Observations upon the Comet That Appeared in the Months of September and October 1757, Made at the Royal Observatory by Ja. Bradley, D. D. Astronomer Royal, F. R. S. and Member of the Royal Academy of Sciences at Paris

LII. Observations upon the Comet that appeared in the Months of September and October 1757, made at the Royal Observatory by Ja. Bradley, D.D. Astronomer Royal, F.R.S. and Member of the Royal Academy of Sciences at Paris. Read Dec. 22, 1757. Deferred to give an account of my observations upon the Comet that hath lately appeared, till I could settle the places of the stars with which it had been compared; several of them not being inserted in the British catalogue, and those which are, requiring some small corrections, which I have since made from my own observations. When I first discovered this Comet, it appeared to the naked eye like a dull star of the 5th or 6th magnitude; but viewing it thro' a seven-foot Telescope, I could perceive a small Nucleus (surrounded, as usual, with a nebulous atmosphere), and a short tail extended in a direction opposite to the sun. Some small stars then appearing in the field of the telescope with the Comet, I measured its distance from them with a Micrometer; and on September 12th at 16h 2' mean time, I found it to be $1^\circ 13' 5''$ distant from a small star, whose right ascension was afterwards found to be $89^\circ 49' 40''$ and declination $36^\circ 11' 30''$ north: and near the same time the Comet was observed to be $43' 10''$ from another star, whose right ascension was $90^\circ 20' 0''$ and declination $35^\circ 12' 0''$ north. Hence Hence I collected, that the Comet's right ascension was $89^\circ 29' 10''$ and its declination $35^\circ 0' 20''$ north. September $13^d\ 12^h\ 37'$ mean time (which is likewise made use of in the following observations), the Comet had the same right ascension with a small star, whose right ascension was $93^\circ 5' 30''$ and declination $34^\circ 36' 40''$ north; and it was about two minutes more northerly than the star. Hence the Comet's right ascension was $93^\circ 5' 30''$ and its declination $34^\circ 38' 40''$ north. September $14^d\ 14^h\ 0'$ the Comet preceded $\theta$ Geminorum $1^\circ 31' 35''$ in right ascension, and was $11' 35''$ more southerly. The apparent right ascension of $\theta$ Geminorum was then $99^\circ 11' 40''$ and its declination $34^\circ 13' 25''$ north. Hence the right ascension of the Comet was $97^\circ 40' 5''$ and its declination $34^\circ 1' 50''$ north. Sept. $17^d\ 13^h\ 0'$ a small star (whose right ascension was $109^\circ 55' 20''$ and declination $31^\circ 27' 40''$) preceded the Comet $47' 10''$ in right ascension, and was $12' 30''$ more northerly. Hence the Comet's right ascension was $110^\circ 42' 40''$ and its declination $31^\circ 15' 10''$ north. Sept. $19^d\ 15^h\ 17'$ a star (whose right ascension was $118^\circ 29' 40''$ and declination $28^\circ 9' 45''$) preceded the Comet $1^\circ 14' 0''$ in right ascension, and was more southerly $15' 45''$. Hence the Comet's right ascension was $119^\circ 43' 40''$ and declination $28^\circ 25' 30''$ north. Sept. $23^d\ 15^h\ 57'$ a star (whose right ascension was $134^\circ 59' 45''$ and declination $22^\circ 15' 55''$ north) preceded the Comet $12' 30''$ in right ascension, and was $29^\circ 0''$ more northerly. Hence the Comet's right ascension was $135^\circ 8' 15''$ and its declination $21^\circ 46' 55''$ north. Sept. 24d $15^h 21'$ the Comet had the same declination with a small star that preceded it $10^\circ 15''$ in right ascension. This star's right ascension was afterwards found to be $138^\circ 13' 45''$ and its declination $20^\circ 5' 20''$. Hence the Comet's right ascension was $138^\circ 24' 0''$ and its declination $20^\circ 5' 20''$ north. Sept. 28d $16^h 22'$ the Comet followed Regulus $1^\circ 7' 12''$ in right ascension, and was $14' 45''$ more northerly. The right ascension of Regulus being then $148^\circ 51' 13''$ and its declination $13^\circ 8' 35''$ north; the Comet's right ascension was $149^\circ 58' 25''$ and its declination $13^\circ 23' 20''$ north. Sept. 30d $16^h 24' \delta$ Leonis (whose right ascension was $155^\circ 0' 10''$ and declination $10^\circ 32' 53''$ north) followed the Comet $18' 45''$ in right ascension, and was $7' 53''$ more northerly. Hence the Comet's right ascension was $154^\circ 41' 25''$ and its declination $10^\circ 25' 0''$ north. October 2d $16^h 48'$ the 37th star Sextantif. Hevel. in the British Catalogue (whose right ascension was $158^\circ 21' 25''$ and declination $7^\circ 38' 40''$ north) preceded the Comet $32' 50''$ in right ascension, and was $3' 20''$ more southerly. Hence the Comet's right ascension was $158^\circ 54' 15''$ and its declination $7^\circ 42' 0''$ north. October 3d $16^h 45' \epsilon$ Leonis (whose right ascension was $162^\circ 2' 15'$ and declination $7^\circ 24' 0''$ north) followed the Comet $1^\circ 12' 55''$ in right ascension, and was $56' 40''$ more northerly. Hence the Comet's right ascension was $160^\circ 49' 20''$ and its declination $6^\circ 27' 20''$ north. October 4th 17h 0' d Leonis (whose right ascension was 162° 0' 15" and declination 4° 54' 57" north) preceded the Comet 40' 15" in right ascension, and was more southerly 20' 53". Hence the Comet's right ascension was 162° 40' 30" and its declination 5° 15' 50" north. October 7th 16h 54' the 79th Leonis in the British Catalogue (whose right ascension was 167° 53' 37" and declination 2° 44' 15" north) followed the Comet 13' 0" in right ascension, and was more northerly 38' 35". Hence the Comet's right ascension was 167° 40' 37" and its declination 2° 5' 40" north. October 8th 16h 53' the Comet preceded v Leonis 1° 53' 30" in right ascension, and was 37' 20" more northerly. The right ascension of this star was 171° 7' 45" and its declination 0° 30' 55" north; therefore the Comet's right ascension was 169° 14' 15" and its declination 1° 8' 15" north. October 11th 16h 52' the Comet followed v Leonis 2° 33' 30" in right ascension, and appeared 1° 55' 5" more southerly; but it being near the horizon, the difference of right ascension must have been contracted by refraction about 1' 5", and the difference of declination about 1' 30": so that the corrected right ascension of the Comet was 173° 42' 20" and its declination 1° 25' 40" south. Immediately after this observation a fog arose, which prevented me from repeating it; and several mornings following proving hazy or cloudy, I could not see the Comet again till October 18th, about an hour and a quarter before sun-rising; when the twilight being strong, and the Comet low, it appeared very faint. However, I was unwilling to omit the oppor- opportunity of determining its place, as near as I could, by a single observation, in the following manner. At $6^h\ 59' \ 54''$ sidereal time, I observed the passage of the Comet over the perpendicular wire of my equatorial Sector; then leaving the instrument in the same position till the next evening, I observed, that at $22^h\ 8' \ 15''$ sidereal time, the 17th star of Eridanus in the British Catalogue passed over the same wire (or horary circle) $9' \ 30''$ more southerly than the Comet. And at $23^h\ 45' \ 36''$ sidereal time the star marked $b$ in Eridanus passed, $19' \ 55''$ more northerly than the Comet. I found that the situation of my instrument was not sensibly altered between the 18th and 19th of October; for the transits and the difference of declination of the same stars being observed with it again on the 19th of October, they agreed very well with those that were taken the preceding night. It may therefore be supposed, that the position of the instrument continued the same likewise during the time of the foregoing observations. The right ascension of the 17th star of Eridanus being $49^\circ\ 39' \ 10''$ and its declination $5^\circ\ 55' \ 25''$ south; and the right ascension of $b$ of Eridanus being $73^\circ\ 59' \ 25''$ and its declination $5^\circ\ 25' \ 10''$ south; I collected, that when the Comet passed the wire (or horary circle) which was October $17^{d}\ 17^{h}\ 12'$ mean time, its right ascension was $182^\circ\ 34' \ 0''$ and its declination $5^\circ\ 45' \ 35''$ south. The last time that I saw the Comet was on the 19th of October in the morning; but it then appeared so faint, that I could not observe its place. Its elongation from the sun was then but about 20 degrees; degrees; and from that day to the present it hath always been less; which is the principal reason why it was invisible to us at the time when it was in its perihelion, and hath remained so ever since. The elongation 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 I first saw it, and it will be also four times further from us than it was at that time. The Comet kept nearly at the same distance from the earth for ten or twelve days together after I 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, although it continued to approach the sun, yet its lustre never much exceeded that of stars of the second magnitude, and the tail was scarce to be discerned by the naked eye. All the forementioned observations were made with a Micrometer in a seven-foot Tube, excepting those of the 3d, 11th, and 17th days of October, which were taken with a curious Sector constructed for such purposes by the late ingenious Mr. George Graham; of which Dr. Smith has given a very exact description in his third book of Optics. Supposing the Trajectory of this Comet to be parabolic, I collected from the foregoing observations, that its motion round the sun is direct, and that it was in its perihelion October the 21st, at $7^h\ 55'$ mean (or equated) time at Greenwich. That the inclination of the plane of its Trajectory to the ecliptic is $12^\circ\ 50'\ 20''$; the place of the descending Node $8^\circ\ 4'$. 12' 50"; the place of the Perihelion Ω 2° 58' 0"; 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 0.667636. From these Elements (which are adapted to Dr. Halley's general Table for the Motion of Comets in parabolic Orbits), I computed the places of this Co- met 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 ob- served places. These differences (nowhere exceeding 40") shew, that the elements here set down will be sufficient to enable future astronomers to distin- guish this Comet upon 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 the period thereof. Greenwich, | Greenwich, 1757 | |----------------| | Mean Time | | d. b. | | Comet. Long. | | Observ. | | Latit. Observ. | | Latit. Comp. | | Latit. Comput. | | Diff. Long. | | Diff. Latit. | | Sept. 12 | 16 | 2 | II 29 | 34 | 13 | II 32 | 16 No. | II 29 | 34 | 11 | 32 | 20 No. | II 11 | 12 | 11 | |----------------|----|---|-------|-----|-----|-------|--------|-------|-----|-----|-----|--------|-------|----|----| | | 13 | 12 | 37 | $2 | 35 | 13 | $12 | 13 | 44 | 3 | 10 | 44 | 3 | 10 | 43 | 43 | | | 14 | 14 | 6 | 37 | 45 | 10 | 44 | 3 | | | | | | | | | | Octob. 2 | 16 | 48 | 7 | 37 | 43 | 1 | 5 | 50 | 1 | 32 | 22 | 1 | 56 | 42 | 1 | 56 | 42 | | | 3 | 16 | 45 | 9 | 51 | 36 | 1 | 56 | 42 | 1 | 56 | 42 | 1 | 56 | 42 | 1 | 56 | 42 | | | 4 | 17 | 0 | 12 | 1 | 4 | 1 | 56 | 42 | 1 | 56 | 42 | 1 | 56 | 42 | 1 | 56 | 42 | | | 7 | 16 | 54 | 17 | 51 | 3 | 2 | 56 | 48 | 1 | 51 | 6 | 17 | 51 | 6 | 17 | 51 | 6 | | | 8 | 16 | 53 | 19 | 39 | 45 | 3 | 13 | 7 | 19 | 39 | 33 | 3 | 19 | 39 | 33 | 3 | 19 | 39 | | | 11 | 16 | 52 | 24 | 47 | 22 | 3 | 48 | 49 | 24 | 47 | 47 | 3 | 24 | 47 | 47 | 3 | 24 | 47 | | | 17 | 17 | 12 | 4 | 38 | 58 | 4 | 15 | 42 So. | 4 | 15 | 42 So. | 4 | 15 | 42 So. | 4 | 15 | 42 So. |