SIMON NEWCOMB, F.R.S., LL.D., D.C.L. xi 



The ordinary meridian determinations, before the time of Brad- 

 ley, 1750, were of very Httle use for a refined investigation Hke this. 

 Another class of observations, however, furnished data comparable 

 in accuracy with the best determinations made today, that is, the 

 occultations of stars and planets by the moon. Though but few 

 such had ever been published, it occurred to Professor Newcomb 

 that among the unpublished work at the European observatories, 

 possibly enough such data might be found to repay the labor involved 

 in the search. The result exceeded his most sanguine expectations. 

 At the Paris observatory, in particular, data were brought to light 

 which carried the period of accurate lunar observation back nearly 

 a century, so that now instead of a lunar theory, which like that of 

 Hansen depended on one hundred years of observation, we have 

 available two hundred and fifty years of accurate data. 



It is interesting to know that this work at the Paris Observatory 

 was carried on while the struggle with the Commune was at its 

 height, the windows frequently rattling with the reports of cannon. 



The lunar theory, as it is called, seems to have been the prob- 

 lem of Professor Newcomb's predilection. Besides the researches 

 already mentioned which involved a great amount of time and labor 

 he wrote, together with other papers, an elaborate memoir dealing 

 with the action of the planets on the moon. He did not attempt, 

 however, a complete revision of the subject. His most important 

 services in this direction were in what has been styled the border 

 land between the theoretical and practical, viz., that of assembling 

 all available data and comparison with theory, thus exhibiting in a 

 concise manner precisely what remains to be accomplished in order 

 to bring the two into harmony. 



The last of his published papers is found in the Monthly Notices 

 of the Royal Astronomical Society for January, 1909, exhibiting the 

 deviations of the mean longitude of the moon from the position 

 given by theory. The explanation of these discrepancies now con- 

 stitutes one of the most interesting of the unsolved problems of 

 astronomy. 



The theoretical researches in the lunar theory have been greatly 

 extended by the classic work of Mr. George W. Hill, while the 



