258 



THE POPULAR EDUCATOR. 



situated in the centre of them, one of the arisen being larger than 

 the other. The other fact is that the dimensions of the rings 

 appear to fluctuate from day to day in a way that is explained 

 by supposing the rings to be elliptical, and to rotate around the 

 planet. Sir W. Herschel fixed the period at 10 hours 32 

 minutes Secchi, however, sets it down at 14 hours 23J minutes. 

 This rotation and eccentricity appear to be necessary for the 

 stability of the Saturnian system. 



If we ndw turn our attention from the ring to the planet itself 

 we find that its surface is marked with belts, so as somewhat to 

 resemble Jupiter's. The markings are, however, more faint, and 

 dark spots are very rarely seen on them hence it becomes more 

 difficult to ascertain the true period of rotation. It may, how- 

 ever, be set down at a few seconds short of 10,^ hours. 



In addition to the compound ring, Saturn is accompanied in 

 its journey round the sun by eight satellites. Two of these 

 the first and seventh are so small as only to be seen in the 

 most powerfid telescopes ; and several of the others require a 

 good instrument to show them well. 'Owing to their distance 

 from us, our information respecting them is but limited. The 

 annexed table shows in a concise form the most important facts. 

 The diameters, however, with the exception of Titan's, are 

 doubtful : 



With so large a number of moons, and most of them revolving 

 so rapidly, the sky must present a most remarkable appearance 

 to observers on the surface of Saturn. The varying phenomena 

 of the ring add in no small degree to this. 



"When the sun is shining on the rings they will appear like a 

 huge luminous arc spanning the sky, and the shadow of the planet 

 will be seen moving along them. At other periods the sun will 

 be on the further side of the rings, which will then only be 

 illuminated by the reflection from the planet's surface. In lati- 

 tudes a little removed from the equator there mil also at times 

 be very frequent eclipses, caused by the ring intervening 

 between the planet and the sun. 



The Earth and inferior planets will be quite invisible, as the 

 greatest elongation of the former cannot much exceed 6. Mars, 

 likewise, will seldom be seen, as it cannot be more than 10 

 removed from the sun. 



URANUS. 



Of the two planets beyond the orbit of Saturn comparatively 

 little is known, owing to their great distance from the Earth. 

 The nearer of these, Uranus ($), is only faintly visible to the 

 naked eye. It was discovered on March 13, 1781, by Sir William 

 Herschel, though it was some few months before its planetary 

 nature was recognised. While engaged in scanning some small 

 stars, he was struck with the appearance of one in particular, 

 and, on applying higher powers, he found that it seemed to 

 increase in size and present a faint disc. It also exhibited a 

 proper motion. Herschel accordingly considered it to be a comet, 

 and announced its discovery as such, but it was soon found to 

 be impossible to assign to the wanderer a parabolic orbit that 

 would account for its movements, and it was then ascertained 

 to be a planet moving in an elliptical orbit, at a mean distance 

 from the sun of 1,753,851,000 miles. 



Some difficulty was experienced in deciding on a name for the 

 stranger. The name cf the discoverer was suggested. He him- 

 self, however, proposed to call it Georgium Sidus, out of compli- 

 ment to George III.; but as the names of the other planets were 

 derived from the heathen mythology, it was decided to select a 

 name for this from the same source, and Uranus was at length 

 fixed upon. The other names will, however, occasionally be met 

 with. 



The diameter of Uranus is a trifle more than 33,000 miles. Its 

 period of rotation is not yet ascertained, as no spots or belts 

 have been detected on its surface which might afford a means of 

 determining it. It takes a little more than 84 of our years to 

 complete its journey round the sun, its period being 30,686'82 

 days. One remarkable feature in connection with this member 



of our system is the great inclination of its equator to the plane 

 of its orbit, the poles being very nearly in the plane. As a result 

 of this, the sun is at different periods vertical to nearly all parts 

 of the planet's surface. 



Several satellites revolve around Uranus. The number, how- 

 ever, does not appear to be definitely ascertained. Sir William 

 Herschel stated that he discovered six, and two, within these, 

 have been found by Lassell and Struve, so that the number is 

 by many astronomers set down as eight, and their periods of 

 rotation vary from 2-J to 107i days. Lassell, however, expresses 

 his firm belief that the total number yet discovered is only four, 

 and only these have as yet been named. They all revolve 

 in a direction contrary to that of all other members of the 

 system. We append a list, as has been done in the case 

 of previous planets. Those given here are numbered 1, 2, 4, 

 and 6 in the fuller list : 



Name. Mean Distance. 



Ariel 123,000 miles. 



Umbriel 171,000 



Titania 281,000 



Oberoa 376,000 



Sidereal Period. Diamefcr. 



2d. 12h. 28m. ? 



4 3 27 ? 



8 1C 55 ? 



13 11 6 ? 



The history of the discovery of this planet is perhaps one of 

 the most remarkable pages in the whole history of astronomy. 

 Uranus, as we have seen, was discovered by accident, and it was 

 some time before it was admitted to be a planet. With Neptune 

 the case was vastly different. Nearly half a century ago M. 

 Bouvard attempted to calculate accurately the movements of 

 Uranus, but found unexpected irregularities which could not 

 then be accounted for. 



If the planet alone were revolving round the sun, its place 

 could be easily assigned, but each of the other planets exert an 

 influence on it, and these influences are continually varying. 

 All these were, however, allowed for, and yet there was some 

 disturbing cause which drew it out of its assigned place. 



The opinion at length gained ground that some exterior planet 

 must exist, and cause these deviations. Accordingly, in the 

 beginning of 1843 Mr. Adams began to investigate the matter 

 with a, view to ascertaining the place the exterior planet (should 

 there be one) ought to occupy, and its elements ; after nearly 

 two years of diligent inquiry, he announced to the Astronomer 

 Royal the results of his enquiries. Nothing further was done at 

 the time ; but soon after a French astronomer, Le Terrier, inde- 

 pendently applied himself to the same question, and obtained 

 results closely resembling those arrived at by Mr. Adams. Upon 

 this a search in the locality indicated was resolved upon, but 

 some little time was occupied in commencing it, as the star 

 maps of that part of the ecliptic were but imperfect. 



Le Verrier then published a slightly revised computation, and 

 on the 18th of September, 1846, M. Galle, of Berlin, directed his 

 telescope to the spot thus indicated by Adams and Le Verrier. 

 A small star not laid down on the charts was at once seen, and, 

 on careful watching, proved to be the suspected planet, which 

 had thus, by purely theoretical computations, had its place 

 marked out among the stars. The problem thus solved almost 

 simultaneously by two astronomers in two different countries is 

 perhaps one of the greatest ever grappled with by the human 

 mind, and reflects the greatest honour on both astronomers, 

 though unfortunately the effort is often made by the countrymen 

 of each to rob the other of his share in the discovery. 



Of the planet itself little can be said. It revolves around the 

 sun at the mean distance of 2,746,271,000 miles, which is con- 

 siderably less than that which would be assigned to it by Bode's 

 law. In this orbit it completes its journey in 60,126'7 days, or 

 nearly 164| years. The eccentricity of its orbit is but small. 

 Its diameter is found to be about 36,620 miles, and no polar 

 compression has as yet been noticed. Owing, however, to its 

 immense distance, it would not easily be perceived if it did exist. 

 Its time of rotation on its axis, and the inclination of its equator 

 to its orbit, are unknown, as is also the appearance of its surface. 

 Nothing, indeed, except the large telescope of Lord Bosse can 

 throw much light on these points, and even that may be unable 

 to determine all of them. One satellite has been discovered by 

 Lassell, revolving round Neptune in a period of 5 days, 21 hours, 

 8 minutes, at a distance from the primary of 220,000 miles. 



The more important facts respecting the planets are exhibited 

 in a tabular form in Vol. II., p. 46. 



