1789 apparition Caroline Herschel first observed the comet on 21 December 1788 and it was observed later that night by her brother
William Herschel who described it as looking like a bright
nebula and about 5–6
minutes in diameter, and much larger than the
planetary nebula,
Messier 57. Through December and January the comet was observed by
Nevil Maskelyne at the
Greenwich Observatory and by
Charles Messier at the
Paris Observatory. Maskelyne was the last observer of the comet, his final observation taking place on 5 February 1789. Similar possible
orbits for the comet were calculated in 1789 by
Pierre Méchain and in 1922 by
Margaretta Palmer. Palmer considered that the orbit which best fitted the observations was an elliptical one with a
period of 1,066 years.
1939 apparition Roger Rigollet rediscovered the comet on 28 July 1939; it was described as diffuse and with a
magnitude of 8.0. The sighting was confirmed the next day by
Alfonso Fresa of the
Observatory of Turin (
Italy) and
George van Biesbroeck of the
Yerkes Observatory. The comet steadily faded after August, final (photographic) observations being obtained on 16 January 1940. The comet was also extensively observed from the
Incheon Meteorological Observatory between July and September 1939. W. H. Dirk captured a series of photographic plates of the comet alongside recording its positions from August to September 1939. Following the 1939 rediscovery, the comet's orbit was calculated by
Jens P. Möller (
Copenhagen,
Denmark), and
Katherine P. Kaster and
Thomas Bartlett (
Berkeley, USA). A
perihelion date of 9 August 1939 was indicated. Based on these early orbits,
Leland E. Cunningham of the
Harvard College Observatory suggested that the comet was likely identical with Herschel's Comet of 1788. The final calculation of the orbit, by
Brian G. Marsden in 1974, used 75 positions from both apparitions of the comet in 1788 and 1939–40 in addition to perturbations by planets, and linked the two sightings, with a perihelion date of 9 August 1939 and a period of 155 years. == Closest approaches to Earth ==