VOL. L.] PHILOSOPHICAL TRANSACTIONS. JOQ 



LIT. Observations on the Comet that appeared in Sept. and Oct. 1757, made at 

 the Royal Observatory by Ja- Bradley, D.D., F,R. S. p. 408. 



When Dr. B. first discovered tliis comet, it appeared to the naked eye like a 

 dull star of the 5th or 6th magnitude ; but viewing it through a 7-foot telescope, 

 he could perceive a small nucleus, surrounded as usual with a nebulous atmo- 

 sphere, and a short tail extended in a direction opposite to the sun. By com- 

 paring its situation with some near stars. Dr. B. collected, that the comet's right 

 ascension was 89° 29' 10', and its declination 35° O' 20'' north. 



In like manner he collected the places of the comet on a great many following 

 days, till the 19th of October, on the morning of which day it appeared so faint, 

 that he could not observe its place. Its elongation from the sun was then but 

 about 20 degrees ; and from that day it became always less ; which is the prin- 

 cipal reason why it was invisible at the time when in its perihelion. The elon- 

 gation will indeed soon become greater, and yet it is probable that we shall not 

 be able to see the comet again ; because its real distance from the sun will be 

 greater than it was when he first saw it, and it will be also 4 times further from 

 us than it was at that time. The comet kept nearly at the same distance from 

 the earth for ten or 12 days together after he first saw it; but its brightness 

 gradually increased then, because it was going nearer to the sun. Afterwards, 

 when its distance from the earth increased, though it continued to approach the 

 sun, yet its lustre never much exceeded that of stars of the 2d magnitude, and 

 the tail was scarcely to be discerned by the naked eye. 



Supposing the trajectory of this comet to be parabolic. Dr. B. collected from 

 the foregoing observations, that its motion round the sun is direct, and that it 

 was in its perihelion October the 21st, at 7^ 55"^ mean or equated time at Green- 

 wich. That the inclination of the plane of its trajectory to the ecliptic is 12" 

 50' 20" ; the place of the descending node y 4° 12' 50"; the place of the peri- 

 helion ^ 2'^ 58' O' ; the distance of the perihelion from the descending node 

 88° 45' 10"; the logarithm of the perihelion distance 9.528328 ; the logarithm of 

 the diurnal motion O.667636. From these elements, which are adapted to Dr. 

 Halley's general Table for the Motion of Comets in parabolic Orbits, Dr. B. 

 computed the places of this comet for the respective times of the foregoing ob- 

 servations, as in the following table, which contains likewise the longitudes and 

 latitudes deduced from the observed right ascensions and declinations, and also 

 the differences between the computed and observed places. These differences, 

 (no where exceeding 40") show, that the elements here set down will be sufficient 

 to enable future astronomers to distinguish this comet on another return ; but 

 as they do not correspond with the elements of the orbit of any other comet 

 hitherto taken notice of, we cannot determine at present its period. 



VOL. XI. 



