Astronomical Society. 52 1 



ASTRONOMICAL SOCIETY. 



March S. — The following communications were read : — 



Observed Transits of the Moon, and Moon-culminating Stars, 

 over the Meridian of Edinburgh Observatory, from June 1 to De- 

 cember 31, 1838. By Professor Henderson. 



Lunar Otcultations of Planets and Fixed Stars, and Eclipses of 

 Jupiter's Satellites, observed at Edinburgh Observatory in 1838. 

 By Professor Henderson. 



Moon- culminating Stars observed at the Cambridge Observatory 

 in the Months of November and December, 1838. By Professor 

 Challis. 



Occultations observed at Dulwich and Ashurst, from July 31 to 

 December 27, 1838. By Robert Snow, Esq. 



On the Method of determining the Longitude by Moon-culmina- 

 ting Stars. By Mr. Epps, late Assistant-Secretary to the So- 

 ciety. 



The author remarks, that the advantages of moon-culminating ob- 

 servations, for the purpose of determining the difference of longi- 

 tudes, particularly in the case of distant meridians, are now univer- 

 sall)'' admitted, every other method being found subject not only to 

 greater trouble and difficulty, but to errors far exceeding the limits 

 of those to which the results of moon- culminating observations are 

 liable. But although this method, the merit of introducing which 

 belongs chiefly to Mr. Baily, is justly regarded as the best known, 

 yet the result of a single observation, or of a few observations, is 

 liable to a considerable amount of error, even when the observations 

 are made by the best instruments and the ablest observers ; and the 

 object of the present communication is to show the extent to which 

 the error may be expected to reach. 



The moon's proper motion, on which the method entirely depends, 

 is such, that an error in the observation is necessarily greatly aug- 

 mented in the resulting longitude ; in fact, the error in longitude 

 will always be from 21 times to 35 times the amount of the error of 

 observation. If, then, it be assumed that 0^'2 of time is the pro- 

 bable error of an observed transit, and that the obser\'ed interval be- 

 tween the transits of the moon and star is liable to an error of 0^'4, 

 the resulting difference of longitude will be in error from 8 to 14 

 seconds of time. This is the probable effect, from observations made 

 at one station only ; and as it is quite possible that the effect may 

 be doubled by an equal amount of error, in a contrary direction, at 

 the other station, the author thinks lie will be within limits in as- 

 suming 10 or 12 seconds of time as the probable error of longitude, 

 resulting from the comparison of a corresponding interval observed 

 at two stations, admitting that the best means of making the obser- 

 vation exist at each. 



In order to show that the probable error is not overrated, by as- 

 .«uming it to amount to 10 or 12 seconds, the author refers to two 

 lists of residts given by Professor Henderson, in tlic introduction to 

 the first volume of the Edinhurrjh Astroiionikul (Jbscricilionii. One 



