia- 

 tion towafds the outer edge of the 

 interior bright ring — the brightest 

 part of the system. 



The white spot bordering the 

 shadow of the planet on the ring, 

 to which Dr. Terby called the 

 attention of the scientific world in 

 March 188'.i, is another optical 

 phenomenon. Although entirely 

 Qlusory, this spot was so obvious 

 a featm-e to the writer at the time 

 to be readily accessible to a 



as 



Fig. 2. — Irradiation 

 and Contrast Effects in 

 the Shadow of Saturn, 

 1890, May 26 (3-in. O. G., 

 power 150). 



three-inch telescope (Fig. 2). In 

 1894, with a moderate opening of 

 the system, the spot was also very 

 remarkable, this time on the northern surface of the ring 

 (Fig. 4). In 1895 it was less distinct, while scarcely any 

 traces of it were noted in 1896, 1897, and 1898. It would 

 thus appear that a moderate opening of the system is, in 

 some way, essential to the production of this appearance 

 of contrast. 



As in the case of Jupiter, the temperate regions of Saturn 

 are marked by a dusky band of apparently variable intensity. 

 The equatorial zone is separated from the temperate lati- 

 tudes of both hemispheres by a prominent dusky belt, of 

 which broad duplicity seems a characteristic feature. One 



* Observers do not seem to pay sufficient attention to optical 

 products generally, and to contrast effects in particular. Contrast 

 explains the bright borders to the Martian '' Seas," the exaggerated 

 cusps of Yenus in dichotomy, and I have recently shown {Monlhlt/ 

 Notices, R.A.S., March, 1898) that the 22.5d. rotation period of Veuus 

 rests on a misinterpretation of contrasts between light and shade. 

 The self-same reason which prevents our seeing stars by daylight masks 

 the real surface of the planet Venus. Hence the hopeless defeat of 

 all attempts to determine her rotation period by visual methods. 



t BuUetin de Id SociAd Astronomique de France, 1897, p. 485. 



