December Monocerotids

Observing

This shower has a duration which persists from November 9 to December 18. Maximum occurs around December 11, with the radiant then being at α=101°, δ=+10°. Though some photographic data has been obtained, the overall visual rate of this shower is only 1-2 per hour. However, radar studies indicate the shower makes annual appearances, so that optical aid may be necessary to better observe the activity produced by this stream.

History

This stream was discovered by F. L. Whipple in 1954, during a search through 144 meteor orbits detected during the photographic surveys of Harvard College Observatory during 1936 to 1951. Two meteors (designated 2313 and 2405) had been photographed on December 13 and 15, 1950, and possessed very similar orbits that indicated a radiant at RA=103°, DEC=+8°. What made the find especially significant was the fact that the indicated orbit was very similar to that of comet Mellish (D/1917 F1)---a comet with an orbital period of about 145 years (uncertainty of 0.6 years).

In 1961, R. E. McCrosky and A. Posen, both of Harvard College Observatory, published a list of 2529 meteor orbits obtained during the Harvard Meteor Project during February 1952 to July 1954. They found 3 meteors, photographed between December 10 and 17, that belonged to the December Monocerotid stream, thus offering the first confirmation that a stream with the orbital elements suggested by Whipple was present.

During the 1960's, the true nature of this stream began to be realized as four major radar surveys revealed further evidence supporting the stream's existence. The first was conducted by C. S. Nilsson at Adelaide, Australia, during the period of December 1960, to December 1961. Although the computer technique designed to process the 2101 radar meteor orbits did not reveal a stream with the orbit determined by the photographic data, Nilsson studied the radiant data and located 2 meteors from December 1960, and 6 meteors from December 1961. The latter meteor orbits allowed a good determination of the stream's orbit. It also indicated a duration of December 5 to 12, with an average radiant of RA=101.6°, DEC=+9.6°.

Interestingly, Nilsson's computer technique did reveal a stream during both 1960 and 1961, which possessed a similar orbit to the photographic determinations, but with an inclination some 10 deg to 15° lower. This resulted in an average radiant of RA=95°, DEC=+15°---meaning it was shifted nearly 9 deg to the northwest. What made this stream particularly significant was the fact that it seemed to possess a twin during September 23-29, at RA=162°, DEC=+14°. Nilsson noted that "care must be taken not to confuse the separate and newly determined radiant at 95°, +15° with the Monocerotid radiant at 102 deg, +10 deg. The values of geocentric velocity are similar, and the orbit of the former is also near parabolic. However, the inclination of the orbit is definitely smaller than that of the Monocerotid stream." He added the the similarity of the orbits might suggest a connection.

In 1973, Z. Sekanina published the results of the Harvard Radio Meteor Project conducted at Havana, Illinois, during 1961-1965. This project revealed a very distinct stream with an average radiant at RA=91.4°, DEC=+15.0°---very similar to the strong, lower inclination stream noted by Nilsson. Sekanina commented that "the photographic stream associated with P/Mellish has both its mean optical radiant and its theoretical radiant in Monoceros, but the mean radiant of the rich radio stream detected in our sample, which is beyond any doubt also related to the comet, lies in Orion...." A further surprise revealed by this radar study was that the indicated duration of this stream was from November 9 to December 18. During 1968-1969, Sekanina revived the Radio Meteor Project, this time with a greater sensitivity than that earlier attained, and among the 275 streams detected was the same stream detected during 1961-1965. The indicated duration was November 16 to December 14 and the average radiant was RA=93.8°, DEC=+14.4°. Due to the similarity of these streams to the meteor orbits found in the photographic studies, Sekanina referred to the streams as the Monocerotids.

Up to this point the radio meteor surveys seemed to indicate that the most prominent shower in this part of the sky during the first half of December was 9 deg away from the radiant indicated by photographic meteors; however, in 1975, G. Gartrell and W. G. Elford published their results of a radar survey conducted during 1968-1969. They wrote that "although no Monocerotids were found to be associated by the systematic stream searches, three meteors were observed with radiants and velocities corresponding to this stream." Their radiant was RA=106°, DEC=+6°.

The first study of this meteor stream was conducted by M. Kresakova during 1974, as she studied comet Mellish and its possible relationship to the Geminids. While studying comet Mellish and the lists of photographic meteors, it was noted that two apparent minor streams were present. The first, designated "component A", possessed nearly identical orbital elements to the Monocerotids first suggested by Whipple. It was based on 16 meteors acquired from both American and Russian sources. The second stream was composed of 9 photographic meteors and was identified with the 11 Canis Minorids first announced by K. B. Hindley in 1969 (see the 11 Canis Minorids later in this chapter). The Monocerotids of Kresakova lay very close in radiant position to that predicted for P/Mellish.

A preliminary examination of previously known data by the author had seemed to indicate two distinct streams being present with a separation of about 10° in inclination; however, the photographic meteors listed by Kresakova and the original radar orbits acquired during the two Radio Meteor Projects directed by Sekanina, seem to show no sign of distinct streams, but, instead, one very diffuse stream. Nevertheless, one cannot ignore the fact that photographic and radar studies tend to indicate distinctly different orbital inclination averages. Thus, it would seem that the fainter and smaller radar meteors tend to possess more lower inclination orbits than the brighter, larger photographic meteors. Meteor showers visible to the naked eye will probably originate from the photographic radiant, while telescopic meteors will tend to lean toward the radar radiant.

Based on the number of meteors detected both by photographic and radio-echo techniques, it would seem the Monocerotid shower may be stronger telescopically than visually. Coordinated attempts to observe this stream visually have never been conducted, but Meteor News published details of many individual attempts to observe this shower between 1977 and 1985 (see issues 40, 49, 53, 57, 61, 65, 68 and 73). Typical observations indicate hourly rates of only one to two per hour. During December 11/12-13/14, 1982, Robert Lunsford (San Diego, California, USA) observed ten Monocerotids and gave the average magnitude as 3.2.

Surprisingly, the December Monocerotids may be responsible for the solving of a major puzzle in meteor astronomy. During the International Astronomical Union's Symposium No. 33 in 1967, I. S. Astapovich and A. K. Terent'eva submitted a paper entitled "Fireball Radiants of the 1st-15th Centuries." They discussed their determination of 153 meteor radiants and pointed out a collection of 14 fireballs that emanated from a radiant similar to that of the Geminids during the period 1038 to 1099 AD. They remarked that the "fireballs of the 11th century gave a definite radiant RA=103°, DEC=+26° (December 6-18), the early fireball of 381, December 13 passing 5 deg to the South of it." Their linking of this fireball radiant to the Geminid shower has caused much controversy (see the Geminids earlier in this chapter), but in 1985, K. Fox and I. P. Williams offered a reasonable solution.

Beginning with an orbit possessing the angular elements determined by Kresakova and borrowing the semimajor axis of P/Mellish, Fox and Williams examined the orbital evolution of a particle between 783 AD and 3183 AD. Perturbations from Jupiter, Saturn, Uranus and Neptune were taken into account. Their results were that the ascending node varied by less than 5 deg during the 2400 years examined, while the nodal distance from Earth's orbit "remained fairly constant." The final conclusions of the study indicated that "a Monocerotid shower would still be seen during December in the eleventh century," as well as 1000 years from the present time. Also, Fox and Williams showed that, of the currently known meteor showers, only the Monocerotids were capable of producing a shower close to the radiant of the eleventh century fireballs, though "there is always the possibility that the ancient fireballs came from a stream which is not observable at present."