Observing
This shower is currently active during June 27 to July 5 and possesses a maximum of activity that falls on the 28th. The greatest activity levels only reach 1 to 2 per hour, but the stream is noted for an especially strong display in 1916, and good displays in 1921 and 1927. At maximum, the radiant is diffuse—probably greater than 5° in diameter—with the average position being at α=223°, δ=+58°. The shower is notable in that its meteors are primarily faint, with an average magnitude near 5, however, bright meteors do occur regularly.
History
The entrance of this stream into the lists of active meteor showers occurred as a result of intense activity that was noted on the night of June 28, 1916. Observers in England were treated to a very active display of meteors beginning soon after sunset and, one of the most experienced observers lucky enough to be outside that evening was W. F. Denning.
After a cloudy day, the skies of England began clearing shortly before sunset. Denning went outside at 10:25 p.m. (local time) and very quickly noted that a meteor shower was in progress. “Large meteors came in quick succession from a radiant in the region between Boötes and Draco,” Denning wrote. He described the meteors as “moderately slow, white with yellowish trains, and paths rather short in the majority of cases. Several of the meteors burst or acquired a great intensification of light near the termination of their flights, and gave flashes like distant lightning.”
Denning enlisted the help of a friend in observing the spectacle, while he plotted as many meteors as possible. Observations ended after 1:15 a.m. when clouds again moved in. Denning concluded that the greatest part of the display occurred prior to midnight. He also stated that the exact location of the radiant was impossible to pinpoint. As he projected his plotted paths backward he noted the “directions were from a wide region or area of about 12-15° diameter.” What Denning considered to be the main radiant was given the position of RA=231°, DECL=+54°.
Though clouds prevented observations on June 29, Denning was able to observe between 10:15 and 11:30 p.m. on the 30th, but only one meteor was noted that could have been from the shower of the 28th. This meteor was also detected and plotted by F. Wilson (Totteridge). From both plots, Denning was able to determine the radiant to have been at RA=223°, DECL=+41°.
Shortly after observing this strong shower, Denning wondered if its sudden appearance might be attributable to a comet. After searching through lists of cometary orbits, he concluded that periodic comet Pons-Winnecke was probably responsible. “The radiant was placed in the correct region and the date agreed. Moreover, the comet passed through perihelion on September 1 last year.”
C. P. Olivier (then director of the American Meteor Society) received several charts during the latter half of 1916 which clearly indicated that, although the great shower of June 28 had been missed in the United States, observers had independently noted activity from a similar area of the sky during May, June, and July—J. Koep and P. Trudelle (both of Chippewa Falls, Wisconsin) observed these meteors during the period of May 19 to June 5, while R. Lambert (Newark, New Jersey) detected a radiant of RA=206.7 deg, DECL=61.2 deg on July 4.
Olivier tabulated the known 1916 observed radiants and computed both parabolic and elliptical orbits for each. He upheld Denning’s belief of a relationship with Pons-Winnecke and added that the minor activity seen in the United States during the period of late May to early July, and the strong activity noted on June 29, were probably produced by the same stream.
A later investigation into the link with this meteor shower and Pons-Winnecke was published in 1932. F. W. Smith extended Olivier’s mathematical work to check the similarity between a radiant ephemeris based on the observations and an ephemeris based on the orbit of Pons-Winnecke. Franklin took a typical radiant from the Koep and Trudelle data and noted that it would drift toward the general position of the June 28 activity (as noted earlier, no obvious center of activity was noted on the 28th), however, the link between the orbits of the observed activity and that of Pons-Winnecke was far from perfect. Smith theorized that “the meteors had been separated from the actual orbit of the comet by dispersive forces, which would be expected in this case because of the direct motion of the comet, its short period, and the low inclination of its orbit.”
Indeed, the forces being exerted upon the comet by Jupiter were great. In fact, from the beginning of the 20th century until about 1940, the comet had been locked into a nearly perfect 2:1 resonance with Jupiter. The main effect of this resonance was the rapid increase in Pons-Winnecke’s perihelion distance. From the time of the comet’s discovery in 1819, until shortly after the 1869 perihelion passage, the comet’s perihelion distance remained about 0.2 AU inside of Earth’s orbit. During the next 46 years the perihelion distance quickly moved away from the sun, and in 1915 it was only 0.04 AU inside of Earth’s orbit. By 1921, the perihelion was 0.03 AU outside of Earth’s orbit, and by 1964 it was located 0.22 AU outside of Earth’s orbit. The orbit of this comet has remained in a fairly stable state since 1964.
A search for signs of activity prior to the 1916 appearance has revealed some interesting data. In the same article cited earlier, Denning pointed out that “in 1860 and 1861 June 30 Mr. E. J. Lowe observed ‘many meteors.'” During 1899, Denning published his “General Catalogue of Radiant Points of Meteoric Showers and of Fireballs and Shooting Stars Observed at More Than One Station” which did list two possible early detections of this radiant. The first was detected during June 26 to July 11, 1872, by observers of the Italian Meteoric Association. The plots were examined by Denning, who determined a radiant of RA=216 deg, DECL=+47 deg. It was based on 10 meteors. The second radiant was seen by Denning during June 14-28, 1887, from a radiant of RA=213 deg, DECL=+53 deg. It was based on 4 plotted meteors. This latter radiant possesses very similar circumstances present when the 1916 shower was noted, namely Pons-Winnecke had passed perihelion at the beginning of September of the previous year.
Following 1916, two notable, though weaker, appearances of this shower occurred during the next two perihelion dates of the parent comet. During 1921, observations in the United States and England revealed predominantly weak activity, with only Robert M. Dole (Wilmington, North Carolina) and Denning being able to secure enough plotted meteors to reveal radiants. On June 29.17 Dole plotted 7 meteors from RA=213.6 deg, DECL=+47.2 deg, while on June 30.10, he plotted 8 meteors from RA=213.2 deg, DECL=+47.0 deg. On June 28, Denning plotted 7 meteors from RA=228 deg, DECL=+58 deg. But, while these observers were barely collecting enough data for radiant determinations, Kaname Nakamura (Kyoto, Japan) noted increased numbers in early July.
Nakamura searched for meteors of Pons-Winnecke on several occasions during late June and early July, in weather that varied from clear to mostly cloudy skies. Beginning his observations on June 25, Nakamura observed his first meteors from this shower on the 26th. Although the number of meteors from this shower increased by the 27th and 28th, the greatest rates were said to have occurred on July 3, when 153 meteors were seen in 35 minutes. Cloudy skies were present on the 4th, but observations on the 5th revealed meteors still falling at a rate of 91 per 41 minutes at one point. During the period of June 26 to July 11, Nakamura was able to plot enough meteors to obtain 9 daily positions, with the radiant moving slowly southeastward. On June 28, the radiant was determined to have been at RA=212.5 deg, DECL=+49 deg, while the position on the night of maximum activity levels was RA=212 deg, DECL=+47 deg. Nakamura’s radiants are very similar to those determined by Dole.
Nakamura had been described as possessing “very sensitive eyes,” and his daily estimations of the mean magnitude of these meteors showed the shower to have begun at 5.4, slowly brightened to 3.5 on July 1, then varied between 4.5 to 5.0 during July 3 to 11. Responding to Yamamoto’s letter, Denning showed some doubt about the sensitivity of Nakamura’s eyes unless “Nakamura is able to discern meteors of 6th, 7th and 8th magnitudes.” Despite Denning’s views interesting observations were made during the next appearance of this shower.
During late June and early July 1927, several members of the meteor section of the Russian society Mirovedenie, observing at Tashkent, noted increased activity which reached hourly rates of 500 on June 27. According to their director, Vladimir A. Maltzev, “about 90 per cent of the meteors were fainter than the 5th magnitude, which leads to the conclusion that our observations are confirming those made in Japan in Kyoto in 1921.” According to N. Sytinskaja, the observations revealed maximum to have fallen on June 27.21, while the radiant’s daily motion was noted to be +1.0 deg in RA and -0.3 deg in DECL. Although the number of meteors seen in the United States was much lower than that in Russia, Dole (East Lansing, Michigan) detected 145 meteors during June 26-30. The radiant moved from RA=212.5 deg, DECL=+55 deg on the 27th, to RA=218 deg, DECL=+59.5 deg by the 30th. He remarked that “many brilliant individual meteors” were seen, but, overall, they were “very faint.” Curiously, the radiant motion indicated by Dole’s observations indicate a northeastward motion, while the Russian data shows a southeastward motion. Similarly, Nakamura’s 1921 observations also revealed a southeastward motion, while Franklin Smith’s calculations revealed a northeastward motion for the radiant. An explanation for this discrepancy among observers is not easy to explain, but could be linked to the apparent diffuse nature of the radiant, as well as the faintness of the meteors.
Recent activity from this stream indicates it has weakened considerably since the 1920s. In 1968, Edward F. Turco wrote that observations had revealed recent rates of 3 to 5 per hour, “with meteors being on the fairly dim side.” In 1981, David Swann (Dallas, Texas) wrote that on six occasions during the period 1964 to 1971, he observed this stream and detected rates of 1 to 2 meteors per hour. Concerning the individual meteors, Swann noted that he had “never noticed any trains, even though I have seen several bright shower members.” He added that, considering the low number of meteors currently coming from this radiant, “the possibility of seeing a fireball from this stream seems good.”
Other observers have managed to determine radiants from the shower’s meager activity levels. On June 28.6, 1964, D. Conger (Elizabeth, West Virginia) observed 5 meteors from RA=226 deg, DECL=+59 deg, while on June 28, 1970, Pennsylvania observers Gary Becker (Allentown) and Mark Adams (Warrington) found radiants at RA=223 deg, DECL=+59 deg and RA=224 deg, DECL=+58 deg, respectively.
It should be noted that, since 1916, attempts have been made to observe this shower in several different years. The entire month of June has gained the attention of potential observers of this shower, but only 1916 displayed activity extending several weeks prior to June 28. Based on the then-published theories of a link with Pons-Winnecke, this indicates that the close proximity of the Earth and comet orbits during June 1916 caused Earth to encounter a broad, diffuse cloud of material as early as May 19. The June 28 activity is, however, a very sharply condensed knot of material. Since 1916, it appears that only material from this well-condensed ring has been encountered, though the weakness of the subsequent displays indicates Earth must only be moving through its outer fringes. On the other hand, an alternative theory might be available for the late May-early June activity of 1916 (see Tau Herculids).
Although the June Boötids are present in both visual and photographic records, their presence in radar records is not as pronounced. Admittedly, Australian and New Zealand radar surveys would be too far south to enable an identification of this stream. On the other hand among northern hemisphere radar surveys, only Sekanina’s Radio Meteor Project conducted in two stages during 1961-1965 and 1969 show signs of this stream.
In Sekanina’s early survey, a stream identified as the “July Draconids” was found to be active on July 1 and 2, from an average radiant of RA=239.5 deg, DECL=+68.8 deg. The discrepancy of the radar radiant from that determined visually was due to the radar orbit possessing an orbital inclination of 30.3 deg. The stream was based on only 5 meteors, and this may represent an excellent example of why researchers should place a low value on data supported by small samples. At the same time, it has been shown that the June Bootids possess a fairly diffuse radiant due to the rapid alterations of the parent comet’s orbit, so that this small sample may have represented a random collection of high-inclination members of this stream. It should be noted that during the comet’s transition from possessing a perihelion distance lying within Earth’s orbit to possessing a perihelion outside of Earth’s orbit, perturbations by Jupiter also acted on the orbital inclination. When discovered in 1819, the inclination was only 10 deg, while the present orbit is at 22 deg. Since the June Bootids displayed an obvious diffuse radiant during 1916, 1921 and 1927, which appears due to an orbital inclination spread of about 8 deg, such a diffusion would have been continued, if not enhanced, by the closest approaches with Jupiter that followed. Thus, Sekanina’s orbit seems based on a collection of meteors orbiting near the edges of the main orbit of the June Bootid stream.
Sekanina’s 1969 survey more conclusively defined the orbit of the June Bootids. During the period of June 2 to July 19, 54 meteors were detected from a stream Sekanina identified as the “Alpha Draconids.” The average radiant was determined as RA=207.4 deg, DECL=+64.0 deg and the orbit was shown to cross the ecliptic on June 22. This orbit closely matches that determined for the visually observed radiants detected from 1921 to the present, as well as the 1927 orbit of Pons-Winnecke. A second stream detected in 1969, was referred to by Sekanina as the “Bootids-Draconids.” Possessing a duration extending from July 1 to 4, from an average radiant of RA=233.7 deg, DECL=+52.2 deg, its orbit crossed the ecliptic on July 2. This orbit closely matches that determined by Olivier from the English observations of June 28, 1916.