Orionids

Best Night: October 21-22, with about 20 meteors per hour
Total Duration of Activity: October 15-29

How to Observe

      The point from where the Orionid meteors appear to radiate is located within the constellation Orion and is referred to as the radiant. The radiant is located in the northeastern part of that constellation. The exact location of the radiant in astronomical terms is RA=95 degrees (6 hours 20 minutes), DEC=+16 degrees, but the following chart will also help you find it:

The radiant of the Orionid meteor shower as seen from mid-northern latitudes at about 2:00 a.m. local time. (Image produced by the Author using Starry Night 2.0 and Adobe Photoshop 5.0.)

      The radiant rises around 10:30 p.m. local time. At about 3:00 a.m. the radiant is about 50 degrees above the horizon. The radiant location with respect to the horizon is shown above.
      To best observe the Orionids wear appropriate clothing for the weather. Lie outside in a reclining lawn chair. The two best ways to observe the Orionids is either by pointing your feet southward (the general direction of the radiant) and looking in the region straight up, or pointing your feet southwestward and have your center of gaze around 60° above the horizon. Do not look directly at the radiant, because meteors directly in front of you will not move much and fainter ones might be missed. Other minor meteor showers will be going on at the time and stray meteors, more commonly called sporadics, will frequently be seen that do not belong to a meteor shower. When you see a meteor mentally trace it backwards and if you arrive at the region just northeast of the main body of Orion outlined in the picture above, it is probably an Orionid.

History

      The discovery of the Orionid meteor shower, seems to be best attributable to Edward C. Herrick. 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 were made by Alexander Stewart Herschel on October 18, 1864. Fourteen meteors revealed a radiant at RA=90 deg, DEC=+16 deg. Herschel next observed the shower on October 20, 1865, with a radiant of RA=90 deg, DEC=+15 deg being revealed from 19 meteors. 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. The radiant was generally more diffuse than some of the other annual showers and this created a strong debate during the first quarter of the 20th century. William F. Denning, as has been mentioned on several earlier occasions, was a strong supporter of the stationary radiant hypothesis and the Orionids were considered one of his best examples. Visual studies had generally failed to detect any motion in the radiant's position during the 10-15 days the shower was under observation each year and Denning strongly believed that two prominent radiants were present---the Orionids at RA=91 deg, DEC=+15 deg and the Geminids at RA=98 deg, DEC=+14 deg. During 1913, the pages of the Monthly Notices of the Royal Astronomical Society became a forum for the pens of Denning and Charles P. Olivier as the two debated the stationary character of the Orionids; however, little was resolved and the matter had to await 1923, when the debate was resumed in the pages of The Observatory.
      Based on observations made by himself and three colleagues from Leander McCormick Observatory, Olivier demonstrated how the radiant clearly moved eastward from day to day---the position for October 18, 1922 being RA=91.0 deg, DEC=+15.0 deg, while the October 26 position was at RA=99.2 deg, DEC=+13 deg. The five positions given by Olivier clearly indicated an advance in right ascension, but the declination actually showed no clear movement. Olivier added that recent observations by members of the American Meteor Society for the dates of October 12 to 31, inclusive, had also indicated motion in the right ascension, but, again, the movement in the declination was not determined.
      The very next issue of The Observatory contained three letters by supporters of the stationary radiant theory---Denning, J. P. M. Prentice and A. Grace Cook. Denning stated that the "American observers appear to have failed to discover the minor streams, and until they can do this the real character of the chief radiant must continue to elude them, for it will be easy to make it a shifting position, and especially by the plentiful meteors radiating from 99 deg +13 deg." Prentice and Cook essentially argued in support of Denning, citing extensive observations made by themselves and other British observers.
      In June 1923, Olivier struck back and, as his ammunition, he stressed the importance of a photographic radiant obtained by Professor King at Harvard on October 20, 1922, and a series of excellent radiant determinations made by R. M. Dole between October 17 and 30, 1922. Dole essentially confirmed Olivier's belief that the radiant moved and provided the following list of radiant positions:

Orionid Radiant Ephemeris

      Olivier said King's photographic radiant fell "exactly where a moving radiant would give it on Oct. 20..." with the position being determined as RA=94.1 deg, DEC=+15.8 deg.
      Olivier's belief in a moving radiant gradually won out during the next few years, with several well-known amateur and professional astronomers adding support. Notably, in 1928, Ronald A. McIntosh conducted a very extensive set of observations during the period October 14 to 24, inclusive. Again, the movement of the right ascension was clearly demonstrated, as was the difficulty in determining the motion of the declination. Estimates made over the last 50 years have shown that the declination actually moves slightly northward as each day passes. The 1986 Handbook of the British Astronomical Association lists the radiant's daily motion as RA=+1.23 deg, DEC=+0.13 deg.
      Referring back to McIntosh's 1928 observations, it should be stressed that the 54 Orionids plotted, revealed more details of the stream than just radiant determinations. McIntosh noted that 25 Orionids, or 46%, left trains, and that the average duration of these trains was 1.3 seconds. His estimates of color revealed 24% to be red and 9% to be blue. Although his observations were frequently hampered by clouds, mist and, finally, the moon, McIntosh did observe on two very clear nights---the 15th and 20th. These nights possessed the highest observed rates of Orionids, with hourly totals being 8 and 6, respectively. [Although magnitude estimates were made for each meteor, the published material was not broken done sufficiently enough for a precise average to be obtained; however, an educated guess by the Author, based on McIntosh's rough tabulation, produces an average magnitude near 2.5.]
      The first estimates of the activity 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 brought any enhanced activity. It is, however, interesting to note the change in the ZHR since 1930. Between that year and 1953 the average ZHR was about 15, with individual rates never exceeding 20. Between 1960 and 1974, the average ZHR was about 24, 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. The comet passed closest to the sun in February 1986.
      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 to generally possess a double maximum. The finding was based on observations made during 1944 to 1950, and the maxima occurred at solar longitude 207.8 deg (primary) and 209.8 deg (secondary). 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 Norman W. McLeod, III, has frequently noted it to stretch up to 6 days. What appears to be the best explanation of the Orionids' irregular occurring date of maximum, was made by A. Hajduk (Astronomical Institute of the Slovak Academy of Sciences, Bratislava, Czechoslovakia) in 1970.
      Hajduk examined the reported activity of the Orionids for the period of October 14 to 28 during the years 1900 to 1967. He particularly noted that the "stream density varies along the orbit," and "there is no fixed periodically recurring position of maximum or secondary maxima." Hajduk concluded that the density changes were not random and that the displacement of activity "can be explained by the presence of some filaments along the stream orbit."
      A strong confirmation of Hajduk's filamentary structure was made during 1975, when radar equipment at Ondrejov and Dushanbe was simultaneously utilized during the period of October 17 to 29. The data was analyzed by P. B. Babadzhanov, R. P. Chebotarev (Dushanbe, USSR) and Hajduk. They found radio-echo rates to slowly increase, but, suddenly, at a time generally attributed as the maximum of the Orionids (solar longitude=208 deg), rates drastically declined. Just as curious was the finding that rates had doubled during the next 24 hours, and then were followed by the normal decreasing rates for every day thereafter. The authors concluded that as Earth entered the Orionid stream "we first intersected a halo with a slight variation of density, then a gap corresponding to solar longitude=208, and this was followed by a steep increase at solar longitude=209." It was claimed that this structure confirmed the presence of filaments.
      Hajduk continued the simultaneous observations of the Orionid shower during 1978---this time combining his results at Ondrejov with those obtained by G. Cevolani at Budrio, Italy. On this occasion the period of October 17 to 24 was covered by both stations, while only Budrio continued until October 29. Unlike 1975, the activity curves of both stations indicated a more consistent increase towards maximum. This maximum occurred during October 21 (solar longitude between 207.8 deg and 208.4 deg), with a relatively flat peak being noted. Rates remained above one-half of maximum during October 19-22 (solar longitude between 206.5 deg and 209.5 deg). The Budrio data also seems to have detected a secondary maximum on October 27 (solar longitude=214.6 deg), which was explained as due to an encounter with a filament. The only hint of a filamentary structure near maximum was noted on October 20 (solar longitude=207.5 deg), when a decline in activity was noted at Ondrejov. This decline was not detected at Budrio, causing Hajduk and Cevolani to conclude that, since Ondrejov was capable of detecting fainter and, thus, smaller meteors, a region possessing fewer small particles than normal may have been encountered by Earth.
      An interesting feature of the Orionids is the apparent complexity of the radiant. In 1939, J. P. M. Prentice noted two active radiants: the primary radiant, at a solar longitude of 208 deg, was described as being about 7 deg wide, centered at RA=94.4 deg, while the declination spread from DEC=14.9 deg to 15.5 deg. A secondary radiant was noted at RA=97.8 deg, DEC=+18.2 deg. Prentice's visual study is still rated as one of the most complete ever published on the Orionid shower, although a convincing confirmation of the secondary branch had to await a 1965-1966 telescopic survey conducted in Czechoslovakia.
      V. Znojil (Public Observatory, Brno) outlined the details of the survey and results during 1968. Telescopes were utilized which possessed 80mm lenses and magnifications of 10. This resulted in a field of view of 7 deg 22' and a limiting magnitude of 10.8. During 1965, two stations were separated by 23.91 km, while the separation was increased to 63.60 km during 1966.
      Znojil found two distinct radiants were present, which he referred to as the northern and southern Orionids. Their average radiants were given as RA=95.4 deg, DEC=+17.8 deg and RA=95.6 deg, DEC=+15.9 deg, respectively. The latter radiant agrees very well with the position usually attributed to the Orionids, while the northern branch is very close to that noted by Prentice 30 years earlier. Znojil's further analysis revealed that the northern branch consistently produced fainter meteors than the southern, with the average difference amounting to 1.02 magnitude. Both radiants possessed a spread in right ascension amounting to 5 deg, while their declinations covered only 2 deg of the sky.
      Many of the above features just discussed reveal an interesting structure for the Orionids. They demonstrate what may be expected to occur within an old meteor stream. Although observations of hourly rates are one way to detect filaments, future detections might be made in the study of average magnitudes and the percentages of meteors leaving persistent trains. Some recent observations follow.

Orionid Magnitudes and Trains

      Observers in the previous table are as follows: G. N. Sizonov, lead a team of seven observers at Armavir (USSR), Norman W. McLeod, III (Florida), Felix Martinez (Florida), Bert Matous (Missouri), Western Australia Meteor Section, Robert Lunsford (California), Dutch Meteor Society.
      Colors have been a point of controversy in meteor astronomy: Are observed colors of meteors real or subjective? During 1978, observers of the Western Australia Meteor Section, headed by Jeff Wood, detected 27 Orionids, of which 0% were yellow and 6.7% were blue-green. However, during 1979, the same group noted that 14.4% were yellow, 3.4% blue, 1.7% orange and 0.8% green. In 1983, Mark T. Adams (Palm Bay, Florida) summed up this situation best when he commented that there was "no clear trend in the observed Orionid colors."
      Studies of the evolution of the Orionid shower possess a strong interest due to the stream's link to Halley's comet. This link was indirectly made in 1911, when Charles P. Olivier mentioned the similarity between the orbit of the Orionids and that of the Eta Aquarids of May. Since 1868, this latter stream had been known to be related to Halley's comet (see the Eta Aquarids in chapter 5); 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 uncanny. 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 (Herzberg Institute of Astrophysics, Ottawa, Canada) and Hajduk (Astronomical Institute of the Slovak Academy of Sciences, Bratislava, Czechoslovakia) published details of a new proposed model of the meteor stream produced by Halley's comet. Using the 1981 study published by Donald K. Yeomans and Tao 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

      Using 59 photographic meteors obtained from papers written by Fred L. Whipple in 1954, Richard E. McCrosky and Annette Posen in 1961, C1964, P. B. Babadzhanov and E. N. Kramer in 1967, C1977 and Jack Drummond in 1980, the Author obtained the following orbit:

During the two sessions of the Radio Meteor Project conducted at Havana, Illinois, during the 1960's, Zdenek Sekanina determined the following orbits for the Orionid stream:

Finally, the orbit of Halley's Comet is given as follows:

      The comet was not seen in 391 BC, but, according to calculations by Donald K. Yeomans (Jet Propulsion Laboratory, Pasadena, California) and Tao Kiang (Dunsink Observatory, Republic of Ireland), this was the probable orbit of the comet, according to their elaborate study of the comet's motion. As can be seen it very closely matches the present orbit of the Orionid stream---especially when compared to the photographic orbit.
      Using an orbit similar to the photographic one above (but with a semimajor axis of 15.10 AU), Ken Fox (Queen Mary College, England) projected the orbit of this stream backward and forward for 1000 years. The following two orbits were obtained:

      The orbit of the Orionids thus seems fairly stable. In 950, the date of maximum was four days earlier from that of the present, while the radiant position was RA=90.8 deg, DEC=+15.8 deg. In 2950, maximum will occur only one-half day later than at present and the radiant will be at RA=98.1 deg, DEC=+15.3 deg. Thus, even though the orbit of Halley's Comet has undergone slight changes during the last 2000 years, the ellipse of material that produces the Orionids is fairly stable.

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