Eta Craterids

Observing

This weak shower is visible during the period of January 11 to 22. Maximum activity occurs around January 16-17, when the average radiant is α=176°, δ=-17°. The shower consists of generally rapid meteors, with streaks. The average magnitude is probably not above 4. It can best be seen from the Southern Hemisphere.

History

First evidence for the existence of this stream occurred during the 1890s, with Corder making the first detection of a radiant in 1892, and Niessl detecting a fireball from the same area on January 16, 1895. To date, Corder has remained the only Northern Hemisphere observer to detect a radiant from this shower, but, observers south of the equator have had better luck.

The New Zealand Meteor Section, led by R. A. McIntosh, was very active in the plotting of meteors during the latter half of the 1920's and throughout the 1930's. McIntosh had detected two probable Eta Craterid radiants in 1929: one on January 17, from RA=177.7 deg, DECL=-18.6 deg, and the other on January 21, from α=177°, δ=-11°. His colleague, Murray Geddes, had found one radiant at RA=180.0 deg, DECL=-15.6 deg on January 13, 1932.

No visual work even remotely close to the same magnitude as the New Zealand observations was carried out until the 1960s. During the period of 1961 to 1967, a systematic visual radiant survey was conducted at Waltair, India, by M. Srirama Rao, P. V. S. Rama Rao and P. Ramesh. It covered the months of November, December and January. During January 19-22, 1966, a radiant was found at RA=178 deg, DECL=-12 deg, which they estimated to possess a maximum hourly rate of 3.3 meteors per hour.

The only other evidence supporting this stream's existence is present in the records of the AMS, where a magnitude 5 fireball, was given the designation AMS Fireball 2232. This fireball was detected on January 12.10, 1939, by three ships located in the Caribbean Sea between Jamaica and Colombia. Charles P. Olivier evaluated the observations and concluded that the radiant was at RA=170 deg, DECL=-19 deg.

Not including Denning's grouping of this meteor shower with his stationary radiant called the Zeta Craterids, there have been two formal mentions of a possible annual or periodic radiant in this area. The first was made in 1935, when McIntosh included a radiant at RA=177 deg, DECL= 17 deg among his 320 southern hemisphere radiants (this radiant included McIntosh's two radiants mentioned earlier). The second was in 1964, when Olivier listed 194 "fireball radiants". Radiant 5008 (which included fireball 2232 mentioned earlier) showed a maximum on January 14 (+/-3 days) from RA=170 deg, DECL=-19 deg. It was based on a projection involving 4 meteors.

There has never been a trace of this stream in any of the radar studies conducted in the last four decades. The most ambitious Southern Hemisphere radar studies were conducted by C. S. Nilsson, during 1961, and G. Gartrell and W. G. Elford, during 1968-1969. The former easily covered the dates of maximum activity, while the latter only operated during January 21-23, 1969. It should be pointed out that two sessions of the Radio Meteor Project, conducted by Zdenek Sekanina in the Northern Hemisphere during 1961 to 1965 and during 1968-1969, also showed no evidence supporting the existence of the Eta Craterids. Sekanina's first study did not reveal any members during the period of January 11-19, while his second study revealed no reasonably close matches during January 13-17.

Extensive photographic studies conducted during this century have revealed only one member. On January 21, 1953, a 1.3-magnitude meteor was detected by two Baker Super-Schmidt cameras located in New Mexico and operated by the Harvard Meteor Project. The radiant was calculated to have been RA=177 deg, DECL= 19 deg.

The conclusion that might be considered from the above data is that this radiant possesses a low hourly rate (hardly above the rate for sporadic meteors) and may be erratic or periodic in nature, rather than annual. The stream is nearly devoid of bright meteors and probably possesses an average magnitude no greater than 4. It is definitely a shower that can best be observed in the Southern Hemisphere, since observers in the Northern Hemisphere will probably see less than half the meteors that can become visible from that radiant every hour. The radiant reaches the meridian at about 4 a.m. (local time) on those January mornings of activity, and the altitude for most Northern Hemisphere observers never exceeds 35°.