Observing the Geminids
For a short summary of this meteor shower, click here
Location of the Geminids
Location of the Geminids
The appearance of this meteor shower seems to have been fairly sudden during the 1860s. It was first noted in 1862, when R. P. Greg (Manchester, England) found a radiant in the constellation Gemini for the period of December 10-12. B. V. Marsh and A. C. Twining (United States) independently discovered the activity around the same time. A. S. Herschel noted meteors emanating from Gemini during December 12/13, 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 was active.
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 1896, English observers gave hourly rates near 23 and also observed "a number of bright pale green meteors...."
The reported rates continued to increase through most of the 20th century. During the 1900s, the rates averaged about 20 per hour. The rates averaged near 50 per hour during the 1930s, 60 per hour during the 1940s and 1950s, 65 per hour during the 1960s, and 80 per hour during the 1970s. The rates stayed near 80 per hour during the remainder of that century.
Visual observations have shown this shower to exhibit 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, photography and radar studies have revealed apparent activity spanning the period of November 30 to December 29.
A major advance in the understanding of this meteor stream was made in 1947. F. 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 M. Plavec (Prague), who began investigating how the gravity of the planets changed the orbit.
Plavec found that only two planets affect 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 [date of maximum]." 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 plane of Earth's orbit. 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.
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.
L. 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), S. Green and J. K. Davies found a rapidly moving asteroid in Draco. The next evening, C. Kowal (Palomar Observatory, California, USA) 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 Whipple that this asteroid moved in 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.