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31S ' Prof. Fisher on the Orbit of the Comet of 1819. 



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The observations which were selected, as possessing, on the 

 whole, the most advantages for correcting the first approximation^ 

 were those made on the evenings of July 9th and 19th, and on 



the morning of July 31st. 



In calculating the geocentric latitudes and longitudes of the 

 comet, the following method was pursued. The mean right as- 

 censions and declinations of the stars employed were taken from 



r 



the most approved tables, and corrected for precession, aberration, 

 and nutation, in such a manner as to give their apparent right 

 ascensions and declinations at the times of observation. The alti- 

 tudes of the comet and stars were found with sufficient accuracy 

 for determining their refraction, which was corrected for the state 

 of the barometer and thermometer. The parallax of the comet 

 was nearly estimated from the approximate elements, and applied 

 to its refraction. The true distance was then calculated from the 

 apparent in a mode analogous to that employed for clearing lunar 

 distances. With the aid of the true distances thus found, and 

 rendered simultaneous by the application, to one of them, of a 

 proportional part of the daily motion, the right ascension and de- 

 clination, and from these, the latitude and longitude were deter- 

 mined. To obtain the true from the apparent latitude and 



r 

 ^^^ r 



longitude, a correction for aberration was necessary. This was 



deduced from the daily geocentric motion, and the distance of th 



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comet from the earth, (known nearly from the first approxima 

 tion,) by the method given, Vince's Ast. vol. i. page 3S6. Th( 

 latitudes and longitudes of the comet, and the longitudes of th( 

 sun, obtained by adding 20'' to those given in the Nautical Alma- 

 nac, are subjoined. 



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