25 June 2020
Perihelion Distance (q)
Aphelion Distance (Q)
Click for NASA orbit diagram
Hills Observatory: 1 January 2013 to 10 June 2019
The comet was first discovered on 17 January 1786 by Pierre Méchain (Paris, France) while
searching for comets in the Aquarius region. Mechain said the comet appeared fairly bright
when viewed through a telescope and exhibited a faint, narrow tail. No observations were
possible on the 18th, but Méchain and Charles Messier were able to confirm the comet on the
19th. Unfortunately, the comet's elongation from the sun was rapidly decreasing and no
observations were made after the 19th so no orbit was computed.
The comet's second discovery was made by Caroline Herschel (Slough, England) during a
routine sweep for comets on 7 November 1795. It was quickly confirmed by her brother
William Herschel, who noted it could be seen with the naked eye. The comet was closest to
Earth (0.26 AU) on 9 November and observations continued as the comet's elongation from
the sun decreased. The comet was last detected on November 29.
The comet's third discovery was made by Jean Louis Pons (Marseille, France) on 20 October
1805 with independent discoveries being made by Johann Sigismund Huth (Frankfurt an der
Oder, Germany) on the 21st and Alexis Bouvard on the 22nd. On the 23rd Huth said the comet
was visible to the naked eye and was similar in size and brightness to the Andromeda Galaxy
(M31). The comet was last detected on 20 November. Later calculations revealed the comet
had passed closest to Earth (0.44 AU) on 16 October.
The comet's fourth discovery was made by Pons (Marseille, France) on 27 November 1818.
He described it as very faint and it passed closest to Earth (0.60 AU) on 1819 January 17.
J. F. Encke would ultimately be the first person to recognize that comets discovered in 1786,
1795, 1805, and 1818 were the same comet. Encke first began to notice this relationship while
investigating the orbit of Pons' comet of 1818-19. He published a parabolic orbit in the February
1819 issue of the Correspondance astronomique. The article added that the orbit "resembles a
little that of the first comet of 1805; perhaps later calculations will teach us something on their
identity." Encke published another article in the same issue of the Correspondance astronomique
which gave the first elliptical orbit. In the March 1819 issue of the Correspondance astronomique,
Encke showed the results of his calculations of an elliptical orbit for the 1805 comet. The
comparison between the orbits of the comets of 1805 and 1819 now showed very little
difference, and Encke was sure the two were identical. He continued working on the problem
and, in May of 1819, published details of his attempts to link the comets of 1805 and 1819 to a
comet observed in 1795. During June of 1819, Encke had successfully linked this comet to one
seen on only two nights in 1786. It was this laborious investigation that led to the comet being
During 1819, Encke suggested the comet would likely return during May of 1822. About a year
later, he took the positions from the 1818-19 apparition and determined a perihelion date of 24
May 1822. One of the earliest documented searches was made by J. E. Bode (Berlin, Germany)
on the evening of 15 February 1822. Using a 3.5-foot focal length refractor he swept the area
the comet was predicted to be located, which was the region south of Omega Piscium. No trace
of the comet was found. Calculations now reveal the comet was in the predicted region, but was
then too faint for recovery. It was moving quite slowly because of its distance from the Earth
and sun, and that area of the sky was carried into twilight during March and then daylight by the
end of April. It passed 10° from the sun on April 24, and then slowly moved away. C. K. L.
Rümker (Paramatta, New South Wales) recovered the comet with the aid of Encke's ephemeris
on 2 June 1822. The comet was then situated very low in the evening sky. Rümker confirmed
the recovery and therefore the identity with the expected comet on 3 June. The comet was
about a half day earlier than Encke's prediction.
The comet experienced one close approach to Mercury, eleven close approaches to Earth, and
two close approaches to Jupiter during the 20th century. It will make the following close
approaches during the first three decades of the 21st century. None of these will produce
notable changes to the comet's orbit.
0.48 AU from Earth on 2013 October 17
0.91 AU from Jupiter on 2022 January 27
0.27 AU from Earth on 2030 July 11
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