May 2, 2000 -- May 5, 2000 is a red-letter day for many astronomy enthusiasts thanks to the upcoming alignment of five planets with the Sun and the Moon. Unfortunately, the alignment won't produce much of a sky show. The Sun will be right in the middle of the cluster -- blinding sunlight will make it nearly impossible to see the other members of the planetary get-together.

If that sounds like discouraging news and you're thinking it might not be worth star gazing this Friday evening, wait! There could be a sky show on May 5 after all -- the annual eta Aquarid meteor shower.

Right: This fanciful picture by Duane Hilton shows an eta Aquarid meteor streaking across an aurora-filled sky. No auroral storms are predicted for May 5, but such displays are becoming more common with the approach of solar maximum.

"This week will provide one of the few good views of a meteor shower this year," says Robert Lunsford, the North American Coordinator for the International Meteor Organization. "Moonlight will spoil most of the major meteor showers in 2000, but the eta Aquarids will occur with the moon near new and out of the way."

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The nominal peak of the eta Aquarids occurs near 1700 UT on May 5, but "the display will not have a sharp peak of activity .... instead good rates will occur for a week centered on May 5."

Experts expect the eta Aquarids to produce 15 to 20 shooting stars per hour for lower-latitude observers in the northern hemisphere and up to 60 per hour in the southern hemisphere. The best times to look will be in the hours before dawn on Thursday, Friday and Saturday, May 4-6.

The eta Aquarid shower isn't the most important annual meteor display for northern skywatchers -- the Leonids, Perseids, Geminids and Quadrantids are all more intense -- but it is interesting for another reason. Every eta Aquarid meteoroid that streaks across the sky on May 5 is a tiny piece of history's best known cosmic snowball -- Halley's Comet.

Our planet passes close to the orbit of Halley's comet twice a year, once in May and again in October. Although the comet itself is very far away -- presently beyond the orbit of Jupiter -- tiny pieces of Halley are still hanging around the inner solar system. These particles are leftovers from Halley's close encounters with the Sun every 76 years; each time the comet returns solar heating evaporates about 6 meters of ice and rock from the nucleus. The debris particles, usually no bigger than grains of sand, gradually spread along the comet's orbit until it is almost uniformly filled with tiny meteoroids. When these meteoroids strike Earth's atmosphere they produce the eta Aquarid meteors in May and the Orionid meteors in October.

Above: Comet Halley moves around the Sun in an elliptical retrograde orbit, opposite to the direction of Earth's motion. As a result, eta Aquarid meteoroids and the Earth approach one another at high speed, much like two cars in a head-on collision. Although the meteoroids are very small and light they give rise to bright shooting stars because they speed through the atmosphere at nearly 66 km/s (148,000 mph). The comet itself won't return to the inner solar system until 2061. The size of Earth's orbit is exaggerated by a factor of two in this diagram.

Like most meteor showers, the eta Aquarids are named after the constellation containing the shower's radiant, a point in the sky from which the shooting stars appear to stream. Because there are several annual showers that come from the direction of Aquarius, the May 5th shower is called the eta Aquarids (the radiant is very close to the star eta Aquarius) to distinguish it from the others. The eta Aquarid's sister shower in October is called the Orionids, from the constellation Orion.

meteoroid -- a tiny space rock (usually a speck of debris from a comet or asteroid) while it is in space or flying through Earth's atmosphere meteor -- a streak of light in the sky caused by a disintegrating meteoroid meteorite -- a rock from space that hit the ground because it did not burn up entirely in the atmosphere.

The radiant of the eta Aquarid display never climbs very far above the horizon in the northern hemisphere. That makes it a better shower for stargazers south of the equator. Although the rate of shooting stars is not very high in the north, skywatchers can hope to see some spectacular "earth grazers" while the radiant is low on the horizon. These are meteors that skim horizontally through the upper atmosphere. "Earth grazers" are typically slow and dramatic, streaking far across the sky.

Meteorites hitting the Moon?

When the Earth passes through Comet Halley's meteoroid stream, so will the Moon. There's no air on the Moon so meteoroids don't burn up in the atmosphere as they do here on Earth. Instead, they simply plummet to the ground, disintegrating with a brief flash of light on the Moon's surface. Scientists calculate that a rocky meteoroid the size of a grapefruit or larger could produce a 3rd magnitude flash, easily seen by the naked eye or through binoculars.

During the 1999 Leonid meteor storm, several amateur and professional astronomers were on the lookout for signs of meteoroids hitting the moon. Few really expected to see anything, but surprised observers captured half a dozen flashes on videotape. It turns out that the Moon passed through a dense region of the Leonid meteoroid stream, resulting in numerous visible impacts.

Above: This video of a lunar meteorite impact was captured by David Dunham on the night of the 1999 Leonid meteor shower. At peak brightness, the flash was about 3rd magnitude. [more information from LunarImpact.com]

The eta Aquarid meteor shower may offer another opportunity to spot lunar meteoroid strikes. The average space density of particles in the eta Aquarid debris stream is probably 100 to 1000 times less than that of the Leonid stream. Nevertheless, there are indications of dense filaments within the eta Aquarid stream [ref] that might carry large numbers of meteoroids. If the Moon passes directly through one of these, flashes could be visible.

"The eta Aquarid debris stream will hit the Moon near its western (sunlit) limb this year," says Prof. George Lebo at the University of Florida Department of Astronomy. "The best place to watch for meteor strikes will probably be about midway between the Moon's sub-Earth point (the middle of the moon's disk) and the slender sunlit crescent. Because the Moon is nearly New there will be lots of dark terrain where flashes might be visible."

Watching for meteorite strikes on May 5 will be difficult, continues Lebo, because the Moon will be so close to the Sun. On the days that follow, however, the Moon will rapidly move away -- by May 6, it will set two hours after the Sun, and by May 10 the Moon will still be above the horizon 6 hours after sunset.

Although the eta Aquarids peak around May 5, activity is high for about a week centered on the maximum. Brian Cudnick, coordinator of the Lunar Meteoritic Impact Search Program sponsored by the American Association of Lunar and Planetary Observers (ALPO), is organizing a watch for eta Aquarid impacts from May 6 through 10, 2000. The ALPO website includes a lunar blank and report form that observers can use to plot and report impact sightings.

For more information about lunar meteorite impacts and how to make scientifically useful recordings of meteorite flashes, visit LunarImpact.com.

The nominal peak of the eta Aquarids shower is 1700 UT on May 5. The shower is expected to produce up to 20 meteors per hour for northern hemisphere observers and 40 to 60 meteors per hour in the southern hemisphere. The difference results from the fact that the radiant is higher above the horizon in the south than it is in the north. The eta Aquarids are usually the best annual meteor shower for southern sky watchers.

The eta Aquarids are noted for unpredictable behavior, with secondary peaks that occur before or after the nominal maximum [ref]. Scientists attribute this to filamentary structures within the cometary debris stream. Brief outbursts of 40 - 60 (or more) meteors per hour could occur anytime during the week centered on May 5. No matter where you live, the best times to watch will be during the hours before dawn. That's when your sky will be headed directly into the densest part of the meteoroid stream (

).

The eta Aquarid radiant peeks above the eastern horizon at approximately 2:30 a.m. local time at mid-latitudes in both hemispheres. The radiant is located near Fomalhaut, a 1.3 magnitude star in the constellation Piscis Austrini. Fomalhaut will be below the horizon for most northern hemisphere observers, but it is a good finder star for skywatchers in the south. In Sydney, Australia, for example, Fomalhaut will be visible at 4 a.m. at an elevation of +25 degrees, just above and westward of the shower's radiant. All of the stars in Aquarius are fairly dim -- the brightest is Sadalmelik at magnitude 3.2.

Left: This image shows the area of sky around the eta Aquarid radiant (indicated by a red dot) as seen from Sydney, Australia at 4 a.m. on May 5, 2000. At 4 a.m. local time -- that is to say, when it is 4 in the morning where you live -- the following statements will be true: The radiant will be about 30 degrees above the eastern horizon if you live at a mid-latitude site south of the equator. The radiant will be about 15 degrees above the eastern horizon if you live at a mid-latitude site north of the equator. Northern observers should rotate this sky map by 180 degrees to see the arrangement of stars in their hemisphere.

You won't need binoculars or a telescope to observe eta Aquarid meteors; the naked eye is usually best for seeing meteors which often streak more than 20 degrees across the sky. The field of view of most binoculars and telescopes is simply too narrow for casual meteor observations.

eta Aquarids at a Glance The meteor shower is active from April 21 until May 12. Maximum activity is expected around 1700 UT on May 5, 2000. The radiant is at RA=22h28m, DEC=-1o Atmospheric velocity=66 km/s Typical magnitude = 3

Experienced observers suggest the following viewing strategy: Dress warmly. Bring a reclining chair, or spread a thick blanket over a flat spot of ground. Lie down and look up somewhat toward the east. Meteors can appear in any part of the sky, although their trails will tend to point back toward the radiant.

Above: The rate of meteor activity is usually greatest near dawn because the earth's orbital motion is in the direction of the dawn terminator. Earth scoops up meteoroids on the dawn side of the planet and outruns them on the dusk side.

Web Links

The eta Aquarid Meteor Shower -from Gary Kronk's Comets & Meteor Showers web site

The Orionid Meteor Shower -from Gary Kronk's Comets & Meteor Showers web site

Lunar Impact .com -find out more about things hitting the moon

International Meteor Organization

North American Meteor Network -includes tutorials about meteor watching and a meteor observer's calendar

American Meteor Society