Astronomical and Nautical Collections, 121 



vi. Further Remarks on Lunar Transits, in illustration of Mr. 

 Henderson's Paper. 



1. It must first be observed, that when a difference of longitude 

 is expressed in time, the time intended is sidereal, and not solar. 



2. Thus the difference of longitude is equivalent to the difference 

 of absolute sidereal time at which the same star comes to the me- 

 ridian of two places. 



3. But the local sidereal time of the transit of any star is the 

 same at all places ; while the difference of the local sidereal time 

 of the transits of two stars, whether at the same place or at diffe- 

 rent places, gives the difference of their right ascensions ; and the 

 same is true of the moon and a star, or of the moon at two places. 



4. Let this difference of the moori's transit, in sidereal hours, be 



called a; it will be expressed in degrees of space by 15°a, which 



will be the difference of the moon's right ascensions ; and if the 



change of the moon's right ascension in an hour of solar time be h, 



and in an hour of sidereal time ^/<, the sidereal time required for 



,, . , .„ , 15°a , . 365.24 



this change will be , p bemg = 



^ ^h ^ '^ 366.24 



5. In this difference of absolute time will be comprehended 

 the difference of longitude c?, and the additional difference of local 



1 'i^/y 1 ^O 



time a, so that will be equal to c? + a, and d = _— — a ; 



^h ^h 



which agrees with Mr. Henderson's equation d =^ aR — a. 



h 



If we employ the Nautical Almanac, where the moon's place is com- 

 puted for apparent time, Mr. Henderson's Table may often be of 

 use in expediting the calculation. Mr. Bouvard seems to have ex- 

 pressed the difference of longitude in solar time, instead of side- 

 real, since 9™ 20* of solar time are equal to 9" 21.5' of sidereal, 

 answering to 2° 20' 22" of space. 



6. With respect to the diversity noticed by Mr. Henderson, between 

 the right ascensions deduced from the observations at Greenwich, 

 made with the transit instrument and the mural circle, it must be 

 remarked, that the Astronomer Royal does not attach the slightest 



