Radiant Energy

By Bob Berman|Thursday, August 01, 1996
We live in a universe where nothing stands still, and yet only one kind of celestial object displays any obvious movement to the naked eye. Punctuating the night’s starry still life, meteors alone remind us that faster-than-bullet velocities are commonplace even in the sleepy backwater of our solar system.

August is the classic meteor-viewing month because of its famous Perseid shower, during which the background rate of six shooting stars each hour increases tenfold. This year’s shower promises to be particularly dazzling because the moon--that nemesis of meteor gazing--will be utterly absent until a harmless thin crescent rises at dawn.

Timing is everything: the intensity of a meteor shower depends on the position of the radiant--the point in the sky whence meteors seem to emanate. Before the radiant rises, the only meteors visible to us are the small percentage that shoot far enough upward from that source to clear the horizon. But after the radiant rises, we see them flashing outward in all directions like sparklers leaking from some dimension beyond.

The Perseid shower originates, of course, in the constellation Perseus, which emerges in the northeast after 11 p.m. For December’s Geminids, the year’s other great meteor display, the curtain rises earlier, soon after nightfall, when Gemini appears in the northeast. The Geminids will also occur in moonless skies, and if clear weather smiles on both showers, a nice comparison is possible. The dense Geminid meteoroids are slow and leisurely as they track across the sky, with only 3 percent leaving glowing trails. But this month’s spectacle features twice-as-fast, 37-mile-per-second streakers; one out of three leaves behind lingering dust trails.

Perseids begin slowly in late July, when our planet enters the fringes of the debris shed by comet Swift-Tuttle. When that comet passed through our neighborhood in 1992 after a 130-year absence, it theoretically restocked its orbit with apple-seed-size ice pellets. Those low-density stony ices make for flimsy meteoroid specimens unlikely to survive their passage into our atmosphere.

About a tenth of the meteors seen on the peak night of August 11- 12 will be sporadics (or strays), built of hardier material than mere cometary debris. Recovered meteorite specimens have come from the moon, from Mars, and from asteroids such as Vesta. These are the meteors we see when there isn’t a shower, and you can tell them from the Perseids because they do not emanate from Perseus and usually appear slower. And unlike the plain vanilla Perseids, the stony, metallic sporadics can sometimes display brilliant colors as they burn.

For the past few years on August 11, observers in various parts of the world have reported brief bursts of Perseids, three to five times the normal one-a-minute rate. This year may bring such intense displays-- the result of Earth’s intercepting narrow bands of the comet’s debris. But, more likely, a somewhat richer-than-average shower will occur, with perhaps 90 meteors an hour. Given the ideal moonless conditions, that should be exciting enough.

This is naked-eye astronomy at its easiest and most productive. Find a site with a wide expanse of sky, away from city lights. Then simply lie back and keep your eyes glued upward. If you’re so inclined, keep careful count of the number seen per hour, and whether they’re Perseids or sporadics. Or take along a friend and count nothing at all but the shared experience of being lost in a magical night of shooting stars.
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