Sky-Watcher Alert: Meteor Show Peaks This Week

John Roach
for National Geographic News

July 27, 2004

The South Delta Aquarids top the bill of several meteor showers converging in the night sky this week, giving night owls good reason to stay up after the moon sets to catch an eyeful of shooting stars.

"They're the strongest of several low-key showers, the combined activity of which makes the last week of July a productive time," said Neil Bone, director of the British Astronomical Association's meteor section.

Peak rates for the South Delta Aquarids top out in the early morning of July 28 at around a dozen shooting stars per hour for viewers in North America and Europe. Rates are higher in southern latitudes.

Lew Gramer is a Cambridge, Massachusetts-based amateur astronomer who maintains a popular meteor-observation mailing list for the North American Meteor Network. He said the activity from several other simultaneous showers could bring rates to between 25 and 30 shooting stars per hour.

The other showers include the South Iota Aquarids, North Delta Aquarids, Alpha Capricornids, Pisces Austrinids, and even a few early Perseids, which peak on the dark-moon night of August 12.

"Not just the SDAs [South Delta Aquarids], but these lesser showers as well, and other meteors which come from no known shower at all—so-called sporadics—all contribute to that upper bound of 25 to 30 meteors," Gramer said.

According to Bone, the Alpha Capricornids in particular could treat observers to the pleasing sight of long, slow, and bright meteors.

Observing the Show

Robert Lunsford, operations manager for the American Meteor Society, laments the bright moon will swamp many of the faint shooting stars at the end of July. Favorable conditions for viewing the South Delta Aquarids return in 2006.

For the determined, however, he offers this advice:

"Wait until the moon is low in the west or beneath the horizon. On July 27, with the moon only four days before full, the moon will set an hour or two before the start of morning twilight [on July 28], depending on one's latitude. This would be the best time to try to see any activity."

Moonlight can cut observers' hourly rates by a half, three-quarters, or more if the nearly full moon is in the sky and shining directly in their eyes. Therefore, night owls who stay up to the wee hours of the morning will have the best chances of seeing any shooting stars, Gramer said.

For observers who cannot wait for the moon to set, Gramer said they can still catch sight of a shooting star. They'll have to find a spot where their view of the sky is unobstructed but the moon is blocked out by trees or a building.

Bone describes the moonlight interference as "frankly awful" but said the upside is a dark sky for the Perseids, which he expects to be excellent this year. Peak rates for the Perseids may reach outbursts at ten times the rate of the South Delta Aquarids.

Scientific Queries

Meteor showers are named after the constellations they appear to be coming from. In the case of the South Delta Aquarids, the constellation is Aquarius; the Leonids, the constellation Leo; the Perseids, the constellation Perseus; and so on.

But the showers have little to do with the constellations they are named for. Rather, they are produced by bits of dusty debris shed by orbiting comets and asteroids. The Leonids are produced by comet Tempel-Tuttle, the Perseids by comet Swift-Tuttle.

The scientific community is not certain what the parent body of the South Delta Aquarids is. Several competing theories point to different sources.

"The best candidate appears to be a short-period comet known as 96P/Macholz," Bone said.

96P/Macholz is also thought to be the parent comet of the annual Quadrantid meteor shower in early January. But this theory too is controversial, which makes the South Delta Aquarids even more intriguing, Gramer said.

Astronomer Peter Jenniskens with the NASA Ames Research Center in Moffett Field, California, recently proposed that a celestial body—a defunct comet or asteroid—known as 2003 EHI could be the parent of the Quadrantids.

The South Delta Aquarids also appear to be related to North Delta Aquarids, which occur at about the same time but appear to be coming from the northern portion of the constellation Aquarius.

"The river of dust which forms the SDA meteor shower—and the one that forms its sister NDA shower—has probably been diverted and redirected by the gravity of the planets, so that it may be very challenging to ultimately tie it back to whatever its original parent body or bodies were," Gramer said.

Amateur observations of the South Delta Aquarids can help scientists address questions about the relationship between the South Delta Aquarids, North Delta Aquarids, and the other simultaneous showers and the parent body of the meteor shower. Such observations may also reveal how to predict elevated rates in coming years.

"Amateur studies actually provide a majority of the knowledge of what is currently known of this shower," Lunsford said.

According to Bone, the thrill of amateur meteor observing is just watching and recording the activity. "Sometimes nominally well-understood showers can produce unexpectedly high activity, and then it becomes a matter of professional interest to try and understand the underlying reasons," he said.

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