218 



PLANKTS 



\Vlirn looked at throngh a telescope it is seen to be 

 winsiderahly flattened at the poles, owing to iu 

 rapid revolution on its own axis ; and ite surface 

 is crooned in a direction parallel to its equator by 

 three or four dUtinct and strongly-marked belte, 

 and a few othent of a varying nature. S|>ote also 

 ap|iear and remain for some time on its surface, by 

 means of which it* revolution on its axis has lieen 

 ascertained. Separate spots give, however, slightly 

 different periods (spine even move past one 

 another), but a period of 9 hr. 5o min. 21 see. 

 is generally accurate. Jupiter has five satellites 

 the fifth discovered by Barnard at Lick Observ- 

 atory in 1892. The four discovered by Galileo, 

 eosify olmervable through an ordinary telescope, 

 have rendered immense service in the determina- 

 tion of longitudes at sea, and of the motion and 

 velocity of light. They were proved by Sir William 

 Herschel to revolve on their own axes in the 

 same time that they revolve round their primary. 

 The smallest is alxmt the same .-!/< as our Moon, 

 the others are considerably larger. The year of 

 Jupiter is 4332 -">H4 days. 



Haturn, next in position, is about 745 times 

 larger in volume, though only about ninety times 

 greater in mass, than the earth. Its apparent ilia- 

 meter when in opposition is 20'3", and there is a 

 considerable flattening towards the poles. Its 

 surface is traversed by dusky belts much less 

 distinctly marked than those of Jupiter, owing 

 doubtless in great part to its inferior bright ness ; 

 its general colour is a dull white or yellowish, but 

 the shaded portions, when seen distinct ly, are of 

 a glaucous colour. The most remarkable peculi- 

 arity of Saturn is its ring, or series of concentric 

 rings, each one parallel and in the same plane with 

 the others and with the planet's equator. The ring 



Fig. 2. Saturn, as observed by Trouvclot with the 

 26-inch Washington Kcfractor. 



is distinctly separable into three parts ; the two 

 outermost are bright like the planet itself, while 

 the innermost (called the 'Dusky' or '('rain-' 

 ring) U of a purplish colour, and is only discernible 

 throngh a powerful telescope. They are most prob- 

 ably coni|M>sed of n multitude of small satellites in 

 rapid revolution round the planet. They are not 

 always visible when Saturn is in the ' opposite' 

 half of its orbit, for when the plane of tin 1 linirs 

 (whose outer diameter is liWi.lKJil milesi ; 

 tween the Earth and the Sun, I heir dark surface 

 is turned towards us, and when the sun is in their 

 plane only the narrow edge is illumined : in both 

 of these CMOS the ring is invisible from the Earth. 

 Its plane being inclined at an angle of 28 to the 

 ecliptic, we see the two surfaces of the ring alter- 

 nately for |>eriods of fifteen years at a time ; and 

 at the middle of each period the rings attain their 

 maximum obliquity to the ecliptic, nnd are then 

 bent seen from the Earth. It is hardly necessary 

 to remark that at the end of each period they 



become invisible. Saturn has also no less than 

 ei^lit satellites, seven of which revolve round it in 

 01 1 pits little removed from the plane of the ring, 

 while the eighth, which is the second in size, is con- 

 siderably inclined to it. Two of the satellites were 

 discovered by Hersche) in 17H7 and 1789, four by 

 Cassini in lt>7'2 ami I(>S4, one by lluyejicns in 1055, 

 one by Mr Lassell in Kngland and 1'rofcssor Bond 

 in America in 1848. The satellites are all situated 

 out Mile of the ring, and the largest of them is nearly 

 equal to the planet Mars in size. The year of 

 Satum is 10,759-219 days. 



Uranus, the next planet in ]>osition, was dis- 

 covered accidentally by the elder llerschel on 13th 

 March 1781, and was named ' the Georgium Sidus' 

 and 'Herschel,' but these names soon fell into disuse. 

 It is aboutseventy-two times greater than the Earth 

 in volume, and thirteen times in mass; but, though so 

 large, its distance is so much greater in proportion 

 that astronomers have been unable to gain much 

 information concerning it. No spots 01 belt- have 

 hitherto been discovered on its surface, and conse- 

 quently its time of rotation and the position of ite 

 axis are unknown. It is attended by a number of 

 satellites, but so minute do these "bodies appear 

 that astronomers hitherto have been unable to 

 agree as to their exact number; Sir William 

 licrscliel reckoned six, while other astronomers 

 believe in the existence of four, live, and eight 

 respectively. That there are at least four is beyond 

 doubt. The year of Uranus is 30,686-820 days'. 



Neptune is the next and outermost member of 

 the solar system, and, at a distance of nearly 3000 

 millions of miles from the centre of the system, 

 slowly performs its revolution round the sun, accom- 

 plishing the complete circuit in about 165 solar 

 years. It is about ninety times larger than the Earth, 

 but from ite extreme remoteness is of almost map 

 I U eciable magnitude when seen through an ordinary 

 telescope. It was the disturbance in the motion of 

 Uranus caused by the attractive force of this planet 

 which led Leverrier and Adams to a calculation of 

 its size and position, on the supposition of its 

 existence ; and the directions which were given by 

 the former to Dr (ialle of Berlin, specifying its 

 exact position in the heavens, led that astrono 

 mer to ite discovery on 23d September 1846 

 (see ASTRONOMY). Mr Lassell of Liverpool dis 

 covered that Neptune is attended by one satellite. 

 The satellites of Uranus and Neptune differ from 

 the other planets, primary and secondary, in the 

 direction of their motion, which is from east to 

 west, and in the case of the former in planes nearly 

 perpendicular to the ecliptic. Both Uranus and 

 Neptune were olwerved UMUI lieforc the times of 

 Herschel and Leverrier, but they were always sup- 

 posed to lie stars. Uranus is known to have been 

 observed l>y Flamsteed between 1690 and 1715, and 

 Neptune by Lalande in 1795. 



In astronomical tables, almanacs, &c. the planets 

 are for convenience denoted by symlnds instead of 

 their names, as follows : Mercury, ; Venus, 2 ; 

 Earth, ; Mars, <J ; the Planetoids, in the order 

 of their discovery, , , , &c. ; Jupiter, V r \ 

 SMI urn, |j or 5 ; Uranus, $ ; Neptune, V or tf ; 

 the Sun, O : the Moon, ([. 



MIMIK I'I.ANKTS, the name given to that 

 numerous group of very small planets which is 

 situated in the solar system between Mars and 

 Jupiter. Till the 19th century they remained 

 undiscovered ; but for some years liefore their 

 existence had been suspected, mainly owing to 

 the remarkable hiatus in the series of the planetary 

 distances when compared with the law of Bode 

 ('|.v.i. On 1st January 1801 the first of them 

 (Ceres) was detected by Pia/zi of Palermo, and 

 his success roused his brother astronomers to 

 search for more planets. Their search was success- 



