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LXVII. On determining the Longitude by Occultations of the 

 fixed Stars. By Mr. Thomas Squire. 



To the Editors of the Philosophical Magazine and Annals. 

 Gentlemen, 

 r^CCULTATIONS of the fixed stars by the moon are 

 ^^ phenomena that would seem to offer the best means for 

 determining the longitude of places on the earth's surface of 

 any yet known, at least as far as regards observation; as the 

 instant of immersion or emersion can mostly be obtained to 

 the fraction of a second. But, on the other hand, the com- 

 putations which are necessary for obtaining the desired re- 

 sults may, under certain circumstances, require data that are 

 not so well established as the nature of the problem demands ; 

 and therefore, though the observations may be taken with the 

 greatest care, yet the longitudes thence obtained may not al- 

 ways prove so satisfactory as the known accuracy of the ob- 

 servations might lead us to expect, even when the process of 

 computation is managed with the greatest circumspection. 



In confirmation of the above remarks the occultation of 

 Aldebaran, on the 15th of October 1829, may be very pro- 

 perly cited as an example. The weather proving favourable 

 at the time of this phenomenon, both the immersion and 

 emersion of the star were accurately observed at Greenwich 

 and at Epping ; and from these observations computations 

 have been made upon two suppositions, with the view of ob- 

 taining the respective longitudes. First, by considering the 

 effects of parallax as computed from the altitude of the star, 

 at the same time using the moon's semidiameter without aug- 

 mentation for altitude, and the orbital angle as given in the 

 elements. Secondly, by using the parallactic depression as 

 found from the apparent zenith distance of the moon's centre, 

 also her visible semidiameter, and the correct orbital angle as 

 found for the times of immersion and emersion. 



Hence, according to the first rule the immersion gives the 

 longitude of Greenwich = s , as it ought to do ; and that of 

 Epping = 25 s, 37 E., which is also very near the truth; but 

 the emersion makes the longitude of the former place = 

 23 s, 34 E ! and that of the latter = 49 s -34 E ! 



Again, by the second rule the longitude of Greenwich = 

 S, 8 E., and that of Epping = 25 s, 92 E., according to the 

 times of immersion ; but by emersion the longitude of Green- 

 wich = 23 s "28 E ! and that of Epping = 49 s '49 E ! which 

 numbers agree very nearly with the above. 



Here then we see that the results according to both me- 

 thods are correct, or very nearly so, for immersion, yet greatly 



in 



