TIIR ASTEROIDS JUPITER.] 



ASTRONOMY. 



847 



caught, with the exception of Vesta, which, under favour- 

 able circumstances, has been seen by the naked eye. 



Vesta, before which, however, there are others in the 

 order of succession in the system, is at the mean 

 distance of 225,000,000 of miles from the sun, and 

 performs a revolution in 1,325 days, somewhat more 

 than three years and a-half. 



This planet is much brighter than its compeers, aud 

 appears like a star of the fourth or fifth magnitude. 



Juno is at the mean distance of 254,000,000 of miles, 

 and accomplishes an orbital revolution in four years- arid 

 a hundred and twenty-eight days. 



Juno has a very eccentric orbit, varying in her distance 

 from the sun to the extent of 130,000,000 of miles. This 

 eccentricity has a remarkable effect upon her motion, for 

 she goes through that half of her orbit which is nearest 

 the sun in nearly half the time that she travels through 

 the remainder. The planet has a reddish colour, and 

 appears like a star of the eighth magnitude. 



Ceres, following Leda, revolves round the sun in 

 about four years and two-thirds, at the mean distance of 

 203,000,000 of miles. 



The telescope reveals Ceres as a ruddy star of the c-ighth 

 magnitude, under circumstances which leave the impres- 

 sion of an extensive atmosphere. 



Pallas is at nearly the same mean distance from the 

 sun as Ceres, and has nearly the same period of revolu- 

 tion. 



liscovery of the asteroids was not an accidental 

 circumstance, but the result of a search conducted upon 

 the presumption that the harmony of the solar system 

 required the presence of a planetary body between Mars 

 and Jupiter. It was observed, that the distance be- 

 tween the orbits of Mars and the earth is about double 

 that between those of the earth and Venus, or 50,000,000 

 of miles ; whereas, between the orbits of Mars and Jupiter 

 there is the tremendous interval of 340,000,000 of miles, 

 nearly two-aml-a-half times the whole distance of the 

 former from the sun. Astronomers, therefore, became 

 thoroughly imbued with the notion that an undiscovered 

 planet existed in the gap ; and commencing an active 

 search of the heavens, the asteroids were met with in the 

 vacancy. 



On the ground of these peculiarities, it has been sur- 

 mised that these inferior bodies have diverged from a 

 common node, and therefore originally formed a single 

 large planet, which some mighty convulsion shattered. 

 This bold hypothesis, first started by Olbers, has received 

 a very general sanction. It obtains evidence from the 

 powerful explosive forces in action in the interior of our 

 own globe, to which its volcanic vents act as safety- 

 valves ; from the phenomena of meteoric showers, re- 

 specting which we have no supposition better than that 

 they are debris which the earth encounters in its orbit, 

 fusing upon contact with its atmosphere. 



Of the telescopic appearance of this group of planets, 

 now increased to above eighty in number, it would be 

 difficult to say anything. A keen eye can detect, in a 

 good telescope, the difference between one of these aste- 

 roids (when not less than the eighth or ninth magnitude) 

 and a star of the same degree of brightness ; but this per- 

 haps is the extent of the power of the telescope. Their 

 diameter is very doubtful. Some observers consider 

 them as a few hundred miles in circumference, others as 

 many thousands. The hazy appearance surrounding 

 Ceres, which was noticed by Herschel and Schroeter, 

 whence they surmised that the asteroids were partly of a 

 cometary nature, has not been noticed by modern ob- 

 servers, and may be due to the imperfection of their 

 reflecting telescopes. 



JUPITER. Tliis is one of the finest planetary objects 

 to which the telescope can be directed ; and even with a 

 small instrument, its disc, band, and satellites can be 

 plainly seen ; and at each opposition there is but little 

 difference in the brightness. Whenever viewed with a 

 telescope, the singular appearance which its disc presents, 

 being always surrounded by two or more dark belts or 

 bands, cannot fail to strike the beholder, and the changes 

 to which they are subject are still more curious. The 



equatorial portions are generally the brightest, and it 

 is at this part the bands are most distinctly visible. 

 They are generally directed parallel to the equator ; but 

 this is not always the case. The polar portions are for 

 the most part darker than any other, but it is only on 

 rare occasions they are equally so ; and indeed the whole 

 cloudy covering of the planet is of such a shifting and un- 

 steady character, that it may almost be said to vary from 

 night to night. In additiou to the dark bands, there are 

 occasionally to be seen, on the disc of the planets, dark 

 spots of irregular form, which are subject to the same 

 changes as the bands, though they are frequently of long 

 duration, and remain for months together at nearly the 

 same place. A telescopic view of Jupiter, and the ap- 

 pearance of its surface under favourable circumstances, 

 will be seen at Fig. 68. 



Fig. 68. 



By attending to the motions of those spots, it has been 

 found that Jupiter, like the planets hitherto described, 

 rotates upon its axis ; but the length of its day is much 

 shorter than that of the planets in the vicinity of the 

 earth. It is rather difficult to tell the exact duration, 

 for the spots themselves have got a motion on the disc ; 

 and Herschel found that the time of rotation appeared to 

 bo sometimes 9h. 55m. 40s. , at other times 9h. 54m. 53s. , 

 from observations of the same spot ; whilst, by a different 

 spot, it was sometimes 9h. 51m. 45s., and at other times 

 9h. 50m. 48s. As the equator of the planet is but very 

 slightly inclined to the plane of its orbit round the sun, 

 it follows that there is but very little variation of the 

 seasons ; so that during its long year, the length of the 

 day and night varies very slightly, either being at any 

 latitude about five hours in length. 



From the rapid motion which those spots undergo near 

 the equator, Herschel thought it was not improbable 

 that at these portions of the planet's disc there existed 

 currents of air similar to the trade winds, the effect of 

 which would likewise be to form the loose vapours on its 

 surface into the parallel belts. From Herschel's obser- 

 vations on these accidental clouds as he was of opinion 

 the spots were he considered that they were occasion- 

 ally driven along at the rate of ninety-six leagues per 

 hour, a velocity far exceeding that of our most violent 

 hurricanes. 



It might be thought, at first sight, that the dark bands 

 represented in Fig. 68, were the clouds, the overcast por- 

 tion of the planet's disc, and the brighter belts were parts 

 of its surface. Herschel, however, considered that the 

 contrary was the case, and in this he is followed by all 

 modern observers. He supposed that the more brilliant 

 portions were the zones, in which the atmosphere was 

 charged with clouds, the darker parts those on which it 

 was quite clear and serene. The latter allow the solar 

 rays to pass through to the surface of the planet, where 

 the reflection being less powerful than from the clouds, 

 less light was consequently returned. 



The disc of Jupiter is considerably flattened .it the 

 poles, and bulged out at the equator. This is immediately 

 seen when the eye is directed through a telescope to the 

 planet ; and the extent to which it occurs will be seen by 

 the diagram (Fig. 68). The belts appear always parallel 

 to the greater axis. 



