Charles Messier (France) had just completed an observation of Jupiter and was in the process of observing several nebulosities he had seen in 1764, when he discovered this comet in Sagittarius on 1770 June 14.95. He said the comet was very faint and scarcely visible in a two-foot focal length telescope, although the nucleus was vivid and white. Messier added that the comet was rather similar to the nebula situated between the head and bow of Sagittarius [Probably M22, which has a total magnitude of 5.1 and a diameter of 24'].
At the time of the comet's discovery, the comet was approaching both the sun and Earth. Messier's own observations revealed the coma increasing very rapidly in size during the remainder of June. He gave the coma diameter as 5' 23" on the 17th, 18' on the 22nd, 27' on June 24, and 54' on the 29th. Messier also noted the comet first became visible to the naked eye on June 20, while on the 24th he noted the comet was as bright as a star of magnitude 2. On June 26, the Chinese observed "an extraordinary star" that was described as greyish-yellow, and was "as large as a pellet." On that same date, David Rittenhouse (Norriton, Pennsylvania, USA) first observed the comet in the evening sky and commented, "from its situation at that time, I expected it would have been visible for many weeks, if not months; and therefore did not prepare, with such expedition as I might have done, for observing its place with accuracy." On June 27, Samuel Dunn (England) independently discovered the comet on June 27. He said there was no tail, but the coma was about one-half degree across and contained a silver-colored nucleus. Dunn added that the portion of the coma closest to the sun was brighter. On June 28, William Earl (Stirling, Scotland) independently discovered this "new star" about 78° from Polaris. He described it as "larger than a star of the first magnitude, of a dull light, with a bright speck or nucleus, in the center. I take it to be a comet, and that its tail is from us." On June 30, the Chinese said the comet "travelled 32 degrees to the north." Messier saw the comet with the naked eye through breaks in the clouds.
The comet passed only 0.0146 AU from Earth on July 1, which is the closest a comet has passed our planet in recorded history. Messier said the coma was 2° 23' across. Earl wrote, "It seemed to me to be increased in size, the shape rather more oval than circular, the nucleus no longer in the center, but advanced towards the northern part of the whole appearance." James Six (Canterbury, England) said the comet crossed 42° of the sky during the previous 24 hours. He wrote that the nucleus "appeared as large as the planet Jupiter, surrounded with a coma of silver light, the brightest part of which was as large as the moonÕs orb, and, gradually diminishing its splendor, continued visible 3 or 4 deg. at least from the nucleus, and, as near as I could judge, (from the short time I viewed it) equal on every side."
The comet attained its most northerly declination of +81° on July 2. Rittenhouse said the comet exhibited "some appearance of a tail on the side opposite to the sun." On July 3, Messier searched for the comet through breaks in the clouds. He said the coma was still so large that, at one point, he noticed it protruding above and below a cloud which covered the comet's center. Rittenhouse also saw the comet. On July 4, Six was the only apparent observer of the comet, as Messier said the comet was lost in the sun's rays and Earl unsuccessfully searched until morning twilight. The comet attained its smallest solar elongation of 17° on July 8. The Chinese wrote that the comet "went out of sight" on July 12. Messier was greeted with cloudy skies from July 4 to 11. He searched diligently for the comet on July 12 and 13, in both morning and evening twilight, but found nothing.
While the comet was lost in the sun's glare orbital computations were made by A. G. Pingré, and an ephemeris was determined for the period of July 19 to September 1. Messier began searching for the comet in twilight on July 19, but failed to find anything. He continued searching during the mornings that followed, but again met with failure. Clouds prevented searches during the period of July 23 to August 2, but Messier finally recovered the comet on August 3, when he said it was seen with great difficulty with the naked eye in the morning sky. Messier added that a small telescope revealed a brilliant nucleus, measuring 54" across, surrounded by a coma 15' across. Observations continued throughout August and September. As October began Messier was experiencing much difficulty in seeing the comet. He described it as "nearly invisible" on October 2 and "extremely faint and very difficult to observe" on the 3rd. The latter date marked the final time the comet was seen.
Fairly similar orbits have been computed by several different astronomers and mathematicians. About half are parabolic, with perihelion dates around 1770 August 9 and 10, while the rest are elliptical, with perihelion dates around 1770 August 13 and 14. The parabolic orbits were computed by Pingré (1770), Rittenhouse (1771), Eric Prosperin (1776), F. A. Widder (1776), Slop Von Cadenberg (1782), and Johann Heinrich Lambert (1782). Prosperin noted that Messier's observations could not be represented by a parabola and suggested that the comet moved in an elliptical orbit. The elliptical orbits were computed by Anders Johan Lexell (1776, 1777, 1778, 1782), with a period of 5.58 years, Pingré (1784), with a period of 5.43 years, J. K. Burckhardt (1807), with a period of 5.57 years, Thomas Clausen (1842), with periods of 5.61 and 5.60 years, and Urbain Jean-Joseph Le Verrier (1844, 1848), with periods of 5.63 and 5.60 years. Lexell noted that the comet may have been placed in its 1770 orbit as a result of a very close approach to Jupiter in 1767.