vi 



Before long Hansen's eminent services to science attracted the notice of 

 the astronomical world ; and in the year 1825, when Encke left the 

 Seeberg at Gotha to take the superintendence of the Observatory of the 

 Berlin Academy, Hansen was selected to conduct the Observatory on 

 the Seeberg, with the title of Professor, a post which he retained for 

 nearly half a century, namely, to the end of his life. The small salary 

 (of 600 Thalers) attached to this appointment obliged him for a number 

 of years to undertake calculations for Ephemerides for the Danish and 

 English Governments. 



He lived in the Observatory itself from the year 1825 to 1839 ; but 

 as the arrangements and fittings were no longer adapted to the re- 

 quirements of science, and besides the building itself was too much out 

 of repair to bear much longer the wear and tear of weather, Hansen, 

 with the consent of the Duke, removed to the town of Gotha, and in the 

 southern suburb built himself a house with a little private observatory, 

 in which the meridian circle was set up. Here he worked from the 

 year 1842 to 1857, until the new Ducal Observatory was established, 

 which was fitted up under his superintendence in such a perfect manner, 

 though on a very modest scale, that it has since served as a model to 

 several larger institutions. 



The work Hansen accomplished at Gotha embraces almost every branch 

 of practical and theoretical astronomy ; and if no regular and compre- 

 hensive series of observations has been made under him, the cause of 

 this deficiency lies in the insufficient funds at his disposal, which neither 

 admitted of the payment of assistants nor of the purchase of large 

 instruments. But though Gotha could not in these respects rival other 

 great obserratories, it possessed an astronomer who was enabled by his 

 mechanical genius to do much in improving the art of observation by 

 ingenious improvements in the arrangements and use of his instruments. 

 The apparatus and methods which he invented for the investigation of 

 errors of division, for the prevention of flexure, for the registration of 

 observations, for the parallactic motion of telescopes moimted hori- 

 zontally, as well as numerous original contrivances, such as those which 

 were applied in the building of the present observatory, obtained the 

 general approbation of astronomers. His works on the use of the Eraun- 

 hofer Heliometer, the Transit-instrument, and the Equatorial have be- 

 come classical in spherical astronomy. 



But it was especially Hansen's rare mathematical ability that enabled 

 him to carry out the great works which make an epoch in the department 

 of physical astronomy known as the Theory of Perturbations. As early 

 as the first years of his residence on the Seeberg, he published in the 

 * Astronomische Nachrichten ' the main principles of his new theory of 

 perturbations, which in the course of years he employed in the accurate 

 investigation and calculation of the motion of the moon, of the sun, of 

 the greater and lesser planets, and of the comets. 



