Caroline Herschel (Slough, England) found this comet in the evening sky on 1788 December 21.8. She had been searching for comets with a small Newtonian telescope, when she found the nebulous object near Beta Lyrae. On December 21.83 William Herschel (Slough, England) observed the new comet with a larger telescope and said, "it had the appearance of a considerably bright nebula; of an irregular, round form; very gradually brighter in the middle; and about five or six minutes in diameter." W. Herschel also noted it was "a much larger object than" the planetary nebula M57. Higher magnifications could not be used on the comet because of its low altitude above the northwestern horizon. W. Herschel observed the comet again on December 22.23, low over the northeastern horizon, and noted it had moved northeastward. He wrote, "This and several evenings afterwards I viewed the comet again with such powers as its diluted light would permit, but could not perceive any sort of nucleus which, had it been a single second in diameter, I think, could not well have escaped me."
Roger Rigollet (Lagny, France) discovered this comet low in the northeastern sky before sunrise on 1939 July 28.09. It was described as diffuse, with a central condensation and a magnitude of 8. The daily motion was given as 4 degrees to the northeast. The comet was confirmed on July 29.08 by A. Fresa (Pino Torinese Observatory, Turin, Italy). George van Biesbroeck (Yerkes Observatory, Wisconsin, USA) independently confirmed the comet later on the 29th and determined the total magnitude as 8.0. He said there was a sharp nucleus and a broad, fan-shaped tail extending 3 arc minutes in PA 290°.
Apparition of 1788: Observations were only made by English observers through the end of December and into January, with the Reverend Nevil Maskelyne (Greenwich, England) obtaining his first view of the comet on December 26. Charles Messier (Marine Observatory, Paris, France) found out about the comet from a letter he received on January 3. Although he failed to find the comet that evening because of moonlight, he succeeded in finding a small mass of luminosity just above Alpha Lyrae on the morning of the 4th. He described it as extremely faint and wrote, "within the refractor, the mass disappeared from time to time ...." Messier said the object exhibited no nucleus, or tail, but was just a faint, even light. He noted the configuration of the stars and one hour later noted that the object had moved, but twilight then ended observation. No position was obtained. Messier searched for the comet in the evening sky, but the brightness of the moon still prevented an observation. Messier next saw the comet on the mornings of January 5 and 7 and found it difficult to see on both occasions. On the former date he noted the comet disappeared with the slightest illumination of the wires of his micrometer. Interestingly, Messier lost the comet after the 7th because of poor skies and finally abandoned additional searches after the 12th. Despite Messier's observations coming to an end, fellow Parisian P. F. A. Méchain (Royal Observatory) was able to obtain his first and last observations on January 15 and 18, respectively. Thereafter, Maskelyne was the lone observer of the comet as he continued to observe it through the remainder of January and into February. His final observation was obtained on February 5.21.
Very similar orbits were computed by Méchain (1789) and Margaretta Palmer (1922), with Méchain supplying two parabolic solutions and Palmer one. Palmer also computed a hyperbolic orbit, with an eccentricity of 1.001809, and three elliptical orbits, with periods ranging from 1066 to 8558 years. Palmer said, "owing to the inaccuracy of the existing observations of the comet ... and the short period of visibility, during which the relative positions of the Earth and comet were unfavorable for an accurate determination of the orbit, it is impossible to determine the elements with any degree of certainty." She said the orbits which fit the observations best were the parabolic one and the elliptical solution with a period of 8558 years. The elliptical solution with a period of 1066 years least represented the observations.
Apparition of 1939: During this apparition, observers reported a maximum magnitude of about 7 during August, while the visual tail length was frequently estimated as between 1° and 2°. The comet steadily faded after August. The final observations were obtained photographically by Jeffers and Adams with the 91.4-cm f/5.8 Crossley reflector on 1940 January 16.42 and January 16.49. They said the nucleus was about magnitude 19 and seemed situated "on the northern edge of a faint and diffuse coma." This coma was measured as 1x 0.3 arc minutes. The moon was full on January 24.
The first orbits computed following the 1939 rediscovery were by Jens P. Möller (Copenhagen, Denmark), and Katherine P. Kaster and Thomas Bartlett (Berkeley, California). They were published on August 2 and indicated a perihelion date of 1939 August 9. Based on the early orbits, Leland E. Cunningham (Harvard College Observatory, Massachusetts) suggested that this comet was probably identical with Herschel's 1788 comet. Following several additional parabolic solutions, the first elliptical orbit was calculated by Allan D. Maxwell and Kaster during the first half of September. They began by assuming an orbital period of 150 years, based on Cunningham's suggested link, and determined a perihelion date of August 9.46. Using precise positions obtained on July 29, August 6, and 13, F. W. Hoffman computed an elliptical orbit which was first published in early November 1939. The perihelion date was determined as August 9.49 and the orbital period was about 125 years. Maxwell and Kaster published a revised orbit near the end of 1940 which used positions spanning a five-month arc. They found a period of 156 years. B. G. Marsden (1974) used 75 positions obtained during the 1788 and 1939-40 apparitions, as well as perturbations by five planets, and approximately linked the two apparitions. The perihelion date was determined as August 9.46 and the period was 155 years.
Close approaches to planets: Only two close approaches to Earth occur during the period spanning the 18th through the 21st centuries. (From the orbital work of Kazuo Kinoshita)
- 0.80 AU from Earth on 1788 November 4
- 0.82 AU from Earth on 1939 July 30
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