WASHINGTON — Washington. TOMORROW NIGHT the earth spins through the flaming wake of debris left by Halley's comet. Look up in the sky and you'll see a night slashed with shooting stars.
Each year in October, the earth plows through the rock fragments, ice, and dust that mark Halley's ghostly orbit. Thousands of pieces of the comet's debris, called meteoroids while traveling in space, collide with the earth's atmosphere some 80 to 60 miles high and, ignited with the heat of friction, arc down the sky like falling stars. Many burn up in less than a second and vaporize to gas. Some 20 percent stay burning as long as one or two seconds.
There's no need to worry about being hit by a meteor. Most burn up completely in the atmosphere, or are so microscopic in size that they can float around for months or years. The meteors that hit earth are usually fragments from the asteroids circling the sun in the orbit of a planet between Mars and Jupiter -- or they might be pieces cast off from the moon. Of the 400 tons of meteorite dust that fall on earth each day, only a few pieces survive the fiery descent and land on earth as chunks of metal, melted, fused and pock-marked -- smaller than a pea or larger than a truck.
Meteor showers are named for the constellations from which they seem to burst. The Orionids seem to be falling from the constellation Orion, that hunter with his dagger in his belt just now beginning to leap sideways over the eastern horizon after Taurus. Other meteor showers at different times of the year seem to originate from other constellations; the Leonids in mid-November seem to fall from Leo; Geminids in mid-December from Gemini, and the Perseids near mid-August from Perseus.
The luminous meteors radiate from a single point much as snowflakes or raindrops burst outward from a small area in the sky as you watch them through the windshield of a moving car. The meteors are actually traveling along lines parallel to one another, but from our small spot on earth, they seem to come from one place -- an optical illusion similar to what you see when you look along railway tracks or rows of trees or buildings. All parallel lines seem to diverge from a single point in the distance -- a concept that architects, artists, designers, and computer graphic people readily understand. Do you?
For a period of about two days and nights each October, Halley's debris falls in luminous showers through the earth's atmosphere. Every 76 years, this comet travels in a huge elliptical path through our solar system.
It was recorded for the first time in the Western hemisphere -- not by a mathematician or astronomer, but by a tapestry weaver who stitched its awesome presence in green, yellow, and red wool on the French Bayeux tapestry illustrating the invasion of England by William the Conqueror in 1066.
In the following centuries, the bright comet was noted in 1531, 1607, 1683, with appearances so regular and orbits so similar to one another that the English astronomer Edward Halley predicted its arrival in 1758. In his honor, the comet was given his name. Once the comet was identified and tracked, scientists poring through ancient chronicles and documents from China found records of the comet as early as 240 B.C.
To watch the Orionids, stand in an open field, or better still, sit in a lawn chair, as far from city lights as possible. With your feet pointing toward the east, feel the whole world move forward with you plowing through the stars, as if you are a deck-chair passenger on the prow of a ship cutting through the sea.
Meteor showers are best seen in the early hours after midnight. At this time, you are on the forward part of the spinning earth and meeting the shower head-on. The combined speed of the meteors shooting toward you and the earth moving forward creates greater friction so that they burn brightly. Before midnight, you are on the trailing side of the earth. The meteors are shooting above and behind you and you see fewer of them -- as you do not see many snowflakes landing on the back of your car as you drive through a snowfall.
At this time of year, our planet passes through the inbound path of Halley's orbit. In May, we pass through the Aquarind meteor showers -- debris left by Halley on its path away from the sun, back into its habitat: the enormous Oorts Cloud at the cold dark edge of the solar system, some 100 times farther from the sun than the most distant planet, Pluto. Millions of comets reside in this massive cloud until one is shaken out of its lair by a passing molecular cloud and, pulled by the gravity of the sun, sweeps toward it and around it and back to the cloud.
The other bright autumnal meteor shower, the Leonids, occurs beginning November 15 and lasts about seven days. These meteorites travel in bunches, circling the sun in the tail of the comet they once were: Comet 18661. In certain years, the earth passes through their path at a time when the stragglers are most closely packed together. That's when spectacular displays of shooting stars occur. One of the most sparkling events yet recorded was in 1833 when some 250,000 meteors were counted at one spot between midnight and dawn. The most recent dense display took place in mid-November 1966, when meteors were counted in the southwestern United States falling at the rate of 140 per second.
Barbara Tufty, a science writer, is conservation editor of the Audubon Naturalist Society.