The duration of this shower extends from May 19 to June 19. Maximum occurs on June 9, from an average radiant of α=236°, δ=+41°, with the average magnitude then being about 4.
The discovery of this meteor shower occurred as a result of the discovery of its parent comet. The latter discovery occurred on May 2, 1930, when plates exposed by A. Schwassmann and A. A. Wachmann (Hamburg ObservatoryBergedorf, Germany), during a regular minor planet survey, revealed the diffuse image of a comet. The saga of the Tau Herculids unfolded a short time later at Kwasan Observatory (Kyoto, Japan).
Observations of Schwassmann-Wachmann 3 were made daily at Kyoto, and, after a few positions had been obtained, Watanabe computed a preliminary parabolic orbit. From this orbit, Shibata deduced a radiant point and predicted a shower would soon occur.
Beginning on the night of May 21, several Japanese observers, most of whom were located at Kwasan Observatory, began watching the skies for the predicted shower. Searches were fruitless until May 24, when a careful watch around Boötes revealed a stationary meteor at α=230°, δ=+48°. The next evening, several more meteors were seen radiating from a point slightly less than a degree east of the previous night's position.
Observations of this shower ceased for a short time following May 25, since Schwassmann-Wachmann 3 had attained naked-eye visibility at the end of May, but by June 3 the Tau Herculids were again under scrutiny and were noted coming from a radiant of α=232°, δ=+46°. Further observations were made on the 6th and 7th as the radiant continued its southeastward trek, but the fairly weak activity observed up to this point suddenly changed on June 9, when 59 meteors (most of which were described as fainter than 4th magnitude) were detected in one hour. Rates were slightly higher on the next evening when 36 meteors were seen in 30 minutes. On the former date, short-trailed meteors were used to determine the radiant position as RA=236.25 deg, δ=+41.5 deg. Weak displays had again returned when observations resumed on June 12 and 13, and the shower was last detected on June 19, from a radiant of α=244.5°, δ=+39°.
It should be noted that by the first days of June the prediction of a possible strong meteor shower had been published in newspapers around the world, but Kaname Nakamura was the only observer to note a strong display. Even Issei Yamamoto, director of the Kwasan Observatory, noted that "Mr. Nakamura was practically the only observer" among staff members of the observatory. However, Yamamoto did point out that a radiant position of α=237.5°, δ=+41° had been determined by K. Siomi (Hukutiyama, Japan) on the night of June 12-13. Thus, the shower seems to have been confirmed. Unsuccessful, however, were the meteor section members of the British Astronomical Association. They noted that bright moonlight interfered with their observations on June 5, 7 and 9, so that no display of any kind was noted. From this data it seems the Tau Herculids possess a very sharply defined maximum, with meteors being predominantly faint.
Attempts to locate past appearances of this shower have revealed few possibilities. Most promising, however, are a series of radiants determined by John Koep and Philip Trudelle (both of Chippewa Falls, Wisconsin) between May 22 and June 5, 1916. The radiants are as follows: May 22.1, α=224.5 deg, δ=+25.3 deg (Koep), May 27.18, α=230.3 deg, δ=+27.4 deg (Trudelle), May 27.75, α=231.0 deg, δ=+27.5 deg (Koep), May 28.17, α=232.1 deg, δ=+26.8 deg (Trudelle), May 30.64, α=232.7 deg, δ=+28 deg (Koep), June 4.2, α=234.4 deg, δ=+27.5 deg (Koep), and June 5.18, α=235.8 deg, δ=+25.6 deg (Trudelle).
Admittedly, the declination is nearly 20 deg too far south in May and about 10 deg too far south in June, however, as will be shown in the "Orbit" section below, the shower's orbit is closer to the Tau Herculids.
The only other pre-1930 detection of this shower came during June 3-7, 1918, when William F. Denning detected 4 very slow "Theta Coronids" from a radiant of α=230 deg, δ=+34 deg.
Following the 1930 shower, attempts at further observations proved fruitless. J. P. M. Prentice (Stowmarket, England) watched on May 20, 22, 23 and 24, 1931. His total observing time amounted to 11 hours and 20 minutes, but no activity was noted. Stars to magnitude 6 were visible some of the time. Additional attempts to reobserve this shower---usually during the years of the comet's predicted perihelions---occurred on several occasions during the 20 years following 1931, but no traces of the shower were ever noted. However, using photography during the early 1950's, the Harvard Meteor Project revealed the stream to still be producing meteors.
The first detection of the photographic Tau Herculid meteors among the 1950's data was made by Richard B. Southworth and Gerald S. Hawkins in 1963. They detected just two meteors, but the similarity of the derived orbit to Schwassmann-Wachmann 3 was close. A further look at possible photographic meteors associated with this comet occurred in 1971, when B.-A. Lindblad (Lund Observatory, Sweden) identified 14 meteors from the Harvard Meteor Project. The average orbit "suggests good agreement in all orbital elements, and the proposed comet-meteor relation may now be considered very probable."
The last extensive search for meteors of this stream came in 1974, following a prediction by K. Kono. With Schwassmann-Wachmann 3 coming toward a mid-March 1974 perihelion, Kono predicted that a shower might occur on June 1.0 from α=146°, δ=+54° (the comet's orbit having underwent some changes since its 1930 apparition). Harold Povenmire noted that observations for 2 hours on June 1 by members of the Florida Fireball Network revealed only one meteor from the radiant, though moonlight was interfering.
Other recent observations of this shower continue to show extremely weak activity:
1976---During 19 hours and 20 minutes of observing during June 1 to 25, Bert Matous (Grandview, Missouri) saw only 4 Tau Herculids.
1977---John West (Bryan, Texas) saw 2 Tau Herculids in 4 hours on May 26 and 28, C. Smith (Bryan, Texas) saw 3 in 3 hours. Matous saw 17 in 12 hours 30 minutes during June 6-11, while Norman McLeod (Florida) recorded 5 in 15 hours and 9 minutes during May 21/22-June 11/12.
1979---F. Roy (Pte. Gatineau, Quebec, Canada) saw 3 meteors in 2 hours 15 minutes on June 3/4. (Schwassmann-Wachmann 3 was recovered for the first time since 1930, and passed perihelion on September 2.)
1982---West saw 4 meteors in 4 hours on May 22/23 and 27/28, while M. Zalcik (Edmonton, Alberta, Canada) observed 2 in 3 hours on May 23/24.
1984---David Swann (Dallas, Texas) saw no meteors during 2 hours on May 29/30, one during 2 hours on May 31/June 1 and one during 1 hour 30 minutes on June 1/2. Bill Katz (Willowdale, Ontario, Canada) observed during 6 hours on June 3/4. Tau Herculids were noted each hour with the highest hourly rate being six and the lowest being two. This marked some of the highest rates in recent years. (The comet was approaching a January 11, 1985, perihelion.)
From an examination of the apparent observations of this comet, it seems that a few meteors have managed to venture to the opposite end of the comet's orbit, however, this still amounts to less than one meteor per hour near maximum. On the other hand, it seems activity is still strongest during years when the comet is near perihelion. Since the meteor stream is probably fairly young, it may be possible that a sharply defined and fairly strong maximum might occur during years the comet reaches perihelion, as in 1930 and 1984. The 1916 activity came roughly two years after a perihelion passage of Schwassmann-Wachmann 3.
Orbit "A" represents an elliptical orbit computed from Nakamura's positions of 1930, using the semimajor axis of Schwassmann-Wachmann 3. Orbit "B" represents the orbit obtained from 11 photographic meteors collected from papers published by McCrosky and A. Posen in 1961, Jacchia and F. L. Whipple in 1961, and Babadzhanov in 1963. Also listed is orbit for comet Schwassmann-Wachmann 3 in 1930.
Using an orbit midway between "A" and "B," Ken Fox determined the stream orbit for 1000 years into the past and future during 1986. The following were determined for the Tau Herculids:
This study revealed a very interesting variation of the Tau Herculid activity over 2000 years. About 1000 years ago the shower's maximum occurred during the latter half of June from α=220.6°, δ=+72.4°, while 1000 years in the future maximum will occur in mid-August from α=154.7°, δ=-29.4°. Even more interesting is the similarity between the orbit of 950 and the present orbits given under the June Boötids. The June Boötids and produced by periodic comet Pons-Winnecke. Perhaps the June Boötids and Pons-Winnecke are somehow related to the Tau Herculids and Schwassmann-Wachmann 3.