Vlll 



founded. Petersen was for a time his companion in this editorial work 

 at Altona. Of the numerous contributions he made to this periodical 

 we need only mention, in addition to those already spoken of on the 

 theory of perturbations, those of early date on the transit-instrument 

 and on the meridian circle, on eclipses and occultations of Stars, on re- 

 fraction of light, on the determination of the latitude, on the calculus 

 of probabilities and the method of least squares, on different geodetic 

 problems, on the disturbances of Encke's comet by a resisting medium, 

 &c. ; and at a later period especially the papers on the calculation of special 

 perturbations by mechanical quadratures and the reduction of places to 

 the ecliptic of the date. An immense number of essays on various sub- 

 jects of interest have appeared in the Eeports of the Transactions of the 

 Mathematical and Physical Class of the Saxon Society ; of these we need 

 merely allude to the papers on the solution of a system of linear equa- 

 tions ; on spherical harmonics ; on ideal coordinates ; on Keppler's 

 problem ; the ecliptic tables, &c., together with the analysis of the same ; 

 on the arrangements of the new Ducal Observatory at Grotha ; on the 

 determination of the orbit of a heavenly body from three observa- 

 tions ; on the secular variation of the mean longitude of the Moon, and 

 the alteration of the day's length by the gradual decrease of the rate of 

 rotation of the Earth (April 1863) ; on the computation of tri angu- 

 lations ; on the centre of gravity of spherical triangles ; on a new 

 telescope-stand ; on the application of photography for observing the 

 Transit of Yenus, &c. 



Hansen published papers in various other periodicals, as, for instance, 

 in the ' Comptes Eendus ' of the Paris Academy, in the Monthly Eeports 

 of the Berlin Academy, in the Monthly Notices of the London Astro- 

 nomical Society, in the P-esults of the Magnetic Society, in Schumacher's 

 Astronomical Jahrbuch, the Mathematical Works of Jacobi, the Mathe- 

 matical Journal of Gutten in Wilna (liber das Eepsold'sche Aequatoreal). 

 the Memoirs of the Naturforschenden Gresellscbaft in Danzig (which 

 latter Society awarded him the prize for his treatise, "Theorie der Pendel- 

 bewegung mit Eiicksicht auf die Gestalt und Umdrehung der Erde," 1853), 

 and so on. In conclusion we may add to this short summary the Memoir 

 on the Eraunhofer Heliometer, 1827, and the ' Commentatio de gradus 

 praecisionis computatione,' written on occasion of Olbers's Jubilee in 1830. 



Hansen did not escape controversy. Some of his works and their results 

 have been attacked by G-erman, Erench, English, and American astro- 

 nomers ; to these attacks such men as Pontecoulant, Lubbock, Encke, 

 Brlinnow, Peters, Baeyer, Weingarten, Newcomb, and Delaunay have lent 

 their names. No one more than Hansen regretted the bitterness which 

 is more or less inseparable from such discussions, though he felt it a 

 duty to science to give distinct expression to his convictions. 



He received various honorary titles and decorations, and numerous 

 recognitions of his scientific position. Most of the learned societies of 

 Germany and other countries elected him a member. The Eoyal Society, 



