Observing the Orionids

Location of the Orionids
For Northern Hemisphere Observers

This represents the view from mid-northern latitudes at about 1:00 a.m. local time around October 21. The graphic does not represent the view at the time of maximum, but is simply meant to help prospective observers to find the radiant location. The red line across the bottom of the image represents the horizon. (Image produced by the Author using SkyChart III and Adobe Photoshop.)

Location of the Orionids
For Southern Hemisphere Observers

This represents the view from mid-southern latitudes at about 2:00 a.m. local time around October 21. The graphic does not represent the view at the time of maximum, but is simply meant to help prospective observers to find the radiant location. The red line across the bottom of the image represents the horizon. (Image produced by the Author using SkyChart III and Adobe Photoshop.)

History

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.

The first estimates of the activity level of this shower came in 1892, when observations placed the hourly rate at 15. During the next 30 years observers became aware that the activity of the Orionids was not consistent, with estimates ranging from a low of 7 in 1900 to a high of 35 in 1922. Unfortunately, there were no organized observations of this shower between 1903 and 1922, so it is not known whether the appearance of comet Halley in 1910 brought any enhanced activity. It is, however, interesting to note the change in the hourly rate since 1930. Between that year and 1953 the average peak rate was about 15 per hour, with individual rates never exceeding 20. Between 1960 and 1974, the average rate was about 25 per hour, with rates of 30 to 40 occurring on 4 occasions. Finally, between 1975 and 1985, the average rate dropped to 18, with the highest rates being only 24. Comet Halley passed closest to the sun in February 1986, so there appeared to be no direct correlation of meteor activity with the comet's appearance.

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.

Studies of the evolution of the Orionid shower are of particular interest because of the stream's link to Halley's comet. This link was indirectly made in 1911, when Olivier mentioned the similarity between the orbit of the Orionids and that of the Eta Aquarid of May. Since 1868, this latter stream had been known to be related to Halley's comet; however, this link between Halley's comet and the Orionids is not considered definite, as pointed out by J. G. Porter in 1948.

Porter considered the 0.15 AU separation between the comet's orbit and Earth's orbit enough of a deterrent to make a connection with the Orionids impossible. Although others agreed with Porter's hypothesis, the similarity in the characteristics of the Orionid and Eta Aquarid meteors and activity rates was considered something more than a coincidence. Thus, despite the orbital distance of Halley's comet at the time of the Orionid maximum it is generally accepted that the two must be related.

In 1983, B. A. McIntosh (Canada) and A. Hajduk (Czechoslovakia) published details of a new proposed model of the meteor stream produced by Halley's comet. Using a 1981 study published by D. K. Yeomans and T. Kiang, which examined the orbit of Halley's comet back to 1404 BC, McIntosh and Hajduk theorized that "the meteoroids simply exist in orbits where the comet was many revolutions ago." Further perturbations have acted to mold the stream into a shell-like shape containing numerous debris belts possessing stable orbits. These belts are considered as the explanation as to why both the Orionids and Eta Aquarids experience variations in activity from one year to the next.

Orbit

Taking 59 meteors orbits published in various papers spanning the period of 1954 to 1980, the Author obtained the following orbit:

During the two sessions of Z. Sekanina's Radio Meteor Project conducted at Havana, Illinois, during the 1960s, the following orbits were determined for this stream. The first orbit was published in 1970, while the second orbit was published in 1976.

Finally, the orbit of Halley's Comet is given below. The first orbit represents the comet's orbit in 391 BC, while the second is for the 1986 apparition.

The comet was not seen in 391 BC, but the calculations of D. K. Yeomans and T. Kiang, reveal that this is the most likely orbit based on an elaborate study of the motion of Halley's Comet. As can be seen, that orbit closely matches the present orbit of the Orionid stream‹especially when compared to the photographic orbit.