ASTRONOMY OF THE NINETEENTH CENTURY. 419 



astronomers, although the law is entirely empirical, and not referable 

 to any known cause. The German astronomers were delighted at a dis- 

 covery which confirmed so strikingly the conjectures of their country- 

 man, and the articles in their journals announcing the discovery of 

 Ceres were headed by such titles as "The long-expected Planet between 

 Mars and Jupiter." 



The new planet Ceres being supposed to complete our system, there 

 was no reason to look for more planets. Yet on the 28th of March, 

 1802, Dr. Olbers of Bremen noticed a small star, which he soon dis- 

 covered was not a fixed star ; but he found it impossible to make its 

 observed positions agree with the supposition of a circular orbit, and its 

 appearance was unlike that of a comet. After observing it for three 

 weeks, Gauss undertook to determine the orbit without making any 

 preliminary hypothesis, and he found that the new body was a planet 

 (Pallas), having an elliptical orbit, very close to that of Ceres, but dis- 

 tinguished by a great degree of eccentricity, and inclined to the plane 

 of the ecliptic at no less an angle than 34. So struck was Olbers with 

 the nearness with which the orbit of Ceres and of Pallas approached 

 each other at the intersection of their planes, that he started the lucky 

 hypothesis that these planets were fragments of a larger one which had 

 been shattered by some internal convulsion, or by collision with some 

 other body. Two consequences were immediately deducible from this 

 hypothesis, namely, that other fragments probably existed, and if so, 

 their orbits must pass near the points in which the orbits of Ceres and 

 Pallas intersect. The parts of the heavens corresponding to the two 

 intersections were accordingly closely observed by the German astro- 

 nomers. On the 2nd of September, 1804, Harding discovered at 

 Lilienthal a third planet, which has- received the name of Juno. The 

 elements of this planet agreed very fairly with Olbers' hypothesis, if 

 allowance be made for the perturbations caused by Jupiter. Olbers 

 himself followed out his idea by every month carefully scrutinizing 

 certain parts of the heavens ; and after having carried on these obser- 

 vations for three years, his perseverance was rewarded by the discovery 

 of Vesta on the 2gth of March, 1807. This planet is similar in all 

 essential particulars to the three previously discovered. The same 

 system of examination was long kept up, and Olbers has declared that 

 so regularly did he examine the parts of the heavens already referred 

 to, he was certain that no new planet had passed over them between 

 1808 and 1816. 



After an interval of thirty-eight years, a fifth member of the asteroid 

 group was discovered by Hencke at Driesen on December 8th, 1845 ; 

 a sixth by the same observer in 1847 ; and two more in the same 

 year by Hind in London. Since 1847 further additions have every 

 year been made to the planets between Mars and Jupiter. By the 

 end of the year 1865, eighty-five asteroids had been observed and 

 their elements computed. The whole number now known (1880) 



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