Observations of a Cornel. 65 



iabctiracy, 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 64 7<191> and the rest of the given 

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

 ascending 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 

 compute by these data the angle at the sun. Then by cal- 

 culating in the next place the radius vector of the comet, 

 aud 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 • 1 69 1 92, the mean 

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

 comet was observed to subtend an angle of 1", which brought 

 to the mean distance of the Earth gives i'*l 69, and since 

 we also know that the Earth's diameter, which, according 

 to Mr. Dalby, is 7913-2 miles*, subtends at the same di- 

 stance an angle of 17"*2, we deduce from these principles the 

 real diameter of the comet, which is 53S miles. 



Having thus investigated the magnitude of our comet, we* 

 may in the next place also apply calculation to its illumina- 

 tion. The observations relating to the light of the comet - 

 were made from the 4th of October to the lyth. 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 defined on its borders* 

 Now as that part of the disk which was then visible to us 

 could not possibly have a full illumination from -the sun, I 

 have calculated the phases of the comet for the 4th and for 

 the 19th ; the result of which is, that on the 4th the illumi- ' 

 nation was 119° 45' 9", as represented in Plate II. fig. 4, 

 and that on the 19th it had gradually increased to 124° 22' 

 40", of which a representation is given in fig. 5. Both phases 

 appear to me sufficiently defalcated, to prove that the comet 

 did not shine by light reflected from the sun only; for, had 



* Sec Philosophical Transactions for I 791, p. 239. Mr. Dalby gives the 

 two semiaxes of the Earth, from a mean of which the above diameter 

 79 13-1682 is obtained. s 



Vol. 33. No. 129. Jan. 1809, E this 



