140 ANNUAL REPOKT SMITHSONIAN INSTITUTION, 1912. 



globe there are many extended regions which have no points so accu- 

 rately determined. 



The year 1911 saw pubUshed the tables of the moon based upon the 

 theory of Delaunay and forming volume 7 of the Annales du Bm-eau 

 des Longitudes. An introduction of 112 pages, where not a line is 

 superfluous, allows us to form an idea of the mangitude of the task. 

 Delaunay, Tisserand, and R. Radau have successively given to the 

 task the last years of their lives and were assisted by Schulhof. An 

 analogous undertaking was carried out in America by Prof. E. Brown; 

 the French astronomers by finisliing first have honorably maintained 

 the tradition established by Laplace and Le Verrier. All the errors, 

 although very small, known in the works of Hansen and Delaunay, 

 have been corrected. JBut it was impossible to break away from all 

 empiricism and there remains an inequality of long period (273 years) 

 discovered by Newcomb. Newcomb gave up trying to find an expla- 

 nation. More optimistic, R. Radau believes the cause can be found 

 in cosmic dust and the infra-mercurial planets. 



A new determination of the parallax of the moon, due to the collabo- 

 ration of the Cape and the Greenwich Observatories has been carried 

 out after six years of work.^ The value generally used was confirmed. 



III. 



The result, at first sight rather small, of a considerable effort, is not 

 to be understood as minimizing the desire often expressed of substi- 

 tuting for the classic method of charting the sky more rapid and more 

 accurate methods. What is aimed at everywhere is the suppression 

 of the measures of moderately large angles by the readings of divided 

 circles. More confidence is placed in measures of time intervals and 

 the extension of such a method is to be expected. 



W. E. Cooke proposed, for the determination of right ascensions, 

 not to use a meridian circle but rather a telescope whose optic axis 

 when rotated about a vertical axis intersects in the heavens a small 

 circle parallel to the horizon. We could by such means divide the 

 celestial equator, or any parallel circle with an accurac}^ comparable 

 with the precision of our best clocks. The declination of the stars 

 would be determined from the times when they reached a determined 

 altitude. The axis of the telescope must therefore be maintained at 

 a constant altitude. At various times the realization of this condi- 

 tion has been attempted with telescopes on floating mounts. Cooke 

 places faith in a vertical axis and level. 



Among the disadvantages of the meridian instrument is the neces- 

 sity that each celestial point has to be separately determined and that 

 great intervals can not be measured as accurately as small ones. 



> Session of the Royal Astronomical Society, May 12, 19U. 



