Arrival of the Perseids

The title may give the impression of a 1950's sci-fi alien invasion movie, but it's actually a meteor shower that occurs every year in the middle of August. A meteor, or "shooting star", is a bright streak of light in the night sky that results from a bit of space debris (a meteoroid) hitting the Earth's atmosphere and burning up from the kinetic energy of that collision. During the course of any clear night, you might be able to see one to a few random meteors. To qualify for a meteor shower, there needs to be many meteors that seem to originate from the same part of the sky with the same speed and occur at the same time each year. Their names come from the constellation from which the meteors appear to originate (such as Leonids, Orionids, Geminids, etc). However, the term "shower" doesn't mean that the sky will be lit up with meteors. Some of the weak annual showers only produce 5 to 10 meteors per hour. The Perseids are one of the more active ones, producing 50 to 80 meteors per hour at its peak during the nights of August 11th and 12th with a relative speed to the Earth of 50 kilometers per second.

The cause of the Perseids, like most meteor showers, is a comet. In this case, it is comet Swift-Tuttle, which was discovered on July 16, 1862, by Lewis Swift in Marathon, NY, and then independently by Horace Tuttle three days later at the Harvard College Observatory in Cambridge, MA. The 130-year orbit of this comet brings it just inside the orbit of the Earth at its closest point to the sun (perihelion), to well outside Pluto's orbit at its farthest (aphelion). Also, the tilt of its orbit is nearly perpendicular to the plane of the solar system. It's actually a little beyond perpendicular, thus giving it a retrograde orbit. When it visits Earth's neighborhood, it makes a looping path above the sun and across our northern sky. As it travels, it sheds bits of rock and dust which form a ring of debris along its entire orbit. When Earth passes through this debris, we get our meteor shower. Three years after this comet's discovery, it was the Italian astronomer Giovanni Schiaparelli who determined that Swift-Tuttle was the Perseids' progenitor by discovering that the orbit of the comet and the orbits of the meteoroids that cause the shower are the same. The radiant, or the spot in the sky where the meteors appear to originate, is in the northern constellation of Persus. When we look toward Perseus at this time of year, we are gazing down the path of Swift-Tuttle's orbit and will see the meteors diverging across the sky from this point. The above image simulates the area of the Perseid's radiant and the number of meteors over a 10 minute period.

It takes a full month, from July 23 to August 22, for the Earth to fully pass through the particle stream left behind by Swift-Tuttle. Since we are traveling about 30 kilometers per second around the sun, that means that the stream is about 72 million kilometers thick. At the very beginning or end of this period, we may expect to see 1 to 2 meteors per hour. But during the mornings of August 12 and 13th, the particle stream is at its thickets, and we'll see its peak rate of 50-80 per hour. Even then, the material in the stream is very sparse. The particles range form very small dust grains to pea-sized pebbles with about 150 kilometers separating each one.

Though you can certainly view the Perseids from anywhere, you'll do better by giving yourself a dark sky by getting as far away from city lights as possible. Fortunately, this year the moon will set shortly after sunset, so its glow will not hinder you from seeing the fainter meteors. Using your naked eye is best since the meteors will be traveling quickly and cover a large part of the sky. A telescope or binoculars would only be useful in observing the ionized vapor trails lingering behind by the brighter meteors. Furthermore, it's not necessary to know exactly where the radiant is. Just looking high in the northeast sky is sufficient. And even though the meteors will be visible at any time of night, you'll see more of them after midnight when you'll be riding the side of the Earth that'll be facing its direction of motion as well as allow full view of the radiant.

So, either take a moment to look up on August 12th, or park yourself in a lawn chair for some extended viewing of one of the year's best meteor showers.