Image: Bootid sky map
This image shows the area of sky around the June Bootid radiant, indicated by a red dot. At 10 p.m. local time in late June, the radiant will be directly overhead as seen from midnorthern latitude observing sites.
By Night Sky Columnist
updated 6/18/2004 8:27:22 PM ET 2004-06-19T00:27:22

Meteor enthusiasts will likely be out in force in the coming nights, hoping to catch a glimpse of an on-again, off-again meteor display. Special emphasis will be placed on two specific nights: June 22-23 and June 26-27.

Ironically, the month of June is usually not noteworthy for any major meteor showers.

Yet six years ago, during the final weekend of June 1998, sky watchers worldwide were caught off guard by an unexpected shower of bright meteors and fireballs.

From Japan, for instance, came reports of meteors that were visible even through heavily overcast skies. Reports from visual observers in other regions suggested that this surprise meteor display produced meteor rates of anywhere from 50 to 100 per hour and lasted more than half a day.

Similar bursts of June meteor activity were noted many decades ago, in 1916, 1921 and again in 1927. Because the meteors seemed to fan out from a region of the sky near the northern border of the constellation of Bootes the Herdsman, they became popularly known as the "June Bootids."

Comet bits
It was also in 1916 that the legendary British meteor observer William Denning first suggested that these meteors were bits and pieces shed by the Comet Pons-Winnecke, a rather small, dim object and a member of the Jupiter family of comets. Such comets are so named because they have their aphelia — the points in their elongated orbits that place them farthest from the sun — at roughly the same distance as the planet Jupiter.

The orbits of Earth and the comet were relatively close to each other during the early 20th century.

In fact, Pons-Winnecke’s closest point to the sun — its perihelion — slowly shifted from just inside Earth’s orbit in 1916 to just outside it during 1921 and 1927. Astronomers assumed that it was this close proximity between the two orbits that accounted for the enhanced meteor activity seen in those three years.

But since then, due to a series of relatively close encounters with Jupiter’s powerful gravitational field, the orbit of the comet has significantly changed.

Meteoric mystery
Since 1921, the gap between the orbits of Earth and the comet has been gradually increasing, becoming more than 22 million miles (35 million kilometers) by 1998. Because of this large gulf between the two orbits, it seemed logical to believe that any future enhanced meteor activity from Comet Pons-Winnecke would be all but impossible. That argument certainly held up … until June 1998. How, then, could that meteor shower have taken place with the two orbits so widely spaced?

The answer came from astronomers David Asher of Armagh Observatory in Ireland and Vacheslav Emel’yanenko, of South Ural University, Chelyabinsk, Russia. Their studies showed that the 1998 meteors were the result of meteoroids released from Comet Pons-Winnecke back in the year 1825.

Asher and Emel’yanenko pointed out that the planet Jupiter completes one orbit around the sun in the same time that it takes the meteoroids shed from Comet Pons-Winnecke to complete two. In other words, while Jupiter takes 12 years to go around the sun, the meteoroids from Pons-Winnecke take six years — a 2-to-1 ratio. So instead of spreading around the whole orbit, the meteoroids were kept clustered closely together by Jupiter’s gravitation.

Computer simulations by Asher and Emel’yanenko also demonstrated that the comet and its ejected particles from 1825 were apparently disturbed by Jupiter in different ways, so that in the ensuing years the comet and the particles that it shed became widely separated from each other.

Ultimately, however, in June 1998, the meteoroids ended up cutting right through Earth’s orbit, producing the unexpected bevy of bright meteors.

Another good shower?
So if the meteoroids that produced the bright 1998 display are still basically moving around the sun in a 6-year orbit, does that mean that we’ll be in for a repeat performance in 2004? Viewpoints are mixed.

Jürgen Rendtel, president of the International Meteor Organization, believes that 2004 could be another good year to look for the June Bootids.

Rendtel points out that on Sunday, June 27, at 01h GMT (which corresponds to Saturday, June 26, at 9 p.m. ET) the Earth should be passing through essentially the very same region of the meteoroid stream as in 1998.

That time would be the middle of the peak activity seen in 1998, but since that display lasted for many hours, observers worldwide should stay alert through Saturday night on into Sunday morning for any unusual or enhanced meteor activity.

On the other hand, David Asher’s belief is that little or nothing will be observed, based primarily on the simple argument that unusual numbers of bright meteors should also have been seen in 1992 and 1986, but nothing apparently occurred.

In recent days, a different forecast for the June Bootids has been issued by Jerimie Vaubaillon of the Institut de Mécanique Céleste et de Calcul des Éphémérides in Paris and Russians Sergey Dubrovsky and Sergey Shanov.

Their calculations suggest that Earth will interact with a swarm of meteoroids that were ejected by Comet Pons-Winnecke at not just one, but several of its past visits to the sun, most notably in 1819, 1825, 1830, 1836 and perhaps 1875. In addition, the predicted peak for this activity comes several days earlier than Rendtel’s suggestion: Wednesday, June 23, at 11h GMT (7 a.m. EDT).

What to watch for
Western North America and the Pacific Ocean will still be in darkness at that time, and are favored with the best possible views. But should the activity last for many hours, then it could be worthwhile to carefully watch the sky from Tuesday night, June 22, on until the first light of dawn on Wednesday, June 23.

Whether you plan to look for the June Bootids on the night of June 22-23 or again on the night of June 26-27, keep in mind that the constellation of Bootes will be excellently positioned as darkness falls. It will appear nearly overhead and high up in the northern sky and will remain in view through the night as it descends toward the northwest.

Fortunately, the moon will be a rather wide crescent and will set just before midnight (local daylight time) on the night of June 22-23. It will, however, be more of a hindrance on the night of June 26-27 when it will have increased in brightness to a bright gibbous phase and not setting until after 1:30 a.m.

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