Geminid History
The appearance of this meteor shower seems to have been fairly sudden during the 1860s. It was first noted in 1862, when Robert P. Greg (Manchester, England) found a radiant at RA=100 deg, DEC=+33 deg for the period of December 10-12. B. V. Marsh and Prof. Alex C. Twining (United States) independently discovered the activity in 1862, while Alexander S. Herschel noted one very probable radiant at RA=105 deg, DEC=+30 deg, during December 12 to 13, 1863, as well as three fireballs from near the same radiant in 1863 and 1864. During the 1870s, observations of the Geminids became more numerous as astronomers realized a new annual shower had been discovered.
The first estimate of the strength of the Geminids came in 1877, when the hourly rate was given as about 14. The same rate was also given by observers in England during 1892, but it was noted that almost twice as many bright meteors were present than had been seen in 1877. In addition, the 1892 observations also revealed three radiants to be active in Gemini, with the most active being located near Pollux (Beta Geminorum). In 1896, English observers gave hourly rates near 23 and pointed out that the greatest activity came from near Castor (Alpha Geminorum). They also observed "a number of bright pale green meteors from the radiant...." During the beginning of the 20th century, the hourly rate of the Geminid stream was being reported as 15 to 30 per hour---with the average being over 20.
Hourly meteor rates during the 1930s ranged from 40 to 70, and, although these rates continued to rise during the next 50 years, the increase was not as dramatic as it had been between 1890 and 1930. For the 1940s and 1950s, hourly rates averaged about 60. During the 1960s, they were near 65 and the 1970s brought rates near 80. Rates between 1980 and 1985 have ranged from 60 to 110.
As with other major showers, the first person to begin sorting through the visual data was William F. Denning. As early as 1885, Denning had evidence that the radiant moved slightly eastward as each day went by, and, in 1923, he published a radiant ephemeris. His analysis revealed a daily motion of +1.25 deg in RA and -0.10 deg in DEC. In June 1926, Alphonso King essentially confirmed Denning's findings as he published an ephemeris that revealed a daily motion of +1.23 deg in RA and -0.10 deg in DEC. Despite the fact that two researchers had arrived at similar conclusions, the matter of the motion of the radiant came under fire in 1931. Vladimir A. Maltzev criticized King's conclusions---claiming an inadequate treatment of the basic data. His subsequent ephemeris revealed a daily motion of +1.05 deg in RA and -0.06 deg in DEC. The general correctness of Maltzev's daily motion has since been confirmed by Allan F. Cook and Robert A. Mackenzie. Cook published a paper in 1973 which revealed a motion of +1.02 deg in RA and -0.07 deg in DEC after an examination of photographic meteors, while Mackenzie correlated visual observations of the British Meteor Society and found a motion of +0.97 deg in RA and -0.08 deg in DEC.
Visual observations have shown this shower to possess a very sharp peak of activity, with hourly rates remaining above a value of half the maximum for about two days. Although visual evidence of this shower indicates activity persists from December 6 to 19, definite photographic members of this shower have been detected as early as December 4, while radar studies have shown activity as early as November 30 and as late as December 29.
One of the most complete studies of the average magnitude of the Geminid shower was conducted in 1982, by George H. Spalding. Using meteor magnitude estimates made by members of the British Astronomical Association during the period 1969 to 1980, Spalding showed that for solar longitudes of 254 deg to 255 deg (December 7) the magnitude is about 2.14. It brightens slightly to 1.63 by the time the sun reaches longitudes of 256 deg to 257 deg (December 9), then proceeds to steadily fade to a magnitude of 2.41 at longitudes of 260 deg to 261 deg (December 13). Maximum occurs shortly thereafter, and the magnitude brightens during the next several days, so that by the time of solar longitude 265 deg to 266 deg (December 18), the average magnitude is near 1.60. Spalding said "in the two days before maximum there is a moderate concentration of small particles, but ... the Earth then moves into a region of larger particles."
In 1984, P. B. Babadzhanov and Yu. V. Obrubov also stressed the correlation between the solar longitude and the magnitude of the meteors. According to their calculations, which they say agree with observations, they found that meteors of magnitude 6 reach maximum at a solar longitude 0.9 deg earlier than the maximum of meteors of magnitude 1. Meteors of magnitude -4 tend to reach maximum 1.3 deg later than meteors of magnitude 1. This survey tends to confirm the British study, except for the fact that Spalding said the Geminids produced brighter meteors on December 9 than on the 7th or 13th.
Generalized estimates of the average magnitude have been published over the years, as well as the estimated percentages of meteors showing trains. Some examples are given in the following table.
Geminid Magnitudes and Trains
Year(s) Ave. Mag. # Meteors % Trains Observer(s) Source
1950 2.62 50 --- Regina JRASC, 46, p. 37
1954 2.38 24 --- Montreal JRASC, 49, p. 171
1955 2.49 2951 --- Czechs BAC, 9, p. 13
1971-1984 2.83 4325 --- McLeod Personal Comm.
1974 2.11 151 1.9 Simmons MN, No. 25
1976 2.43 --- 1.2 Martinez MN, No. 36
1976 2.66 --- 1.2 Matous MN, No. 36
1980 2.29 449 --- Lunsford Personal Comm.
1982 2.56 893 2.4 Lunsford Personal Comm.
1982 2.10 1101 3.0 NMS WGN, 12, No. 2
1983 2.39 604 9.1 Lunsford Personal Comm.
1983 2.78 3036 7.0 WAMS WGN, 12, No. 3
1985 2.87 4960 4.0 International WGN, 14, No. 2
The observers cited are Norman W. McLeod, III (Florida), Regina Astronomical Society (Canada), Montreal Centre of Royal Astronomical Society of Canada, 7 observers in Czechoslovakia, Karl Simmons (Florida), Felix Martinez (Florida), Bert Matous (Missouri), Robert Lunsford (California), Nippon Meteor Society and the Western Australia Meteor Section.
A major advance in the understanding of the already mentioned intricacies of this meteor stream was made in 1947. Fred L. Whipple had been involved in the Harvard Meteor Project, a photographic survey aimed at better understanding meteors and their origins by obtaining data that could be used to calculate orbital elements. While analyzing meteors associated with the Geminids he found an orbital period of only 1.65 years, as well as a high eccentricity and a low inclination. Such an orbit attracted the attention of Miroslav Plavec (Prague), who began investigating the effects of perturbations on the orbit.
Plavec found that only two planets effect the orbit of the Geminids---Earth and Jupiter, though the former was considered negligible compared to the effects of the giant planet. "From the observer's point of view," he wrote," the most important phenomenon is the rapid backward shift of the node." The degree of this shift was calculated to cause the date of maximum to occur one day earlier every 60 years. Another interesting conclusion involved the point of intersection between the stream's orbit and the ecliptic. For the year 1700, it was found that the intersection point was placed 0.1337 AU inside Earth's orbit. For 1900, the intersection point was located 0.0178 AU inside Earth's orbit and in 2100, the point would be 0.1066 AU outside of Earth's orbit. Thus, Plavec not only showed why the activity of the Geminids was steadily increasing, but he also demonstrated that the activity would eventually decline and that sometime in the future Earth would no longer contact the stream's orbit.
Despite Plavec's calculations, the fate of the Geminid stream was still considered a matter that was up for grabs. In 1967, during the International Astronomical Union's Symposium No. 33, I. S. Astapovich and A. K. Terent'eva submitted a paper entitled "Fireball Radiants of the 1st-15th Centuries." They discussed their determination of the radiants of 153 meteor showers. According to their findings, a total of 14 fireballs were detected between 1038 and 1099 AD from a radiant similar to the Geminids', while additional fireballs were noted in 381 and 1163. They remarked that the "fireballs of the 11th century gave a definite radiant RA=103 deg, DEC=+26 deg (December 6-18)." They said the 11th century radiant was situated south and east of the present radiant and concluded that the radiant indicated that "apparently there has occurred a secular increase of the orbital inclination and a change in the line of apsides." They added, "the node of the orbit remained practically unchanged in the course of nine centuries."
Controversy over the Geminids' past continued throughout the 1970s, though astronomers generally seemed to favor the work of Plavec. In 1982, Ken Fox, Iwan P. Williams and David W. Hughes published a paper entitled, "The evolution of the orbit of the Geminid meteor stream." They essentially confirmed Plavec's findings of a nodal retrogression rate of about 1.6 deg/century, as well as his recognition of the relative newness of the shower in historical records---thus, eliminating the link to the fireballs of the 11th century (see the December Monocerotids). However, the confirmation of the nodal retrogression rate led to a problem: the observations did not confirm the predicted change in the date of maximum that amounted to one day in about 60 years. The authors theorized that the predicted change was actually being altered and proceeded to analyze several possibilities.
Since the orbit of the Geminid stream passed through the asteroid belt, the British researchers looked for an asteroid that may periodically pass near the stream's orbit. They found that asteroid 132 Aethra actually passed only 0.0003 AU from the Geminid orbit. However, they quickly discovered that for the asteroid to account for the variations noted would require it to have a mass only slightly less than that of Jupiter! Another possibility was that of general relativity---an affect noted in several planetary orbits---but the result of the calculations was a slight increase in the nodal retrogression rate, rather than the expected slowing down.
The final possibility considered was "the shape of the cross-section of the intersection of the meteor stream with the ecliptic plane." A computer simulation predicted the meteor rate profile was skew. Fox, Williams and Hughes further elaborated on this distribution in a paper published in 1983. "At the present time the Geminid shower slowly builds up to maximum rate and then drops away from maximum relatively sharply. About 50 yr ago the skewness should have been exactly the opposite with a sharp build up to maximum rate and a much slower falling away." The proposed model indicated Earth's orbit would intersect the Geminid stream only between 1800 and 2100. It also explained the currently observed mass segregation within the stream.
A major question concerning the Geminid stream involves its origin. It was long known that no parent comet for this stream was present in current catalogs, but, since the exact size and shape of the stream were not known until 1947, few conjectures were made. In 1950, Plavec theorized about the Geminid stream's parent body and pointed out that the "existence of a parent comet in such a short-period orbit, even in the past, seems to be not very probable. Planetary perturbations could scarcely have reduced the semimajor axis so much. More probably, the Geminids were separated from a parabolic comet by the close approach of the comet to the sun." Concerning a possible candidate for the parabolic comet mentioned, Plavec considered the great comet of 1680 (after a suggestion made in 1931 by Maltzev) and concluded that the close approach of the two orbits at a point slightly beyond the Geminid perihelion point, made a possible connection impossible to exclude.
Lubor Kresak strengthened the comet link to this meteor stream's formation, but instead of offering a theory as exotic as Plavec's, he favored a more direct formation of the Geminids. In 1972, he wrote that the parent comet "must have previously occupied the present orbit." He stressed that the compact nature of the stream would eliminate the possibility of it having formed in a different orbit and then been perturbed into the present orbit. Eleven years later, Kresak's theory would gain considerable strength.
On October 11, 1983, during a search for moving objects amidst the data gathered by the Infrared Astronomical Satellite (IRAS), Simon Green and John K. Davies found a rapidly moving asteroid in Draco. The next evening, Charles Kowal (Palomar Observatory, California) confirmed the body by photographing it with the 48-inch Schmidt telescope. The asteroid received the preliminary designation 1983 TB.
As early orbital calculations were being made, the International Astronomical Union Circular for October 25, 1983, relayed the opinion of Fred L. Whipple that this asteroid possessed an orbit almost identical to that of the Geminid meteor stream. Additional observations confirmed the link and the asteroid eventually received the permanent designation of 3200 Phaethon. The excitement of having found the parent body of the Geminid stream was almost dwarfed by another realization, this was the first time an asteroid had been definitely linked to a meteor shower and it subsequently serves as an important link between comets and meteor streams. |
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