Orionids

Observing the Orionids

For a short summary of this meteor shower, click here

Moon Forecast for 2010: On the morning of maximum, the moon is full and will severely hamper the meteor display.

The Orionid meteor shower is the second of two showers that occur each year as a result of Earth passing through dust released by Halley's Comet, with the first being the Eta Aquarids. The point from where the Orionid meteors appear to radiate is located within the constellation Orion.

The Orionids generally begin on October 15 and end on October 29, with maximum generally occurring during the morning hours of October 20-22. The Orionids are barely detectable on the beginning and ending dates, but observers in the Northern Hemisphere will see around 20 meteors per hour at maximum, while observers in the Southern Hemisphere will see around 40 meteors per hour. The maximum can last two or three nights, although there is evidence of some fluctuation from year to year.

There are other, weaker meteor showers going on around the same time as the Orionids. The Orionids generally appear to move fast. When you see a meteor, mentally trace it backwards. If you end up at Orion then you have probably seen an Orionid meteor! If you are not sure where Orion is in the sky, the following charts will help you find it from both the Northern Hemisphere and Southern Hemisphere:

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.

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.