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15P/Finlay

Past, Present, and Future Orbits by Kazuo Kinoshita

Discovery

     W. H. Finlay (Royal Observatory, Cape of Good Hope, South Africa) discovered this comet with a 7-inch equatorial on 1886 September 26.83. He described it as round, one arc minute across, and "very slightly more condensed towards the centre." He added that the comet was faint, and about magnitude 11, but exhibited no tail. He was able to determine additional positions on September 27.80 and 27.85, which confirmed the object was moving.

Historical Highlights

  • The comet was fairly widely observed as September came to a close, with observations being reported from Europe, Australia, and the United States. The comet remained a faint object until the last days in October, when observers reported the comet was bright and about 2.5 arc minutes across. A more significant condensation began being reported at about the same time. The comet reached a diameter of about 3 arc minutes around mid-November. Fading had set in as November came to a close. As December progressed, more and more observers remarked on how faint the comet was. The coma diameter was back to 2 arc minutes during January and the few observers managing to see the comet in February usually described it as excessively faint. The comet was last seen on April 12.
  • The first parabolic orbits were published during the first days of 1886 October, with H. Oppenheim (Berlin) pointing out a similarity between this orbit and that of de Vico's lost periodic comet of 1844. Lewis Boss (Dudley Observatory, Albany) determined one of the earliest elliptical orbits during late October and gave the orbital period as 4.32 years. He noted the large discrepancies between the orbit of this comet and de Vico's of 1844 were probably due to planetary perturbations. After further observations had been made, Boss determined a revised orbit during 1887 January. He then conducted a meticulous check for the planetary perturbations necessary to have changed the orbit from the possible 1844 apparition to the 1886 appearance. Boss concluded that no such perturbations existed and that de Vico's comet could not be the same as Finlay's.
  • L. Schulhof (Paris) provided a prediction for the comet's 1893 return and Finlay recovered the comet on May 18. He described it as very diffuse, circular, and 1 arc minute across. He added that the comet appeared faint, with a magnitude of about 11, and exhibited no tail. The comet was followed until September 22.
  • The comet was missed at the unfavorable 1899 return, but was recovered at the very favorable 1906 return. The comet passed 0.27 AU from Earth on 1906 August 16, at which time the coma diameter was given as 12 arc minutes. The comet continued to brighten thereafter as it approached perihelion and reached magnitude 6 during the last days of August and first days of September.
  • The comet passed 0.45 AU from Jupiter during 1910 June, which increased the orbital period from 6.54 years to 6.69 years. The 1913 return was then impossibly placed for recovery, but hopes were high for the 1919 return. In fact, the 1919 return was widely publicized in various astronomy and science magazines because of its potential favorable nature. Searches were made, but the recovery was elusive. On 1919 October 25, T. Sasaki (Kyoto Observatory, Japan) reported the discovery of a new comet, which astronomers quickly realized was none other than Finlay's periodic comet, which was further off its predicted path than expected.
  • The Jupiter encounter of 1910 had placed the comet in an orbit which made for steadily worsening apparitions after 1919. The 1926 return was the comet's faintest since discovery and the next three returns were extremely unfavorable. The comet was next recovered in 1953 and has been observed at every return since.
  • The comet experienced another Jupiter encounter in 1993 September (0.96 AU), which decreased the perihelion distance from 1.09 AU to 1.04 AU. It is next expected at perihelion on 2002 February 7. A significant encounter with Jupiter on 2004 May 8 will decrease the perihelion distance from 1.03 AU to 0.97 AU, which could once again allow the comet to become relatively bright at favorable returns.
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