(meteorobs) The Ursids

For the last three weeks, Richard Taibi, Rainer Arlt, and myself have been working on a two part paper for meteorobs. Richard performed an exhaustive search through several periodicals in an attempt to find any probable Ursid observations prior to the 1970s. Rainer also provided some difficult to find sources, as well as numerous computations. I began applying my orbit integration program, normally used for comets, in an attempt to observe the evolution of the particles within the Tuttle/Ursid orbit. It is unfortunate that my end of the deal ended up being more time consuming than I had anticipated, mainly because each set of integrations revealed new questions that I felt were necessary to answer before publishing our conclusions. As of today, my integrations have generated over 2500 orbits. A total of 71 imaginary particles were placed into the Tuttle/Ursid orbit and the evolutionary patterns over the period of 1803 to 2001 were examined--sometimes in great depth. Although we are probably prepared for both parts to be published, I am afraid a rather formal write-up of "Part 2: The Analysis" will only be offered as a summary sometime tomorrow because we have run out of time. For now, however, here is "Part 1: The History." I would like to point out that "alpha" represents Right Ascension, while "delta" is Declination.

By Gary W. Kronk, Rainer Arlt, and Richard Taibi

Abstract------------------------------------------------------------------

Unlike the Leonids, the systematic study of the Ursids spans less than one century. Although the total period of activity, the date of maximum, and the average hourly rate are fairly well determined, the meteor display has developed a reputation of producing outbursts. But these are not normal increases in activity that occur when the parent comet 8P/Tuttle is near perihelion, as with the Leonids, Perseids, and Draconids. The Ursid reputation is that the outbursts occur roughly six years after comet Tuttle passes perihelion. An explanation is sought to explain the apparent fluctuations in Ursid activity.

History-------------------------------------------------------------------

The discovery of the Ursids is typically attributed to William F. Denning (Bristol, England). In a paper published in The Observatory during 1912, he mentioned he had seen meteor activity on December 21 and 22, one of which came from a radiant at alpha=220 degrees, delta=+78 degrees.(1) In 1921, Denning suggested a relationship between this shower and the periodic comet Tuttle, though the weak nature of the shower prompted him to add that such a relationship "may be an accidental accordance."(2) In 1923, he summarized his observations covering several years early in the 20th century, and determined the shower's duration as December 18-22. The radiant position was given as alpha=218 degrees, delta=+76 degrees.(3)

It is interesting how absent this meteor shower was from papers published during the latter part of the 19th century and the early years of the 20th century. Denning himself did not note any meteors from this radiant in his "General Catalogue of Radiant Points" published in 1899. A collection of over 4000 radiants gathered from numerous observers during the period of 1833 to 1898, only one radiant comes within 15 degrees of the Ursids on the appropriate date; however, as will be shown later on, both comet Tuttle and the Ursid stream are moving in remarkably stable orbits, so this one radiant is most likely not related. Another paper was published by Denning in May 1912. Meant as a supplement to his "General Catalogue", it offered 250 more radiants determined exclusively by Denning during the period of 1899 through September 1911. Again, no trace of the Ursids were present. A. King presented his observations for the period of 1898 to 1915, in a 1916 paper, which also did not include anything in the region of the Ursids.

There is a possibility that two older observations exist for the Ursids that predate Dennings observations by a half century to a century. During 1857, James Hector, then leading a branch expedition in northern Saskatchewan as part of the John Palliser expedition, wrote under the date of December 20, "Just as the sun was setting I observed a very brilliant meteor, so bright that it was distinctly visible even when close to the sun's disc. At sunset and sunrise for several days past there has been a very remarkable number of meteors."(4) Even earlier still, is an observation from Japan on 1795 December 20, when it was reported that "Stars flew like a shower."(5)

During the years following Denning's discussions of the Ursids, few took an interest in the meteor radiant. In fact, during the 1930s, it would seem the only observations acquired were buried amidst the vast work of over 5400 radiants compiled by Cuno Hoffmeister for his 1948 book "Meteorströme". Yet even these observations amounted to only five, with one occurring in 1931 and four occurring in 1933. The activity from the Ursids was so slight, that Hoffmeister did not recognize it and include it in his list of annual showers, even though he did note that comet Tuttle was likely to produce meteors from a radiant of alpha=237 degrees, delta=+74 degrees around December 22.

Ursid Radiants From Hoffmeister (1948)
                           Solar
Year Desig. Max. Date Longitude   R.A.(deg) Decl.(deg)
1914   727    Dec.         268          214       +66
1931 1797    Dec.         265.4        196       +73
1933 2035    Dec.         262.6        215       +58
1933 4858    Dec.         264.2        215       +69
1933 2045    Dec.         265.2        192       +79
1933 2047    Dec.         265.2        242       +71

A real wake-up call for interest in this meteor radiant came on 1945 December 22. On that date a strong outburst was reported by observers in Czechoslovakia. IAU Circular No. 1026 included a notice from A. Becvar (Skalnate Pleso, Czechoslovakia) announcing that a "new meteoric swarm" was observed on 1945 December 22. It was observed from 16:30 to 20:45 UT, with the maximum between 18:15 and 18:25 UT, when the hourly rate reached 169. M. Dzubak was able to determine the radiant as alpha=233 degrees, delta=+82.6 degrees, from 16 recorded paths. It was added, "Connection with the comet 1792 II is highly presumable."(6) R. Rigollet (Paris Observatory) followed with a statement on IAU Circular No. 1027. He re-iterated the suggestion of a relationship to comet Tuttle (1792 II) and said his calculations revealed the comet would produce meteor activity from a radiant of alpha=221.6 degrees, delta=+78.6 degrees.(7) In 1951, Z. Ceplecha re-examined the 1945 data. From the photographic meteors he gave the solar longitude as 270.62 degrees (based on a semi-major axis equal to that of the comet) and gave the radiant as alpha=217.08 degrees, delta=+75.85 degrees for 1945 November 22.773.(8)

During the next few years interest in the Ursids continued, especially at Jodrell Bank radio telescope, where the following details were published with respect to the hourly rate, date of maximum, and radiant position.

Ursid Radio-Echo Radiants
                    Hourly     Solar
Year   Date of Max. Rate    Longitude   R.A.(deg) Decl.(deg)
1948    Dec. 21.3     15      269.4       210±10      +82±8
1949    Dec. 22.3     13      270.2       207.1±8     +77.6±3
1950    Dec. 22       20      269.8       199±8       +77±3
1951    Dec. 23       13      270.5       200         +77
1952    Dec. 22        9      270.4        ?            ?
1953    Dec. 23       11      271          ?            ?

But visual observations were made as well. Becvar again saw the Ursids in 1946, but the peak at 22:00 UT on December 22 produced an hourly rate of only 7, which he suggested would have been near 11 per hour if the radiant had been at the zenith. Becvar determined the radiant position as alpha=203 degrees, delta=+75 degrees.(9) Another 1946 observation was made by V. Vanysek. During the period of 21:47 to 23:05 UT on December 22, he plotted 9 Ursids and determined the radiant as alpha=217.8 degrees, delta=+76.7 degrees.(10) The 1947 return was watched by J. P. M. Prentice (Battisford, England). Excellent conditions on December 22 revealed only 1 Ursid in 1 hour and 43 minutes. The next morning, the sky was mostly cloudy, yet Prentice saw 8 Ursids during a 25 minute period when a small patch of the sky became clear. He estimated that the hourly rate must have then been about 20. Four plotted meteors revealed a radiant of alpha=207 degrees, delta=+74 degrees (1900).(11)

Sadly, following the Jodrell Bank observations of 1953, coordinated observations of the Ursids ended. It was not until 1970 that observers began making an attempt to watch again. In that year members of the American Meteor Society (AMS) sent a total of 91 meteor counts to Meteor News, which allowed the determination that the maximum zenithal hourly rate (number of meteors seen in an hour if the radiant was at the zenith) was between 3.2 and 3.5.(12) Members of the British Meteor Society (BMS) recorded a maximum ZHR of 10(13), while members of the Nippon Meteor Society (NMS) reported 11 counts which indicated a maximum ZHR of 5.7.(14) The same groups also provided observations in 1971, but the variation was larger. The AMS observers reported a maximum ZHR of 2.9(15) and the NMS observers reported a maximum ZHR of 2.4(16); however, the BMS observers reported a maximum ZHR of 22.(17) Hourly rates were low from every location during 1972 and 1973, except from England. Members of the BMS were monitoring for radio meteors during the daylight hours of 1973 December 22, when they recorded maximum rates of 30 per hour at 11:00 UT.(18)

No abnormal activity seems to have been recorded during the next several years. But on 1979 December 22, observers in Sogne, Norway detected Ursids falling at a ZHR of 25 to 27.(19) The next few years saw ZHRs typically less than 10, but on 1986 December 22, two experienced Norwegian observers suddenly detected a rapid increase in the numbers of Ursids, which peaked at over 54 meteors per hour on December 22. The peak was confirmed by an observer in Belgium who was using radio equipment to monitor the activity.(20)

Although the rates of the Ursids appear to have been quiet during the remaining years of the 20th century, there are still occasional reports of high activity from experienced observers. For instance, on the internet discussion group "meteorobs" some interesting reports have been printed concerning abnormal rates. Robert Lunsford (California) recorded 81 Ursids during 5 hours of observing on 1993 December 22. His best hour of observing revealed 26 Ursids, which would indicate a ZHR near 75.(21) Marco Langbroek reported that Jurgen Rendtel and himself independently detected high Ursids ZHRs of 25 to 30 on the morning of 1996 December 22.(22)

[Coming up next: Analysis of the Hourly Rates and Orbit of the Ursids and comet Tuttle]

Endnotes:
1 Denning, W. F., The Observatory, 35 (1912), pp. 90-1.
2 Denning, W. F., The Observatory, 44 (January 1921), pp. 31-2.
3 Denning, W. F., Monthly Notices of the Royal Astronomical Society, 84 (November 1923, p. 52.
4 The Papers of the Palliser Expedition, Edited by Irene M. Spry, Great Britain: Robert MacLehose and Company Limited (1968), p. 194.
5 Imoto, S., Hasegawa, I. (1958): "Historical Records of Meteor Showers in China, Korea, and Japan". Smiths. Contr. Astrophys. 2, Nr. 6, pp. 131-144.
6 Becvar, A., IAU Circular, No. 1026 (1946 Jan. 24).
7 Rigollet, R., IAU Circular, No. 1027 (1946 Feb. 4).
8 Ceplecha, Z., Bull. Astron. Inst. Czechoslovakia, 2 (1951), pp. 156-60.
9 Becvar, A., IAU Circular, No. 1078 (1947 Feb. 4).
10 Vanysek, V., "Les meteores de la Comete Tuttle (1790 II)". Bull. Astron. Inst. Czechosl., 1 (1947), pp. 10-11.
11 Prentice, J. P. M., Journal of the British Astronomical Association, 58 (1948 May), p. 140.
12 Meteor News, No. 5 (1971 Mar.), pp. 7-8.
13 MacKenzie, R. A., Solar System Debris, Dover: The British Meteor Society (1980), pp. 28-30.
14 Meteor News, No. 10 (1972 Mar.), p. 5.
15 Meteor News, No. 10 (1972 Mar.), p. 5.
16 Meteor News, No. 10 (1972 Mar.), p. 5.
17 MacKenzie, R. A., Solar System Debris, Dover: The British Meteor Society (1980), pp. 28-30.
18 MacKenzie, R. A., Solar System Debris, Dover: The British Meteor Society (1980), pp. 28-30.
19 Meteor News, No. 51 (1980 Oct.), p. 5.
20 Van Wassenhove, Jeroen, WGN, 15 (1987 Feb.), pp. 12-13; Roggemans, Paul, WGN, 15 (1987 Apr.), p. 50; Hillestad, Trond Erik, WGN, 15 (1987 Apr.), pp. 59-60.
21 Lunsford, Robert, "meteorobs", (1999 Dec. 17).
22 Langbroek, Marco, "meteorobs", (2000 Dec. 3).