|Discovered by||J.-L. Pons|
|Discovery date||February 23, 1818|
|1818 D1, 1873 V1, 1928 W1, 1956 S1|
|Epoch||July 18, 2011|
|Semi-major axis||9.204 AU|
|Orbital period||27.9 yr|
27y 10m 2d (perihelion to perihelion)
|Last perihelion||August 3, 2011|
February 20, 1984
|Next perihelion||May 27, 2039|
Comet Crommelin, also known as Comet Pons-Coggia-Winnecke-Forbes, is a periodic comet with an orbital period of almost 28 years. It fits the classical definition of a Halley-type comet with (20 years < period < 200 years). It is named after the British astronomer Andrew C. D. Crommelin who calculated its orbit in 1930. It is one of only four comets not named after their discoverer(s), the other three being Comets Halley, Encke, and Lexell. It next comes to perihelion (closest approach to the Sun) around May 27, 2039.
The first observation was by Jean-Louis Pons (Marseille, France) on February 23, 1818, he followed the comet until February 27 but was prevented further by bad weather. Johann Franz Encke attempted to calculate the orbit but was left with very large errors.
In 1872, John R. Hind produced a rough orbital calculation and observed it was close to that of Comet Biela, based on these observations, Edmund Weiss later speculated it may have been part of Biela's comet.
The next observation was on November 10, 1873, by Jérôme E. Coggia (Marseille, France), and again on November 11 by Friedrich A. T. Winnecke (Strasbourg, France), but it was lost by November 16. Weiss and Hind took up the calculations and tried to match it again with the 1818 appearance.
A third discovery was by Alexander F. I. Forbes (Cape Town, South Africa) on November 19, 1928, and confirmed by Harry E. Wood (Union Observatory, South Africa) on November 21. It was Crommelin who eventually established the orbit and finally linked the 1818 (Pons) and 1873 (Coggia-Winnecke) comets to it (also see Lost comet).
On its latest return, 27P/Crommelin was recovered on May 12, 2011, at apparent magnitude 18.7 and peaked at magnitude 10.7 at perihelion on August 3. 27P/Crommelin was last observed in January 2012 and passed 1.5 AU from Saturn on July 11, 2015.
The next perihelion will be on May 27, 2039. Near perihelion the comet will be 0.74 AU from the Sun and 1.73 AU from Earth. This is about as far from Earth as the comet can get during perihelion.
On December 22, 2120 it will pass 0.297 AU (44.4 million km) from Earth.