Mr. Squire on determining Longitudes by Occultatio?is. 443 



in excess for emersion, which might lead us to suppose that 

 in the latter case there is some error in the observations ; but 

 as we cannot suspect this to be the case at the Royal Observa- 

 tory, and the discrepancy being nearly the same at both places, 

 it may reasonably be inferred that the time of emersion at Ep- 

 ping is equally correct as that at Greenwich. And, moreover, 

 as both methods of computation are founded upon true mathe- 

 matical principles, it were for these reasons naturally to be ex- 

 pected that the results for the respective places would have been 

 the same at emersion as at immersion. But as that is not the 

 case, some natural cause must have operated in producing an 

 effect in the former instance that did not take place in the 

 latter, and which, being unknown, could not enter as a com- 

 pensating quantity into the elements of computation. 



Before any opinion is hazarded on this point, it may be 

 proper to observe, that in this occultation the immersion took 

 place at the light, and the emersion at the dark border of the 

 moon. Now it is well known that the rays of light suffer a 

 degree of inflection when they pass near the surface of an 

 opaque body. Hence in this case, when a direct ray of light 

 from the star became a tangent to the dark limb of the moon, 

 which was the absolute time of emersion, it was bent towards 

 that body, and thence thrown off at some distance from the 

 spectator ; so that the moon had to advance a few seconds in 

 its orbit before the star could be seen by the observer. 



It is pretty evident that some phenomenon of this kind 

 must have taken place at the emersion by which the occultation 

 was retarded several seconds beyond what the true semidia- 

 meter of the moon, and her visible horary motion, would give. 

 If therefore we increase the moon's radius a small quantity 

 (in the present instance 9"*74), as a compensation for the time 

 it took for her western limb to reach the incidental point of 

 inflection, then we shall have the longitudes of both places 

 as correct at emersion as at immersion*. 



Perhaps, after all, the above may not be considered an 

 adequate solution of the difficulty: — if so, I can only say that 

 I have nothing better to offer just now; and therefore hope 

 some of your scientific readers will have the goodness to give 

 their opinions on the subject, by pointing out the reason why 

 correct observations of the immersion and emersion of a star 

 at any place should not give the longitude the same, with the 

 same method of computation founded upon a true mathema- 

 tical basis. I have not thought it necessary to enter minutely 

 into the method of solution, as that must be evident to most 



* Some observations on the value of the Moon's inflection will be found 

 in No. 29 of the Proceedings of the Astronomical Society, p. 190. — Edit. 



3 L 2 who 



