282 A Short History of Astronomy [-;;. X. 



improvements in methods of calculation and of utilising 

 observations. 



In estimating the immense mass of work which Lacaille 

 accomplished during an astronomical career of about 22 

 years, it has also to be borne in mind that he had only 

 moderately good instruments at his observatory, and no 

 assistant, and that a considerable part of his time had to 

 be spent in earning the means of living and of working. 



225. During the period under consideration Germany 

 also produced one astronomer, primarily an observer, of 

 great merit, Tobias Mayer (1723-1762). He was appointed 

 professor of mathematics and political economy at Gottingen 

 in 1 75 1, Apparently on the understanding that he need not 

 lecture on the latter subject, of which indeed he seems 

 to have professed no knowledge ; three years later he was 

 put in charge of the observatory, which had been erected 

 20 years before. He had at least one fine instrument,* 

 and following the example of Tycho, Flamsteed, and Bradley, 

 he made a careful study of its defects, and carried further 

 than any of his predecessors the , theory of correcting 

 observations for instrumental errors. t 



He improved Lacaille's tables of the sun, and made a 

 catalogue of 998 zodiacal stars, published posthumously in 

 T 775 ') by a comparison of star places recorded by Roemer 

 (1706) with his own and Lacaille's observations he obtained 

 evidence of a considerable number of proper motions 

 ( 203) ; and he made a number of other less interesting 

 additions to astronomical knowledge. 



226. But Mayer's most important work was on the moon. 

 At the beginning of his career he made a careful study of 

 the position of the craters and other markings, and was 

 thereby able to get a complete geometrical explanation of 

 the various librations of the moon (chapter vi., 133), and 

 to fix with accuracy the position of the axis about which 

 the moon rotates. A map of the moon based on his 

 observations was published with other posthumous works 

 in 1775. 



* A mural quadrant. , 



f The ordinary approximate theory of the collimation error, level 

 error, and deviation error of a transit, as given in text-books of 

 spherical and practical astronomy, is substantially his. 



