Discovered: 817 February 5.8 (Δ=0.42 AU, r=1.17 AU, Elong.=105°)
Last seen: 817 February 20.5 (Δ=0.78 AU, r=1.11 AU, Elong.=76°)
Closest to the Earth: 817 January 30 (0.3662)
Calculated path: AUR (Disc), TAU (Feb. 9)
This comet has been the point of minor debate over the years. First, some European sources specifically note that a comet was seen during a lunar eclipse on 817 February 5, while others mention the eclipse and comet in separate sentences, as if they were unassociated events. Second, several major astronomical sources published during the last 200 years report that the comet was seen in Sagittarius, a constellation which did not rise until after the February 5 eclipse had ended. The resolution of these points is very important because the Chinese reported a comet shortly after mid-February.
For all practical purposes, the two points of debate noted above can be solved by consulting the oldest available European sources. The German texts Einhardi Annales (830) and Vita Hludowici Imperatoris (840) both state that on February 5, a lunar eclipse occurred at the second hour of the night, and that a comet was seen in the sign of “Agitatoris”. Agitatoris is Latin for charioteer and the only charioteer in the sky is Auriga. It should be noted that the probable UT of the eclipse was February 5.8. A brief description of the comet is included in the French text Annales Sithienses (823) where it is noted that the tail looked like a sword. Later monastic histories say the comet was in Sagittarius.
As early as the 10th century accounts of this comet were being reported differently. A good example comes from the German text Annales Fuldenses (901) which says “a solar eclipse occurred on February 5. At night a comet resembling a sword was seen.” It can be seen here that not only are the eclipse and comet reported in separate sentences, but the eclipse had been changed to a solar eclipse, which definitely could not have happened on February 5.
The Chinese texts Chiu T’ang shu (945), Tang hui yao (961), and Hsin T’ang shu (1060) say a “broom star” appeared on 817 February 17. It is reported to have appeared south of the Pi [α and ε Tauri]. The date and location indicate an evening sky observation, implying a UT of February 17.5. This comet measured more than 2° in length and pointed southwestward. The texts add that, “after three days it came near Shên-Chhi [ο¹, ο², π¹, π², π³, π4, π5, and π6 Orionis] and went out of sight.”
The above accounts indicate the comet was in the evening sky at the time of all of the observations. It was certainly at naked-eye visibility on February 5, but bright moonlight on either side of this date apparently hid the comet from observers, except for those in Europe who experienced the lunar eclipse. The southwestward pointing tail noted by the Chinese would seem unlikely, unless it was an antitail. On the other hand, the Chinese might have been referring to the direction based on the comet’s shape, so that it was actually pointing southwestward toward the sun and the tail was therefore extending to the northeast.
||817 Mar. 3 (UT)
ABSOLUTE MAGNITUDE: H₁₀=5.5 (Kronk)
Full Moon: February 5
Sources: Annales Sithienses (823), p. 37; Einhardi Annales (830), p.203; Vita Hludowici Imperatoris (840), p. 621; Annales Fuldenses (901), p. 365; Chiu T’ang shu (945), p. 172; Tang hui yao (961), p.172; Hsin T’ang shu (1060), p.172, A. G. Pingré (1783), p. 339, 612; J. Williams (1871), p. 45; G. F. Chanbers (1889), p. 568; Ho Peng Yoke (1962), p. 172; Royal Frankish Annals, edited and translated by Bernhard Walter Scholz, with Barbara Rogers, Ann Arbor: the Universiy of Michigan Press (1970), p. 102; R. R. Newton (1972), p. 677; I. Hasegawa (1980), p. 73;