OBSERVATIONS OF A COMET. 



11 



measured with all the accuracy that can be used ; and when 

 the result of such a measure has given us the diameter of 

 this satellite, it may by calculation be brought to the dis- 

 tance from the Earth at which, in my observation, it was 

 compared with the diameter of the comet, and thus more 

 accuracy, if it should be required, may be obtained. The 

 following result of my calculation however appears to me 

 quite sufficient for the purpose of general information. From 

 the perihelion distance 0*G474<>1, and the rest of the given 

 elements of the comet, we find, that its distance from the as- 

 cending node on its orbit at the time of observation was 

 73° 45' 44"; and having also the Earth's distance from the 

 same node, and the inclination of the comet's orbit, we com- 

 pute by these data the angle at the sun. Then by calculat- 

 ing in the next place the radius vector of the comet, and 

 having likewise the distance of the Earth from the sun, we 

 find by computation, that the distance of the comet from the 

 Earth at the time of observation was 1*169192, the mean dis- 

 tance of the Earth being 1. Now since the disk of the 

 comet was observed to subtend an angle of l", which 

 brought to the mean distance of the Earth gives l<*l69, 

 and since we also know that the Earth's diameter, which, 

 according to Mr. Dalby, is 79-13 ,c 2 miles*, subtends at the 538 miles m 

 same distance an angle of I7""2 we deduce from these ia e r * 

 principles the real diameter of the comet, which is 538 

 miles. 



Having thus investigated the magnitude of our comet, we Its illumina- 

 may in the next place also apply calculation to its iliumina- tion * 

 tion. The observations relating to the light of the comet 

 were made from the 4th of October to the 19th. In all 

 which time the comet uniformly preserved the appearance 

 of a planetary disk fully enlightened by the sun: it was 

 every where equally bright, round, and well denned on its 

 borders. Now as that part of the disk which was then visi- 

 ble to us could not possibly have a full illumination from 

 the sun, 1 have calculated the phases of the comet for the 



• See Phil. Trans, for 1791, page 239. Mr. Dalby gives the two se. 

 miaxes of the Earth, from a mean of which the above diameter 7913-1682 

 |S obtained. 



4th 



