Observing the Orionids
For a short summary of this meteor shower, click here
Location of the Orionids
Location of the Orionids
The discovery of the Orionid meteor shower should be credited to E. C. Herrick (Connecticut, USA). In 1839, he made the ambiguous statement that activity seemed to be present during October 8 to 15. A similar statement was made in 1840, when he commented that the "precise date of the greatest meteoric frequency in October is still less definitely known, but it will in all probability be found to occur between the 8th and 25th of the month."
The first precise observation of this shower was made by A. S. Herschel on 1864 October 18, when fourteen meteors were found to radiate from the constellation of Orion. Herschel confirmed that a shower originated from Orion on 1865 October 20. Thereafter, interest in this stream increased very rapidly---with the Orionids becoming one of best observed annual showers.
The Orionids were frequently observed during the latter years of the 19th century and became the focus of debate during the first quarter of the 20th century. The British amateur astronomer W. F. Denning and the American astronomer C. P. Olivier began using the pages of two astronomical periodicals to debate whether the Orionid radiant, the point from which the meteors seemed to radiate in the sky, moved from one day to the next: Denning argued that it did not, while Olivier argued that it did. Each astronomer had supporters that chimed in, but the argument remained essentially theirs. The problem was that the Orionid radiant was more diffuse than the other well-observed annual meteor showers. Thanks to the use of photography and the very precise plotting of meteors by several amateur and professional astronomers, Oliver was eventually proven correct.
One very unusual feature the Orionids tend to display is an unpredictable maximum. In 1981, observers reported very low rates of less than 10 meteors per hour during the period of October 18 to 21 (maximum predicted for October 21), but high rates of near 20 per hour were noted on the morning of October 23. Interestingly, a study published in Czechoslovakia during 1982, revealed the Orionids generally exhibited a double maximum. The finding was based on observations made during the period spanning 1944 to 1950. Shortly thereafter, several visual studies indicated the presence of a "plateau effect" or a long period of maximum devoid of any sharp decline of activity, instead of a double peak. Most notably, the 1984 observations of the Western Australia Meteor Section, show a nearly flat maximum lasting from October 21 to 24, while N. W. McLeod, III (Florida, USA), has frequently noted it to stretch up to 6 days.
The variation in activity levels around the time of maximum has been attributed to the presence of filaments within the Orionid stream orbit. Each of these filaments represents a previous orbit that comet Halley has followed in the past. Since observations indicate that comet Halley has been around for over 2200 years and since the comet orbits the sun in about 76 years, there are quite a few filaments making up the Orionid stream.