SATURN. 



SATURN. 



290 



the seventh in the order of distance from the central body, 

 following table will be found useful : 



The 



The periods and mean distances inserted in the foregoing table have 

 been taken from Sir John Herschel's ' Outlines of Astronomy.' The 

 period and mean distance of Hyperion have not yet been determined 

 with sufficient precision. 



The light of Japetus has been found to be subject to a variation 

 depending on the position of the satellite in its orbit. From this 

 circumstance it has been inferred, with a strong degree of probability, 

 that the satellite effects a complete rotation on its axis in the same 

 time as that in which it revolves around its primary, as in the case of 

 the moon revolving around the earth. By far the most considerable 

 of the satellites is Titan, which Sir John Herschel considers may be 

 equal in magnitude to the planet Man. 



The appearance which Satum presented to Galileo, when he first 

 ' turned his telescope on the planet, wan utterly inexplicable. The 

 illustrious philosopher remarked, that the planet seemed to consist not 

 of one but of three bodies, which almost touched each other and 

 always maintained the same relative position. The three bodies were 

 arranged in the name straight line, the largest body being in the centre, 

 and the two others on the east and west sides of it. Pursuing his 

 observations he found that the two lateral bodies continued gradually 

 to diminish, until at length, after the lapse of about two years, they 

 entirely vanished, leaving the planet quite round. He now felt much 

 alarmed, lest those who derided his discoveries as optical illusions 

 baring no real existence, should adduce this mysterious transformation 

 of the planet in support of their views. Writing to Welser on Decem- 

 ber 4, 1612, he says, " What is to be (aid concerning so strange a 

 metamorphosis ? Are the twp leaser stars consumed after the manner 

 of the solar spots ? Have they vanished and suddenly fled ' Has 

 Saturn perhaps devoured his own children ? Or were the appearances 

 indeed illusion or fraud, with which the glasses have so long deceived 

 me, as well as many others to whom I have showed them ? Now, 

 perhaps, is the time come to revive the well nigh withered hopes of 

 those who, guided by more profound contemplations, have discovered 

 the fallacy of the new observations, and demonstrated the utter im- 

 possibility of their existence. I do not know what to say in a case so 

 surprising, so unlooked-for, and so novel. The shortness of the time, 

 the unexpected nature of the event, the weakness of my understanding, 

 and the fear of being mistaken, have greatly confounded me." 



It was reserved for Huyghens to ascertain the real nature of the 

 appendage with which Saturn is furnished. He originally announced 

 his discovery in the form of an enigma, in a pamphlet entitled ' De 

 Saturni Luna Observatio Nova,' published in 1656. The logogriphe 

 ran thus : 



aaaaaaa ceecc d eeceo g h iiiiiii 1111 mm nnnnnnnnn 



0000 pp q TT tttt UtlUUU. 



In 1659 he published a work entitled ' Systema Saturnium,' in 

 which he reveals the real nature of his discovery, and gives a detailed 

 explanation of the various circumstances connected with it. Restoring 

 the letters of the logogriphe to their original places, they now stood 

 thus: 



Annulo cingltnr tenni piano, nnnquam cohirrente, ad ecllpticam inclinato ; 

 or, as thus translated : 



Th.- planet is encompassed by a slender flat ring, everywhere uncon- 

 Mated with its surface, and inclined to the ecliptic. 



Not liinsj ran be more convincing or beautiful than the explanation 

 which t|ji.< theory affords of the various pb< nc.ni.'ii.i |. -.nt.il by the 

 planet. \Vln-n tin- position of the planet in ita orbit is siu-h that tin: 

 plane of the ring paHM through the sun, the edge only of the ring 

 is exposed to the solar rays, and the extent of the illuminated surface 

 being very small, it is incapable of producing a sensible impression on 

 the visual organ. In this position, then, the ring is invisible, and the 

 planet presents a round appearance like the sun or the full moon. The 

 ring also disappears when its plane passes through the earth ; for 



ARTS AlfD SCI. DIV. VOL. VII. 



although one of its sides may then be illuminated by the sun, it is 

 only the edge which is turned towards the observer. Besides these 

 two causes of disappearance, which are of a transient nature, and 

 render the ring invisible only for a few days at most, there is a third 

 cause, which generally continues in operation during a longer period 

 of time, and produces a more lasting effect. When the planet is so 

 situated that the plane of the ring passes between the earth and the 

 sun, the unenlightened side of the ring is then turned towards the 

 earth, and consequently during the whole time that the planet is in 

 this position (which frequently extends to several mouths) the ring 

 will be invisible. The same theory affords an equally satisfactory 

 account of the different phases assumed by the appendages of the 

 planet during the period of its visibility. It is manifest that when 

 the plane of the ring passes through the sun, and when consequently 

 the ring ceases to be visible, the planet, if viewed from the sun, would 

 appear in the node of the ring. When the planet revolves from this 

 position, the sun commences to ascend above the plane of the ring, and 

 the latter in consequence becomes visible in the form of a very elon- 

 gated ellipse, gradually opening out in breadth. The ellipse continues 

 to approach towards a circular form until the planet has reached a 

 distance of 90 from the node of the ring, when the elevation of the 

 &un above the plane of the ring has attained its maximum. The ring 

 henceforth begins to contract, and the same succession of appearances 

 in a reverse order will obviously ensue as the planet revolves towards 

 the opposite node, where the ring agaiu will cease to be visible. The 

 ring will therefore complete the cycle of its phases in a period equal to 

 half a revolution of the planet, or in about fifteen years. The appear- 

 ances will not be materially different whether the ring be viewed from 

 the earth or the sun, except during the time that the planet is in the 

 vicinity of either of the nodes of the ring. At such a juncture, the 

 combined motions of the earth and the planet may cause the plane of 

 the ring to pass more than once through the earth, and the ring may 

 in consequence disappear and reappear twice before its plane has 

 entirely swept over the terrestrial orbit. 



Huyghens was originally under the impression that the plane of the 

 ring is parallel to the equator, and he consequently supposed its inclina- 

 tion to the ecliptic to be 23 30'. On a subsequent occasion, however, 

 he determined the inclination by actual measurement in conjunction 

 with Picard and he found it to amount to 31. Huyghens also fixc;l 

 the ascending node of the ring in 1 70 30* of longitude. Several suc- 

 ceeding astronomers determined the elements of the ring. The most 

 recent as well as the most complete investigation of the subject is due 

 to Beesel. The conclusion at which he arrived was that in the begin- 

 ning of the year 1800 the longitude of the ascending node of the 

 ring was 166" 53' 8''-9 and that the inclination of its plane to the 

 ecliptic was 28 10' 44"'7. He also found that the node of the ring 

 retreats upon the plane of the ecliptic at the rate of 46""462 

 annually. 



In 1675 Cassini found that the ring around Saturn consists in 

 reality of two distinct rings separated from each other by a dark 

 interval. This interesting fact was subsequently established beyond 

 doubt by Sir William Herschel. The following are the dimensions of 

 the planet and the two rings as assigned by M. Struve : 



Exterior diameter of the exterior ring . 

 Inner diameter of the exterior ring . , 

 Exterior diameter of the interior ring . 

 Interior diameter of the interior ring 

 Kqnatorial diameter of Saturn 

 Breadth of the exterior ring . . . 

 Breadth of the division between the rings 

 Breadth of the interior ring . . . 

 Distance of the interior ring from the ball 

 Equatorial radius of Saturn 



40"-095 



35"-289 



34"-475 



26"-668 



17"-991 



2"-403 



0"-408 



3"-903 



4"'SS9 



8"-995 



Them measures are supposed to refer to the mean distance of the 

 planet from the sun. At such a distance an object which would sub- 

 tend an angle of only 1 " would measure 4387 miles. This would give 

 78,927 miles for the diameter of the planet. The absolute dimensions 

 of the rings will of course bear a similar proportion to their respective 

 angular measures. Thus, it will be found that the diameter of the 

 exterior ring amounts to 175,928 miles. 



The thickness of the rings around Saturn would seem to be very 

 inconsiderable. This is plainly indicated by the circumstance that 

 when the edge of the ring is turned towards the observer, it is 

 generally found to be invisible, even when the most powerful tele- 

 scopes are directed towards it. From certain lucid protuberances 

 which Sir William Herschel remarked upon the ring, he was led to 

 suspect that it had a rotary motion around the planet. A closer 

 scrutiny of the phenomenon confirmed this impression, and he finally 

 concluded that the ring effects a complete rotation around the planet in 

 a period nl lOh. 32m. ISs'f. 



On the 15th of November, 1850, Mr. Bond, Director of Harvard 

 College Observatory, Cambridge, U.S., discovered a third ring around 

 Saturn, situated between the interior of the two rings already dis- 

 covered, and the body of the planet. The same phenomenon was 

 discovered by Mr. Dawes on the 19th of the same month, and, of 

 course, before intelligence of Mr. Bond's discovery had reached Eng- 

 land. The new ring is much fainter than either 'of the two exterior 



