|Discovered by||John Tebbutt|
|Discovery date||May 13, 1861|
|Orbital characteristics A|
|Epoch||JD 2400920.5 (921.0?)
(May 25, 1861)
|Semi-major axis||55.1 AU|
|Orbital period||409 a|
|Last perihelion||June 12, 1861|
The Great Comet of 1861 formally designated C/1861 J1 and 1861 II, is a long-period comet that was visible to the naked eye for approximately 3 months. It was categorized as a Great Comet, one of eight in the 19th century.
It was discovered by John Tebbutt of Windsor, New South Wales, Australia, on May 13, 1861, with an apparent magnitude of +4, a month before perihelion (June 12). It was not visible in the northern hemisphere until June 29, but it arrived before word of the comet's discovery.
On June 29, 1861, comet C/1861 J1 passed 11.5 degrees from the Sun. On the following day, June 30, 1861, the comet made its closest approach to the Earth at a distance of 0.1326 AU (19,840,000 km; 12,330,000 mi). During the Earth close approach the comet was estimated to be between magnitude 0 and −2 with a tail of over 90 degrees. As a result of forward scattering C/1861 J1 even cast shadows at night (Schmidt 1863; Marcus 1997). During the night of 1861 June 30 – July 1, the famed comet observer J. F. Julius Schmidt watched in awe as the great comet C/1861 J1 cast shadows on the walls of the Athens Observatory. The comet may have interacted with the Earth in an almost unprecedented way. For two days, when the comet was at its closest, the Earth was actually within the comet's tail, and streams of cometary material converging towards the distant nucleus could be seen.
By the middle of August the comet was no longer visible to the naked eye, but it was visible in telescopes until May 1862. An elliptical orbit with a period of about 400 years was calculated, which would indicate a previous appearance about the middle of the 15th century, and a return in the 23rd century.
It is possible that C/1861 G1 (Thatcher) and this comet are related, and that in a previous perihelion (possibly the 1500 one) and that C/1861 G1 broke off of this comet, as the two comets have many similar orbital characteristics.
Observations in writing
Granville Stuart noted the observation of this comet in an journal entry on July 1, 1861 while living in western Montana. The entry reads as follows: "Saw a huge comet last night in the northwest. Its tail reached half across the heavens. It has probably been visible for sometime, but as it has been cloudy lately I had not observed it before."
|“||Light rains this morning, I went with Mrs. Brewer, to visit Mrs. Hampton. O. busy, preparing C. to attend a camp-drill of some weeks above here. A brilliant and beautiful comet appeared tonight in the same part of the heavens as that a few years ago, the train of this it the longest that I ever saw, pointing directly upwards.||”|
July 5, 1861 . "I awoke in the night at 1 o’clock, when I had a glorious sight of the largest comet I ever beheld. The head, or nucleus, was large as Venus, and very bright and blazing, and about 20 degrees above the horizon, pointed to the north, while the bright, long tail reached full half way across the heavens. It was a most wonderful sight." James Riley Robinson, on the schooner Conchita, in the Mexican harbor of Agiabampo. 
- Kronk, Gary W (2001–2005). "C/1861 J1 (Great Comet of 1861)". Cometography.com. Retrieved 2011-08-22.
- C/1861 J1 ( Great comet ) Seiichi Yoshida
- Donald K. Yeomans (April 2007). "Great Comets in History". Jet Propulsion Laboratory/California Institute of Technology (Solar System Dynamics). Retrieved 2011-02-02.
- Horizons output. "Observer Table for Comet C/1861 J1 (Great comet)". Retrieved 2011-08-22. (Observer Location:500)
- Marcus, Joseph C. (2007). "Forward-Scattering Enhancement of Comet Brightness. I. Background and Model". International Comet Quarterly 29 (2): 39–66. Bibcode:2007ICQ....29...39M.
- Hasegawa, Ichiro; Nakano, Syuichi (October 1995). "Periodic Comets Found in Historical Records". Publications of the Astronomical Society of Japan 47 (5): 699–710. Bibcode:1995PASJ...47..699H.
- Stuart, Granville. Forty Years on the Frontier Pg 178.
- Sarah R. Espy (1859–1868). "Private journal". Alabama Department of Archives and History, Montgomery, Alabama. Retrieved 8 May 2013.
- Diary of John R. Robinson, Feb 14 to Sept 15, 1861: His journey to Batopilas, Mexico in Inspect Silver Mines, with a View to Purchase. http://www.torski.com/family/jrrobinson/pgjrr1stdiarymain.htm. Also located in the Huntington Library, San Marino, California.
- C/1861 J1 (Great Comet of 1861) on Cometography.com
- The Comet of 1861, Gallery of Natural Phenomena
- JPL DASTCOM Comet Orbital Elements
- Orbital simulation from JPL (Java) / Ephemeris