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84P/Giclas

Past, Present, and Future Orbits by Kazuo Kinoshita

A. Nakamura image of 84P exposed on 1999 October 20
Copyright � 1999 by Akimasa Nakamura (Kuma Kogen Astronomical Observatory, Japan)

The CCD image was taken on 1999 October 20.74 UT, using a 0.60-m f/6 Ritchey-Chretien telescope.

Discovery

     Henry L. Giclas (Lowell Observatory, Arizona, USA) discovered this comet in Cetus on 1978 September 8.30. He determined the magnitude as 15.6, and described it as diffuse, with a condensation and a possible elongation toward the west. Giclas subsequently found a prediscovery image on a plate exposed on September 3.33.

Historical Highlights

  • Using positions obtained through 1978 September 12, Brian G. Marsden (Central Bureau for Astronomical Telegrams) published both a parabolic and an elliptical orbit on September 14. The parabola indicated a perihelion date in 1979 January, while the elliptical orbit indicated perihelion would occur in 1978 November. The latter orbit also indicated an orbital period of 6.74 years. Marsden commented, "The elliptical elements seem more probable" though he noted the eccentricity, and therefore the orbital period, was uncertain. A few days later the elliptical orbit was confirmed and on September 18 Marsden published an orbit with an orbital period of 7.16 years. The orbit was continually revised during the comet's entire apparition, although Marsden's orbit published on October 2 was the first to very closely match the comet's final orbit. It indicated a perihelion date of 1978 November 21 and an orbital period of 6.68 years.
  • Observations obtained during the comet's discovery apparition indicated a possible brightening following discovery as several observers gave the magnitude as 15 to 15.2 during the remainder of September. This was no doubt a result of the comet's steady approach to both the sun and Earth. The comet passed closest to Earth on 1978 October 2 (0.81 AU) and a slow fading seem to set in thereafter. By mid-October the magnitude was estimated as near 15.5, and it was close to 16 by the end of the month. The comet passed closest to the sun on November 21 (1.73 AU) and by the end of the year the brightness was given as 17.0. Observations finally ended on March 28, when astronomers at Harvard College Observatory's Agassiz station described the comet as weak and diffuse.
  • The comet was next predicted to return to perihelion in 1985. Edgar Everhart (Chamberlin Observatory field station, Colorado, USA) recovered the comet on an exposure obtained by John Briggs with the 0.4-m reflector on 1985 June 22.39. The recovery went unconfirmed until an independent recovery was made by C. Y. Shao (Oak Ridge Observatory) on an exposure obtained by G. Schwartz on July 18.30. Everhart estimated the brightness as magnitude 20, while Shao estimated it as 18. A third independent recovery was made by Tsutomu Seki (Kochi Observatory, Geisei station, Japan) on July 22.76. He estimated the magnitude as 18.5. The precise positions indicated the prediction by S. Nakano required a correction of -0.66 day. Shao and Seki both noted the comet was so condensed it was nearly stellar in appearance. Although the comet was predicted to become no brighter than magnitude 16, amateur astronomers found it slightly brighter than magnitude 13.5 during the latter half of October. By December the brightness had dropped to 14 and the comet was last detected on 1986 January 19, when T. Gehrels and James V. Scotti determined the magnitude as 16.9.
  • The comet was next expected at perihelion in 1992 and Seki recovered it on 1992 June 30.78. He estimated the magnitude as 18 and described the comet as diffuse with a central condensation. The comet brightened and slightly surpassed 15th magnitude during November and December. It was last seen on March 20.45 at magnitude 17.6.
  • During 1995 Brian Skiff (Lowell Observatory, Arizona, USA) began measuring the positions of minor planets on plates taken by Clyde W. Tombaugh (Lowell Observatory) during the 1930's. On plates exposed on 1931 September 12, 16, and 21, Skiff found images of a comet. Precise positions were measured and Skiff sent the information to B. G. Marsden of the Central Bureau for Astronomical Telegrams. Marsden tried to fit an orbit to the positions and found an elliptical one represented the observations best. The positions and orbit were published in 1995 April. IAU Circular 6168 (1995 May 5) announced that R. J. Bouma (Groningen, The Netherlands) suggested "that the observed positions and orbital elements of comet D/1931 R1...are completely compatible with identity with comet 84P/Giclas." This was confirmed by Marsden a short time later when he successfully linked the 1931 observations with those of 84P up to 1993.
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