Originally published 12 August 1985
If it is good luck to see a shooting star, then this could be your lucky night. This is the peak of the Perseid meteor shower, the richest show of the year. If the night is clear and you are patient and can get far enough from the city lights to find dark skies, you might see a few dozen meteors per hour tonight.
The Perseid shower is an annual event, but is more dramatic in some years than others. This year [1985], because the moon is just a slim crescent rising shortly before dawn during the shower’s peak, the skies will be darker than usual and more meteors should be visible. But the intensity of the shower itself also varies greatly, and somewhat unpredictably; a sudden peak, with many more meteors per hour, can occur from one hour to the next next.
Meteor showers are associated with the orbits of comets; the Perseids are the spawn of Comet Swift-Tuttle. The orbits of comets are dirty spaces; they are littered with comet stuff, material thrown off by the comet in its passage — icy grit, bits of stone — all moving in the same long elliptical orbit as the comet, but spread out along the comet’s orbit.
Every year in mid-August Earth crosses the orbit of Comet Swift-Tuttle and sweeps up cometary debris. As these particles encounter the Earth’s atmosphere they are heated by friction and vaporized. It is the streaks of burning vapor that we see in the sky as “shooting stars,” not stars at all but grains of cosmic dust making spectacular swan dives into a sea of air. It has been estimated that there are 100 million tons of particulate matter spread out along the orbit of Comet Swift-Tuttle; tonight you might see an ounce’s worth.
Head on collision
The Perseids are swift. They streak into the Earth’s atmosphere at 40 miles per second — 150,000 miles per hour. They sizzle in the air. They are swift because the Earth encounters the stream of particles head on, like an automobile driving into the rain, rather than being overtaken from behind. During the course of a single night the Earth will fly half a million miles across the stream of the Perseids, sweeping up billions of them. Our planet will add tons to its bulk, tons of sky dirt plastered onto the planet like bugs onto a windshield. Some of that dirt, if it is big enough, will survive its fall through the atmosphere and hit the surface. Scientists can dig down into Arctic or Antarctic ice and collect Perseids that fell to Earth a thousand years ago.
And where is the comet itself, the parent of this prodigious stream? It was last near the orbit of the Earth in 1862. In that year it was a spectacular naked-eye object, with a nucleus, coma, and tail. Astronomers anticipated the return of Comet Swift-Tuttle in 1984, after a century-long journey far out beyond Pluto. That was last year, and still there is no sign of the returning comet.
The orbit of Comet Swift-Tuttle is steeply inclined to the orbit of the Earth. It dives down out of the northern sky and crosses the Earth’s own trajectory, and then swims away to the south. This week’s meteors approach along that same path. The place in the sky where a meteor shower seems to originate is called the “radiant” of the shower. The shower of mid-August has its radiant in the constellation of Perseus: hence, “Perseids.”
Spokes of a heavenly wheel
The “shooting stars” you may see tonight will be in every part of the sky; but the lines of their motion, if traced backwards, will intersect, like the lines in a perspective drawing, in the dark space between Perseus and Cassiopeia. The streaks of lights are like the spokes of a wheel that has its rim on the horizon and its hub in Perseus. When Comet Swift-Tuttle comes — if it comes! — it will approach along the axle of that wheel.
As the night progresses, the radiant climbs higher in the sky and the number of meteors visible per hour should climb with it. After midnight, we are on the “front” of the Earth, the hemisphere that points toward the direction of the Earth’s motion and therefore gets the brunt of the incoming shower. The number of meteors visible per hour just before dawn should be double the evening rate.
A passion for observing
A comet takes its name from its discoverer or discoverers. Comet Swift-Tuttle was first observed by Lewis Swift, a farmer of Marathon, New York, and Horace Tuttle of the Harvard College Observatory. To call Lewis Swift a farmer is not quite accurate; the farm was his livelihood, but his passion was the sky. On every clear night Swift observed the heavens with a telescope mounted on a platform attached to his barn. The discovery that made him famous occurred on the evening of July 15, 1862.
On that night Swift observed a fuzzy blur in the dark sky of the constellation Camelopardalis, just to the north and east of Perseus. The blur was bright, bright enough to be within easy reach of binoculars. Swift thought it unlikely that a comet could have reached such a level of brightness without having been previously noticed by someone else. He reluctantly concluded that what we had seen was the recently discovered Comet Schmidt. Three nights later, Horace Tuttle in Cambridge observed the blur of light in Camelopardalis and immediately recognized its significance. On the same evening, Lewis Swift realized his error and reported the new comet. According to protocol, the two men shared the honor of discovery.
Comet Swift-Tuttle quickly brightened to become a splendid naked-eye object, one of the finest comets of the century. It made its closest approach to the sun on Aug. 23, 1862, and then quickly receded into the southern sky. In 1867, the Italian astronomer Giovanni Schiaparelli announced that the calculated orbit of the comet of 1862 was coincident with the orbit of the Perseid meteors of August. It was the first time that recurring shower of meteors was convincingly linked to a periodic comet.
And where is the comet, now overdue? You can be sure that this very night there will be dozens of present-day Lewis Swifts with their telescopes trained on the dark space to the north of Perseus, hoping to be the first to see the returning comet. If we are lucky, Comet Swift-Tuttle is coming near. In the meantime, we can make our wishes on the bits and pieces of the parent comet that come streaking our way.
Comet Swift-Tuttle made its return visit to Earth in 1992 and is not expected to be seen again until 2126. The Perseid meteor shower peaks every year around August 12 – 13, although in 2019 the light from a near-full moon will reduce the number of visible meteors per hour. ‑Ed.
