Great Southern Comet of 1887

The Great Southern Comet of 1887, or C/1887 B1 using its International Astronomical Union designation, was a bright comet seen from the Southern Hemisphere during January 1887. Later calculations indicated it to be part of the Kreutz Sungrazing group.

A curious feature of the comet was that few, if any observations were made of a cometary head or nucleus. As a result, some older astronomical texts refer to it as the "Headless Wonder".^[2]

Discovery

The comet was officially discovered by astronomer John Macon Thome at Córdoba, Argentina, on January 19, at which point it was located in the constellation Grus.^[3] However, correspondence from William Henry Finlay suggests that it may also have been seen from Blauwberg, South Africa, on January 18.^[3] At the time of discovery the comet had already passed perihelion a week earlier, and its closest approach to Earth had been a month earlier.

Observations

The comet reached first magnitude,^[4] and was widely observed by astronomers in the Southern Hemisphere for the remainder of January. On the 22nd Finlay described it as a "pale narrow ribbon of light, quite straight" of about 35 degrees in length, though no cometary head could be distinguished.^[5] On the 23rd, Thome recorded a tail length of over 40 degrees, but like other observers stated he could not find a nucleus. On January 27, C. Todd recorded seeing the comet's head as a "diffused nebulous mass", but noted a break between the head and the tail (possibly representing what is referred to as a tail disconnection event).

Following the publication of an ephemeris by S. C. Chandler, which suggested the comet could be located 20° from Rigel by the end of February, astronomers in the United States eagerly waited for it to move far enough into northern skies to be visible.^[6] However, the comet faded extremely rapidly, and never became visible from northern latitudes. It was last observed by John Tebbutt from New South Wales on January 30, a relatively short period of observation overall for a large comet.

Analysis

The first, speculative, orbit was calculated by Heinrich Kreutz; however definitive calculation was difficult because no observations were made of the nucleus. By February, Finlay had derived an orbit which linked the comet firmly to the Kreutz Sungrazing group. A more definitive orbit was calculated in 1978 by Zdeněk Sekanina, based on the assumption that the comet's head was on a great circle "through the sun and inner part of the tail".^[7]

Sekanina was subsequently to speculate that the unusual appearance of the comet was due to a "tail formation event", an outburst of cometary dust, about 6 hours after perihelion.^[8] This event and the rapidly fading brightness, Sekanina argued, showed that C/1887 B1, along with C/1945 X1 (du Toit), represented a class of comets in between the "great" sungrazers (such as the Great Comet of 1882) and the many smaller objects discovered by the Solar and Heliospheric Observatory.

References