Sky Lights

Summer's spectacular meteors let you watch a cosmic trompe l'oeil

By Bob Berman|Saturday, August 06, 2005

Spatial perception skills that work well in everyday life often fall apart when it comes to the sky. Case in point: this month’s Perseid meteor shower. Logic says these shooting stars must arrive from deep space, yet when you gaze up they all seem to emanate from a single spot in the atmosphere. Stranger still, that spot is the same no matter where you are on Earth—almost as if the meteors were following you.

No need for paranoia. What you see is a straightforward example of parallel perspective. The classic representation of this illusion is observed when you look down a straight stretch of railroad tracks. If you stand between the rails, the tracks appear to converge in the far distance, although they never do meet for real. Like the train tracks, the meteors’ paths are parallel; they look angled only because of perspective.

The illusory divergence of the meteors tells a great deal about their true origin. Each Perseid meteor is part of a swarm of sand-grain-size bits of space dust that create a white-hot trail of incandescent gas when they crash into Earth’s atmosphere, about 50 to 100 miles up. These swarms follow a looping orbit around the sun; the apparent source of the meteors, known as the radiant, marks the direction from which they hit us.

Under clear, dark skies you should see a meteor a minute when the Perseids peak on the night of August 11. After spotting just a few of them, you can easily locate the radiant: Although the meteors can show up anywhere in the sky, their paths all point back to a spot in the constellation Perseus, which gives the shower its name. In 1866 Italian astronomer Giovanni Schiaparelli noted that the orbit of the Perseids closely matches that of comet Swift-Tuttle. He deduced that the meteors are material shed from the comet and so was the first to make the connection between a meteor shower and its source.

Once you have found the radiant, you will be able to distinguish the Perseids from sporadics, stray meteors that arrive from arbitrary directions at a more or less constant rate of about a half dozen per hour. Sporadics follow varied orbits and derive from diverse sources. Some of them may consist of asteroid fragments or breakaway pieces of the moon or Mars. As a result, they usually have a noticeably different speed, brightness, and color from the Perseids.

A shower’s radiant tells other tales too. If it is located in the direction of Earth’s orbital motion, as is the case this month, the meteors meet our planet nearly head-on and move extremely quickly. The Perseids rip through the air at 37 miles per second, twice the speed at which Earth circles the sun. By contrast, October’s wimpy Draconid meteors catch up from behind, loping across the sky at a mere 12 miles per second.

The radiant’s location also determines the best time to see a shower. Meteors generally look most dramatic when you are facing right into the swarm. Since the Perseids arrive in the direction opposite that of Earth’s orbit, almost perpendicular to the sun, they peak halfway between midnight and noon—that is, around daybreak.

At midnight, when Perseus is just above the northeast horizon, you will be able to see only those meteors heading upward. After 1 a.m. the radiant will have risen high enough to enable you to see meteors streaking downward from that spot as well. In the predawn hours you can see all the meteors emanating from the radiant, and the shower will be at maximum intensity.

Clumpings within the Perseid swarm can produce brief variations within this pattern. Meteor astronomer Peter Jenniskens of NASA’s Ames Research Center forecasts that a narrow, dense filament of material shed by Swift-Tuttle in the year 1479 will trigger an hour-long spike in activity at 4:18 a.m. eastern daylight time on the morning of August 12. During that time meteor rates should quadruple to about four a minute; viewers in eastern states, who at that time see the radiant highest in the sky, get the best view.

With the moon setting at 10 p.m., the late-night skies will be beautifully dark. If it doesn’t cloud up, the Perseids this year should be truly radiant.

What's up in the August sky
August 7: The crescent moon meets Venus, returning to the evening sky, low in the west at dusk.

August 8: Neptune reaches opposition, its closest approach to Earth, in the constellation Capricorn. Even so, it is too faint for the naked eye.

August 9: The crescent moon meets brilliant Jupiter, now slowly departing into the sun’s glare. The pair look best an hour after sunset.

August 11–12: The legendary Perseids—the year’s best and most consistent meteor shower—build after midnight and peak before dawn.

August 20–27: Mercury meets Saturn in Cancer, a pretty pairing in the predawn eastern sky.

August 31: Venus and Jupiter make a beautiful two-day close approach, lighting up the dusk.

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