42 



could be accurately predicted for years in advance : a know- 

 ledge which, as we have seen, was not in existence when 

 Greenwich Observatory was founded. It followed, therefore, 

 naturally that the observation of the moon's place on every 

 possible occasion became a necessary item in the Greenwich 

 programme, and eventually, by the common consent of 

 astronomers, this duty of tne continuous observation of the 

 moon has been entrusted wholly to the Royal Observatory. 

 To carry out this duty more fully the Transit Circle has been 

 supplanted by the "Altazimuth," this last being at the 

 present time a second Transit Circle of similar aperture to 

 the first, but dinering from it in that, on occasion, it can 

 be fixed in other positions than in the meridian, for, as the 

 first Transit Circle is confined to the meridian, the moon 

 cannot be observed with it for several days in each month 

 near the time of conjunction with the sun. In the year 

 referred to in the 1914 Report, 79 observations of the moon 

 were secured by the Altazimuth, of which 12 were taken out 

 of the meridian. 



But something more is required than mere observation of 

 the moon to render it possible for its future movements to_be 

 predicted with accuracy. The observations must be discussed, 

 and the amounts and periods of apparent irregularities in its 

 motion must be ascertained and reduced to system. In recent 

 years this great work has been undertaken by PHILIP COWELL, 

 appointed becond Chiei Assistant at the Observatory in 1^96, 

 a post which he held until he was made Superintendent 

 of the Nautical Almanac in 1910. Cowell undertook the 

 enormous task of analysing the outstanding discordances 

 between the adopted tables of the moon and the long series 

 of Greenwich Observations, for the double purpose of correct- 

 ing the coefhcients of the periodic terms which had been 

 used for the construction of the tables and of detecting the 

 existence of other periodic terms which had not been thus 

 employed. In this examination, the Greenwich Observa- 

 tions lor the latter half of the 19th century were most minutely 

 considered, and for an important part of it the observations 

 from 1750 to 1651 were also included. In other words, the 

 inquiry covered more than a century and a half of the Green- 

 wich observations of the moon. JNo work in this particular 

 branch of practical and mathematical astronomy has ever 

 been carried out on so great and comprehensive a scale at 

 any other observatory. 



This enterprise of employing the moon as the assistant of 

 man in the navigation of the ocean was purely utilitarian 



