17. 



ON NEW TABLES OF THE MOON'S PARALLAX. 



[From the Monthly Notices of the Royal Astronomical Society (1853), Vol. xni., and 



Nautical Almanac for 1850.] 



THE importance of an accurate knowledge of the Moon's Parallax is 

 very evident. No observation of the Moon's place can be compared with 

 the Tables, or turned to any practical use, without undergoing a preliminary 

 reduction of which the amount of the Parallax is the most important 

 element. Now the same theory by which the angular motion of the Moon 

 round the Earth is determined gives likewise the form of the orbit, and 

 therefore the proportion between the Parallaxes at different times ; hence, 

 as the theory is sufficiently perfect to represent the place of the Moon 

 within 10", it cannot be doubted that it would be competent to give the 

 variations of the Parallax within a small fraction of a second, provided the 

 mean Parallax were known. To determine this, however, by theory, it is 

 necessary to know, in addition to the elements furnished by observations 

 of the Moon's motion, the ratio of the Moon's mass to that of the Earth. 

 Hence, conversely, if the mean value of the Parallax be deduced from 

 corresponding observations of the Moon's declination, made at distant points 

 on the Earth's surface, one means is afforded of finding the ratio of the 

 masses. 



The most recent determination of the Parallax by means of observations 

 of this kind is contained in a paper by Mr Henderson in the tenth volume 

 of the Memoirs of the Royal Astronomical Society, and is founded on his 

 own observations made at the Cape of Good Hope, combined with cor- 



A. 12 



