Copyright © 1976 by Gary W. Kronk
This photograph was obtained by Gary W. Kronk on 1976 March 6, with a Yashica 35mm camera. The camera was equipped with a 50mm lens and was mounted on a 15-cm Criterion Dynascope. The film was GAF 500.
The European Southern Observatory at La Silla, Chile, was involved in the southern Sky Atlas project during the mid-1970s. Astronomers would use the 100-cm Schmidt telescope to take long exposures of the sky. Following the exposures, the plates would be gathered up and shipped to Geneva, Switzerland where they would be examined later by other astronomers. While examining a batch of the plates on 1975 November 5, Richard M. West found the trail of a comet on a 60-minute exposure obtained by Guido Pizarro on 1975 September 24.02. The comet was then within the constellation Microscopium and later calculations revealed it was located 2.36 AU from Earth and 2.98 AU from the sun. West estimated the magnitude as 14 or 15, and described the comet as diffuse, with a head 2-3 arc seconds across and a tail extending 10 arc seconds toward the north. West noted the comet was moving rather slowly and when he began looking at other plates of the region he subsequently found a comet within the constellation Grus on plates exposed by Oscar Pizarro on August 10.21 and by Guido Pizarro on August 13.19. This comet also showed a diffuse cometary trail with a possible faint tail extending northward. He estimated the total magnitude as 16 or 17 for the first date, and 16 for the second.
West took the three plates and measured the beginnings and endings of the trails and sent the times and positions to Brian G. Marsden. Marsden was not entirely confident that the six positions all belonged to the same comet, but found they were nicely represented by a parabolic orbit with a perihelion date of 1976 February 24.81 and a perihelion distance of 0.200 AU. Subsequently, on IAU Circular 2860 (dated 1975 November 6), Marsden announced West's discovery and wrote "the comet should be conveniently placed for observation in the northern hemisphere in the morning sky at [a magnitude of about] 5 in mid-March 1976." Later observations during the days following the announcement confirmed the general correctness of Marsden's orbit.
The comet was initially a southern hemisphere object. Observations by various amateur and professional astronomers typically placed the magnitude at between 12.5 and 14 during the latter half of November. But the comet was heading northward, and although a difficult object, it was picked up by the Northern Hemisphere observers Tsutomu Seki (Kochi Observatory, Geisei station, Japan) on December 1 and Henry L. Giclas (Lowell Observatory, Arizona, USA) on the 6th. Seki estimated the magnitude as 12.5. The December 18 full moon ended observations for over a week and by the time dark skies had returned Southern Hemisphere observers had found the comet had considerably brightened. During the last couple of days of December the comet had just barely exceeded magnitude 9.
As 1976 began, the comet had become an impossible object for Northern Hemisphere observers, but was still being followed in the Southern Hemisphere. The comet reached magnitude 8 by mid-January, magnitude 6 at the beginning of February, and was at magnitude 4 by February 13. During the period of February 13 to 19, the comet rapidly brightened to magnitude 1! The first observation of a tail came in mid-January when G. F. Morrisby (Salisbury, Rhodesia) spotted it in a 10-cm refractor. The tail was 1° long around mid-February.
Marsden revised the orbit for this comet on February 12. Using 28 positions obtained between 1975 August 10 and 1976 January 27, he computed an elliptical orbit with a perihelion date of February 25.22, a perihelion distance of 0.197 AU, and an orbital period of about 254 thousand years. E. Everhart (University of Denver, Colorado) took this orbit, applied perturbations by all nine planets, and ran the comet backwards to a point prior to its entry into the solar system and concluded the comet was not fresh from the Oort comet cloud.
The comet's brightness was becoming very impressive during the last few days of February. Charles and Peggy Townsend (Oxnard, California, USA) saw the comet on the 22nd just 23 minutes after sunset. It was then situated a little less than 4° above the Pacific Ocean, and 7x50 binoculars revealed it as a "tear drop shape" some 25 arc minutes in length and of about magnitude 1.0. The 22nd was also the first day the comet was observed with the naked eye, with several observers indicating magnitudes as great as -1. The comet had moved to within 12° of the sun by February 23, and was then seen by John E. Bortle (Stormville, New York, USA) with 10x50 binoculars just after sunset. He determined the magnitude as -1.6 and noted the tail extended 20-30 arc minutes toward PA 100°. On the evening of February 24, Bortle determined the magnitude as -1.9 with the same binoculars, while Peter L. Collins (Harvard College Observatory, Cambridge, Massachusetts, USA) saw it ten minutes after sunset with a 6-cm finder and estimated the magnitude as -0.5. Collins added that it appeared nearly stellar in the finder, but elliptical in a 23-cm Clark refractor at 100x.
The comet passed perihelion on February 25 and then became the first comet since Ikeya-Seki to be observed in broad daylight. Stephen O'Meara (Harvard College Observatory, Cambridge, Massachusetts) was the first person to see the comet in daylight, when on February 25.72 he saw it a few minutes past local noon with the 23-cm Clark refractor. He described it as magnitude -2, with a slightly elongated, very condensed, tailless coma. Collins joined O'Meara on February 25.73 and saw the comet in the 6-cm finder of the Clark refractor. He noted it was easier to see when a pink filter was used. Later that afternoon, O'Meara found the comet with 7x35 binoculars on February 25.81 and 25.92. He noted that the 23-cm refractor showed a bifurcated tail one degree long extending toward PA 100°, while the coma contained two jets pointing due west. Dennis Milon (Harvard College Observatory) saw the comet with 7x35 binoculars on February 25.93, right at sunset. He said the Clark refractor showed a white parabolic head. Bortle found the comet in daylight with his 32-cm reflector on February 25.94. He described it as "a brilliant almost stellar object" 10-15 arc seconds in diameter. It was white in color and similar to a planet in poor seeing in both sharpness and solidity. A faint parabolic fan was located in the antisolar direction with bright jets extending back towards PA 150° and 330°. The tail was 30-40 arc minutes long. Thirty-five minutes later, Bortle found the comet with 10x50 binoculars and estimated the magnitude as -3. He remarked that the comet was "brilliant, like the planet Venus in daytime but with a bright 15' tail!" Finally, just 7 minutes prior to sunset, Bortle made a naked-eye observation of the comet and said it was "an incredible sight!" The comet reached a minimum solar elongation of 6.4° on the 26th, and Townsend saw it on February 26.09, when, just after sunset, he noted the comet was "easily visible with the naked eye at mag. -2." Additional daylight observations were made by Bortle on February 27.50, when at local noon he observed the comet with his 32-cm reflector and 15x80 binoculars, and estimated the magnitude as -2.4 after comparing it with Mercury and Venus.
Widespread observations began on February 29, when the comet became visible around the world in the morning sky. It was then still located 9· from the sun, but estimations of the total magnitude ranged from -1.8 to -1.3. The tail was then 1° long. As each day passed, the comet slowly faded, but the increase in tail length was dramatic as the comet moved away from the sun. By March 3, the solar elongation had increased to 17° and the tail length was visually given as 10° to 15° and photographically given as 25°. The brightness had faded to magnitude 0 by March 4. The maximum reported visual tail length was about 25° on March 7, while the gas tail was then about 6° long.
D. Willmarth (Mount Hopkins Observatory) photographed the comet on March 5.50 and Zdenek Sekanina's (Center for Astrophysics) examination revealed the dust tail consisted of as many as 20 synchronic bands over the area between PA 310° to PA 357°, with the maximum tail length being about 25°. He added, "a fainter glow can also be detected in the north-northeast, apparently terminating in PA 40°." Sekanina said Willmarth's print also showed a plasma tail extending more than 15° in PA 300°.
This comet became the third comet in history to have the spectrum of OH detected by radio telescope. J. C. Webber, L. E. Snyder, R. M. Crutcher, and G. W. Swenson (Vermilion River Observatory) used the 37-m radio telescope during March 12, 13, and 14 to detect this spectral line.
Extra Nuclei: Edward H. Geyer and M. Hoffmann (Hoher List Observatory) visually observed the comet with a 36-cm reflector (250x) on March 5.23 and noted a secondary nucleus separated from the primary by 3 arc seconds toward PA 50°. The magnitude difference was 0.5. By March 9, observers were reporting that two additional nuclei were present. The four nuclei were labelled as "A", "B", "C", and "D", with "A" being the primary. Although "C" was not seen after March 27, the remaining three nuclei remained visible through the end of the comet's apparition.
The comet steadily faded during the remainder of the apparition. The magnitude dropped to 2 on March 9, 4 on March 23, 6 on April 13, 9 by the beginning of August, and 11.5 by the beginning of September. Many observers reported the coma became larger and more diffuse after June, with many diameter estimates during July and August being within the range of 10-12 arc minutes.
The comet was last detected on September 25.1, when Shao said nucleus B was situated 33.2 arc seconds toward PA 334° from nucleus A, and nucleus D was situated 17.6 arc seconds toward PA 338° from nucleus A. The comet was then situated 3.78 AU from Earth and 3.74 AU from the sun.