ASTEROIDS. 



ASTEROIDS. 



64,3 



discorery of further fragments of the planet, that a careful examination 

 should be undertaken, and henceforward uninterruptedly maintained, 

 of the regions in those two constellations which included the common 

 points of intersection of the orbits of the two bodies. In the 

 meantime, his views received additional support from the discovery 

 of a third planet, Juno, by Harding, at Lilienthal, on the 2nd oi 

 September, 1804. The new planet was discovered in the constellation 

 Pisces, ' near the region in the Whale through which Olbers hac 

 maintained that the fragments of the shattered planet would neces- 

 sarily pass. 



Gibers continued with unwearied perseverance to explore the two 

 opposite regions of the heavens in which he expected to discover 

 further fragments of his supposed shattered planet. At length, on the 

 28th of March, 1807, his efforts were rewarded by the discovery ol 

 another planet, Vesta, in the north wing of the constellation Virgo 

 one of the regions of the heavens in which he expected to effect such a 

 discovery. The mean distance, as determined by the calculations 

 of Gauss, plainly indicated that it belonged to the group of new 

 planets between Mars and Jupiter. In accordance with his hypo- 

 thesis, Olbers continued for several years after his discovery of Vesta 

 to search for further fragments of the shattered planet, confining 

 his attention to the two opposite regions through which he expected 

 that they would in each instance necessarily pass in the course ol 

 their revolution around the sun, but no new triumph rewarded his 

 labours. Subsequent discoveries have not served to confirm the 

 somewhat fanciful idea upon which his system of observation was 

 founded. 



The new planets were found to be excessively small, exhibiting 

 hardly an appreciable apparent magnitude even when viewed in the 

 most powerful telescopes. Sir William Herschel having measured the 

 apparent diameter of Ceres under favourable circumstances, found that 

 at the mean distance of the sun from the earth it would subtend 

 an angle of 0"'35, whence it was easy to infer that the absolute 

 diameter of the planet amounted to 161 miles. In a similar way he 

 found that the absolute diameter of Pallas was not more than 110 

 miles. 



In consequence of the strong resemblance which the new planets 

 bore to each other, both in size and in the forms and positions of their 

 orbits, and the contrast which they offered in these respects to the 

 older planets, Herschel proposed that they should be called Atteroidi 

 rather than planet*. This mode of designation was favourably received, 

 and is now generally applied to those bodies. 



With the exception of Olbers himself, it does not appear that any 

 contemporary astronomer was induced to undertake a systematic 

 exploration of the heavens in search of new asteroids. The want of good 

 (tar-charts rendered all operations of this kind exceedingly laborious 

 during the early period of the present century. In the year 1825, 

 however, a circumstance occurred which was destined to exercise an 

 important influence on the progress of such researches. This con- 

 sisted in a proposal on the part of the Berlin Academy to construct a 

 series of star-charts, on which should be laid down all the stars to the 

 ninth magnitude inclusive, comprised In the region of the heavens 

 which extends to a distance of 15 on each side of the equator. It 

 was arranged that the zone hi question should be divided into 

 twenty-four parts, corresponding to the twenty-four hours of right 

 ascension ; and astronomers of all countries were invited to unite 

 in carrying this project into effect, each co-operating astronomer 

 undertaking the execution of one hour of right ascension. The 

 views of the Berlin Academy met with a favourable response from 

 astronomers, and although only a short time has elapsed since they 

 were carried into complete effect by the construction of the last of 

 the twenty-four charts constituting the zone, still the publication of 

 the individual charts from tune to time was productive of important 

 rt'.-'ilf-. 



The utility of star-maps in searching for new planets is obvious 

 enough. The observer compares his map with the corresponding 

 region of the heavens. If a star should appear in the heavens which is 

 not inserted in the map it may be presumed to be a planet which has 

 wandered into the region of the map since the epoch of its construc- 

 tion. On the other hand if one of the stars in the map should be 

 miaring in the heavens, it may be reasonably supposed that the object 

 is in reality a planet which was laid down as a fixed star at the time of 

 the construction of the map, and has subsequently travelled entirely 

 out of the region to which the map corresponds. The observer, aided 

 by this clue, then proceeds to explore the heavens in the immediate 

 vicinity of the region of the map with the hope of discovering the 

 missing object. 



It ia plain from the foregoing remarks that trustworthy star-charts 

 are indispensable in searching for new planets. The Berlin charts 

 were obviously very valuable for- this purpose, since they included 

 stars of a magnitude considerably inferior to that exhibited by the 

 newly discovered asteroids. It was probably from a consideration of 

 the advantages offered by these charts in the exploration of the 

 heavens, that M. Hencke, an amateur astronomer, residing at Driesen 

 in Germany, was induced in the year 1830, to undertake a search for 

 further asteroids. After devoting fifteen years to a persevering scrutiny 

 of the heavens his labours were finally rewarded by the discovery of a 

 fifth asteroid, Astraa, on the 8th of December, 1845; and two years 



ARTS AND 8CI. D1V. VOL. I. 



afterwards, on the 1st of July, 1847, by the discovery of a sixth 

 asteroid, Hebe. 



Henceforward the progress of discovery has continued without 

 interruption down to the present time. The aggregate number of 

 asteroids ascertained to exist at the time when this sheet is passing 

 through the press (1859, March 15), amounts to fifty-six. It is usual 

 to characterise each asteroid by the number, enclosed in a circle, indi- 

 cative of its rank in the order of discovery. The following is a complete 

 list of the various discoveries. 



In the foregoing list the number prefixed to each asteroid indicates 

 bhe order of discovery. With respect to the fifty-sixth asteroid (which 

 lias not yet been named), there appears to be a deviation from this 

 rule, for according to the date of discovery assigned to it in the second 

 column it ought to rank as the forty-seventh in the list. The origin of 

 this apparent inconsistency is interesting. Upon the discovery of 



* Discovered independently, although subsequently in respect of time, by 

 tf. de Gasparis. 



Discovered independently by M. Chacornac. 



; Discovered independently by Mr. Pogson, and also by M. Chacornac. 

 Discovered independently by M. Luther. 



1 T 



