Copyright © 2007 by Rolando Ligustri (CAST Observatory, Talmassons, Italy)
This image was obtained by R. Ligustri on 2007 September 11.92. Seven 180-second exposures were obtained using a 35-cm reflector and an ST10Xme CCD camera.
P. F. A. Méchain (Paris, France) discovered this comet in the evening sky near Omicron Piscium on 1790 January 9.75. Several observers, including Charles Messier and William Herschel, observed the comet for the next three weeks. Ultimately, Méchain obtained the final observation on February 1. Méchain also computed an orbit, but because of the short duration of visibility the comet's short-period nature was not recognised.
Horace Parnell Tuttle (Harvard College Observatory, Cambridge, Massachusetts) discovered this comet on 1858 January 5.01. He described it as "rather faint, but not so much so as to afford any difficulty in observing it with the great refractor." Karl Christian Bruhns (Berlin, Germany) independently discovered this comet on January 11.89. He described it as "a very large diffuse object without nucleus and distinct border." By that time, several observatories had already confirmed Tuttle's discovery, so Bruhns' name was not attached to this comet.
Charles W. Tuttle (Newburyport) used three positions obtained between January 5 and 13, and computed a parabolic orbit with a perihelion date of 1858 February 20.48 and a perihelion distance of 1.070 AU. He noted "so strong a resemblance to [the orbit] of the second comet of 1790, that there would seem to be small reason for doubt of the identity of the comet." Several other astronomers began to also suspect this identity. the first elliptical orbit was computed by James C. Watson (Ann Arbor, Michigan) during late February. He used three positions obtained between January 5 and February 25, and determined a perihelion date of February 23.26, a perihelion distance of 1.026 AU, and an orbital period of 13.96 years. Ultimately the comet passed closest to Earth (0.7604 AU) on January 21. The coma diameter reached a maximum of 3.5 to 4 arc minutes during February and early March. The comet's total magnitude may have been slightly brighter than 7 at maximum.
The comet's first predicted return was that of 1871. On October 13 Alphonse Louis Nicolas Borrelly (Marseilles, France) recovered it. Independent recoveries were made by Winnecke (Karlsruhe) on October 16 and H. P. Tuttle (Harvard Observatory) on the 22nd. The comet may have exceeded magnitude 8 when at its brightest.
This comet has been observed at every return except that of 1953 which was especially unfavorable. Since the 1871 apparition, only one return attained a maximum brightness greater that 7 and that was in 1980, when the magnitude reached 6.5.
Apparition of 1994: G. Tancredi and M. Lindgren (La Palma, Canary Islands) obtained CCD exposures of the comet, using the 2.56-meter Nordic Optical Telescope, on 1992 July 29.06 and July 29.14. They noted a stellar appearance and gave the magnitude as 21.2-21.3. Additional images were obtained with the same telescope on July 29.97, July 30.90, and July 31.20. The comet maintained its stellar appearance on each date and the magnitude remained between 21.2 and 21.4. The positions indicated the predicted orbit required a correction of only -1.7 days. Interestingly, the comet was not observed again, but it was not because of negligence. Although the comet was about 130 degrees from the sun when observed, it ultimately passed perihelion when on the other side of the sun from Earth, making this about the worse apparition possible. Following the final observation, the comet's elongation from the sun generally decreased and finally reached a minimum value of 8 degrees on 1994 June 8. Although the elongation increased to 37 degrees by 1994 September 12, it actually dropped back to 27 degrees by November 27. By the time the comet finally moved out of the sun's glare in 1995, it was too faint for observations.
Apparition of 2007-2008: This was a very nice apparition for this comet. Since its discovery in 1790, this comet has made five particularly close approaches to Earth, with the best being the discovery apparition itself, when it passed 0.37 AU from our planet. The comet beat that value on 2008 January 2, when it passed 0.25 AU from Earth! The comet was an easy binocular object and was visible to the naked eye for observers with very dark skies. It spent most of the latter half of 2007 close to the celestial north pole and then rapidly moved southward in December. By the end of January 2008, the comet was mostly visible to observers in the Southern Hemisphere.
This comet is the parent of the Ursid meteor shower, which reaches maximum on December 22. Rates are typically near 15, although outbursts of about 100 per hour occurred in 1945 and 1986, while an unexpected increase of 30 per hour came in 1973.