Ursids

Observing

The point from where the Ursid meteors appear to come from is located within the constellation Ursa Minor, also known as the "Little Dipper". This meteor shower is active during the period spanning December 17 to 25, but it peaks on December 22/23. At maximum, rates can normally reach 10 per hour. The meteor shower is produced by the periodic comet 8P/Tuttle and can occasionally experience short-lived outbursts of up to 100 meteors per hour.

Northern Hemisphere

This represents the view from mid-northern latitudes at about 1:00 a.m. local time around December 22/23. 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 3.5 and Adobe Photoshop 5.5.)

The radiant is above the horizon throughout the night, except for observers deep in the southern portion of this hemisphere. The best time to observe is from midnight to the beginning of morning twilight, with the radiant rising slightly higher as the morning progresses. These meteors are typically faint.

Southern Hemisphere

The Ursids are not really visible to observers in the Southern Hemisphere. Its highest altitude is 18° below the horizon and that occurs in daylight. The highest altitude in dark skies occurs just before morning twilight begins, at which time the radiant is about 32° below the horizon.

History

This meteor shower seems to have been discovered by W. F. Denning (England), who, during several years around the turn of the century, observed a radiant at RA=218 deg, DEC=+76 deg, which endured during December 18-22. C. Hoffmeister (Germany) listed additional observations of this meteor shower in 1914, 1931, and 1933, in his book Meteorströme. Despite these early observations, coordinated studies did not commence until after Dr. A. Becvár accidentally observed a strong display in 1945.

Becvár was observing at the Skalnate Pleso Observatory (Czech Republic) on December 22, 1945, when he, and other observers, noted meteors falling at a rate of 169 per hour. Many meteors were photographed and M. Dzubak computed a preliminary radiant of RA=233 deg, DEC=+82.6 deg. A reinvestigation of the data by Z. Ceplecha revealed a photographic radiant of RA=217.1 deg, DEC=+75.9 deg.

Coordinated studies of this meteor shower were finally begun in 1946. Becvár confirmed his detection of an active radiant, but found the maximum hourly rates only reached 11 on the night of December 22. Bochnícek and Vanysek confirmed Becvár's observations and both observers also established radiants for December 22.9. Bochnícek estimated it as RA=213 deg, DEC=+75 deg, while Vanysek found it to be RA=217.8 deg, DEC=+76.7 deg.

In 1947, visual studies of this stream were carried out by J. P. M. Prentice, of the British Astronomical Association. During 1 hour 43 minutes on December 22, only 1 meteor from the Ursid radiant was seen, but in 25 minutes on December 23, 8 meteors were detected---making the corrected hourly rate about 20. Four of the eight meteors detected on the latter date were plotted and revealed a radiant of RA=207 deg, DEC=+74 deg. The radiant diameter was less than one degree.

In addition to visual observations of this shower, 1947 was also notable for the first detection of the radiant by radio-echo observations. Aerials at Jodrell Bank (England) first got a bearing on this shower at 3 hours Universal Time (UT) on December 22. Beginning at 9 hours UT, hourly rates could be determined and, until 11 hours UT on December 23, these rates averaged 15. They estimated the equivalent visual rate would have been about 10. Thereafter, activity dropped sharply. The pointing of the aerial to different directions allowed the radiant to be determined as RA=195+/-8 deg, DEC=+78+/-5 deg.

Radio-echo observations became the primary means of studying this shower during the period 1948 to 1953, with observers at Jodrell Bank detecting the meteor shower every year. The rates ranged from 9 to 20 per hour.

The Ursids went through another period of neglect after 1953, but this finally ended in 1970, when the British Meteor Society (BMS) began a period of annual observations. From observations spanning the period 1970 to 1976, the BMS found an average radiant of RA=217 deg, DEC=+76 deg. Maximum occurred at a solar longitude of 270.66 deg (about December 22), with the duration being established as December 17-24. Hourly rates were 10 in 1970, 22 in 1971, 16 in 1972, 18 in 1974, 9 in 1975 and 4 in 1976. The moon interfered in 1973, but during daylight hours on December 22, BMS radio observers detected a short, 1-hour burst of activity that produced a corresponding visual hourly rate of 30.

Beginning in the 1970s, observations became more numerous in the United States, but they indicated the shower was far from what observers saw during the late 1940s and early 1950s. The typical observer spotted 1 to 2 meteors per hour at maximum during the period of 1970 to 1982. Coordinated studies in Japan during 1970 and 1971 confirmed the low rates of the observers in the United States. There does, however, seem to be occasional strong displays of this shower. Observers in Sogne, Norway, noted a strong display during 2 hours of observations on December 22, 1979, with observers reported 10-15 meteors per hour. Veteran meteor observer, Norman W. McLeod, III (Florida) has commented that the Ursids "must be a compact stream like the Quadrantids. You have to be within 12 hours of maximum to see much."

The first half of the 1980's showed no particularly impressive activity from the Ursids. Thus, it was particularly surprising that when an unexpected outburst of activity occurred on December 22, 1986, there were several observers monitoring the shower. L. Gobin (Mechelen, Belgium) reported unexpected "very high rates" while operating radio equipment at 66.17 MHz. Gobin's equipment had detected average hourly echo rates of 60 to 68 during December 19, 20, 21, and 23, but found rates of 171 on the 23rd. This enhanced activity was also noted visually, and seems to have peaked during the nighttime hours over Europe. G. Spalding (director of the British Astronomical Association Meteor Section) reevaluated his observations of December 22 and arrived at a corrected hourly rate of 87+/-29.

T. E. Hillestad (Norwegian Meteor Section) reported the observations of K. Gaarder and L. T. Heen. The former observer detected 94 Ursids in 4 hours, including 37 in the hour following December 22.83, and reported an average magnitude of 1.90. The latter observer saw 75 Ursids in 2 hours, including 54 in the hour following December 22.88, and reported an average magnitude of 2.61. These two observers saw only 4 and 2 Ursids, respectively, during a one-hour interval on the night of December 23, indicating a very rapid decline in activity. Of the 175 Ursids seen, 17.1% left a persistent train.

From observations obtained early in the 20th century, Denning had suggested that this meteor shower was associated with "Mechain-Tuttle's Comet"---now known simply as comet 8P/Tuttle. Upon Becvár's announcement of his discovery of the shower in 1945, he mentioned that a connection with periodic comet Tuttle "is highly presumable."

The correlation between the observed activity rates of the Ursids and the perihelion passages of periodic comet Tuttle is particularly strange. In fact, the strong shower of 1945 actually occurred six years after the comet's perihelion passage---thus, placing the comet near aphelion! No rate estimates are available prior to 1945, and, unfortunately, observations virtually ceased during the latter half of the 1950's and early 1960's, so that the delay cannot be confirmed following the 1953 perihelion passage. Comet Tuttle next passed perihelion on March 31, 1967. No apparent observations were made in that year, but from 1968 onward, the rates generally remained between 10-15 per hour until 1973, when, as noted earlier, radio equipment operating in England detected activity corresponding to visual rates of 30 per hour during a one-hour period in daylight. This increase came six years after the comet's perihelion passage. The comet next passed perihelion on December 14, 1980. Hourly rates appeared normal in the following years, until December 22, 1986, when European observers detected very high rates while using both visual and radio-echo methods. Thus, three of the last four perihelion passages of comet Tuttle have been followed six years later by strong meteor activity.

Comet Tuttle next passed perihelion during 1994. As in the past, the skies were well monitored during the next few years. The six-year delay was announced on the internet by amateur astronomer Richard Taibi on December 1, 2000. During the next couple of weeks there was much discussion of the possible upcoming event. Because of the success in the predicted outbursts of the Leonid meteor shower in 1999 and 2000, Peter Jenniskens and Esko Lyytinen announced that a two to three hour outburst was expected when maximum rates could reach one meteor per minute on the night of December 22/23. Interestingly, the webmaster (Gary W. Kronk) conducted his own analysis of the Ursids. The result was that an outburst probably would not happen in 2000 because perturbations by Jupiter in 1948 and 1960 would have dispersed this area of the Ursid meteor stream.

The nights of December 22 and 23 were interesting. As expected, observations were made from around the world; however, the final picture was a confusing one, primarily because of the introduction of observing techniques that were new to the study of the Ursids. Jenniskens ultimately announced a success in his modeling of the stream and this success was very quickly published by the Central Bureau for Astronomical Telegrams on International Astronomical Union Circular No. 7548. Jenniskens had monitored the meteor shower using visual, radio-echo, and light-intensified cameras. Jenniskens also published all of the positive observations on his web site. But the fact was that there were more negative observations than positive with the visual and radio-echo observations. Jennisken's group had partly cloudy skies in northern California and reported a large number of meteors, which indicated a corrected hourly rate of at least 50 meteors per hour. Other observers with clear skies in southern California and Oregon, some with over 20 years experience, saw no sign of an outburst. Among the radio-echo observations, Jenniskens' proof of an outburst came from a single observer in Finland, which was under auroral activity at that time. Other radio-echo observations made across the United States and Europe, including the much more advanced meteor radar system at Ondrejov Observatory in the Czech Republic, detected no enhanced activity. The only observing technique not contradicted by other comparable observations was the filming of the meteors using light intensified cameras and that was because Jenniskens was the only person in the United States using such cameras. These cameras apparently revealed an abundance of faint meteors when the Ursids reached maximum over the United States and Canada. The problem here was that there were no baseline observations to go on. In other words, no one had monitored the activity rate of faint meteors within the Ursid meteor shower before. So, the jury is still out on whether an outburst really occurred in 2000.