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 the eight greatest comets of 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 (23 Sun-widths) 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. During the Earth close approach, the comet was estimated to be between magnitude 0 and −2 with a tail of over 90 angular degrees. As a result of forward scattering, C/1861 J1 even cast shadows at night (Schmidt 1863; Marcus 1997). During the night of June 30 – July 1, 1861, 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. Ichiro Hasegawa and Shuichi Nakano suggest that this comet is identical with C/1500 H1 that came to perihelion on April 20, 1500 (based on 5 observations). It was hypothesized that C/1861 G1 (Thatcher) and this comet are related, and that in a previous perihelion (possibly the 1500 one), C/1861 G1 broke off of this comet, as the two comets have many similar orbital characteristics. However, this was disproved in 2015 by Richard L. Branham Jr., who used modern computing technology and statistical analysis to calculate a corrected orbit for C/1861 J1. By 1992, this great comet had traveled more than 100 AU from the Sun, making it even farther away than dwarf planet Eris. It will come to aphelion around 2063.

Return

Computing the barycentric orbital period of the comet when it is outside the planetary region (using an epoch of 1900) shows an orbital period of about 406 years which would give a return year of 2267. Propagating the orbit forward also gives a return to perihelion around the year 2267.

Tebbutt's account

In his Astronomical Memoirs, Tebbutt gave an account of his discovery:

Observations in writing

June 30, 1861

Raphael Semmes, commander of the CSS Sumter wrote of the June 30 escape of his vessel from New Orleans: "The evening of the escape of the Sumter was one of those Gulf evenings, which can only be felt, and not described. The wind died gently away, as the sun declined, leaving a calm, and sleeping sea, to reflect a myriad of stars. The sun had gone down behind a screen of purple, and gold, and to add to the beauty of the scene, as night set in, a blazing comet, whose tail spanned nearly a quarter of the heavens, mirrored itself within a hundred feet of our little bark, as she ploughed her noiseless way through the waters." Samuel Elliott Hoskins, a doctor from Guernsey, observed: "At 9 p.m. a large luminous disc surrounded by a nebulous haze became visible in the N.W. horizon. At 9.40 it unmistakably assumed the character, to the naked eye, of a comet, having a large nucleus & a fan-like tail projecting vertically towards the zenith. It was permanently brilliant until sunrise the next morning − travelling with apparent rapidity, but slight declination, from N.W. to N.E."

July 1, 1861

Granville Stuart noted the observation of this comet in a journal entry on July 1, 1861, while living in western Montana: "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." Sarah R. Espy, of Alabama, in her private journal: "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." Emily Holder, wife of Joseph Bassett Holder, while stationed at Fort Jefferson, Florida: "Its appearance was sublime, as it extended over nearly half of the heavens...many wondered if the world was not coming to an end." Martin Bienvenu, an officer on a ship at Bangkok, in his unpublished journal: "A very brilliant comet had been visible in the Northern sky during the preceding week. I measured its tail with a quadrant, the extreme length of which was 93 degrees 50 minutes."

July 2, 1861

Raphael Semmes, commander of the CSS Sumter: "Day passed into night, and with the night came the brilliant comet again, lighting us on our way over the waste of waters. The morning of the second of July, our second day out, dawned clear, and beautiful, the Sumter still steaming in an almost calm sea, with nothing to impede her progress." R.W. Haig, the Chief Astronomer of the 49th Parallel Boundary Commission in British Columbia, wrote in a letter home "We saw a large comet for the first time last night, although I have no doubt it has been seen some days earlier in England." Charles Wilson, surveyor on the Boundary Commission with Haig, wrote "We caught sight of the Comet for the first time the night before last, to our great surprise as the lightness of the evenings had prevented our noticing his approach; he bids fair to eclipse the one of 1858 in size and splendour."

July 5, 1861

James Riley Robinson, on the schooner Conchita, in the Mexican harbor of Agiabampo. "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."

July 7, 1861

S. Watson, a tea inspector for the British firm Bull & Purdon writes from Hong Kong: "There is a very brilliant comet to be seen every evening as large as the one seen in England in 1858." Sarah Lois Wadley, daughter of William Morrill Wadley who managed the Vicksburg, Shreveport and Texas Railroad and served as Confederate superintendent of railroads, writes in her diary from Vicksburg, Mississippi: "There has been a comet visible for some nights past, I went out to see it Friday night, it was very beautiful, but its brightness was beginning to wane. I was sorry that I had not been well enough to have seen it at first."