The Perseids’ radiant sits roughly two-thirds of the way on a line drawn from Polaris — the North Star, located at the end of the Little Dipper — to Algol, the second-brightest star in Perseus. Closer to the radiant are 4th-magnitude Miram (Gamma [γ] Persei), which sits 2.5° southwest of the radiant, and 5th-magnitude Lambda (λ) Persei, just 0.7° south of the radiant. In the hour before dawn on the 12th and 13th, the radiant will be 60° high in the northeastern sky.
What to expect
The strength of a meteor shower is given by the zenithal hourly rate, or ZHR, at the shower’s peak. ZHR is a measure of the number of meteors an observer can expect to see when the radiant is at the zenith, or 90° high in the sky. This year, the Perseids’ expected ZHR at peak is 110 meteors per hour. Because the radiant won’t reach the zenith, however, most North American observers can expect to see a lower rate of about 60–90 meteors per hour, or roughly one meteor per minute, before dawn. Note, though, that a rate of 60 meteors per hour doesn’t mean you’ll see exactly one meteor each minute. Meteors are sporadic and unpredictable; you might see several over the course of just a few minutes, then none at all for the next short while.
If you prefer to watch at night, wait until moonset (around 10:30 P.M. local time on the 12th and 11 P.M. local time on the 13th) for the darkest skies. Note that at this time, Perseus has only recently climbed above the horizon and the shower’s radiant is only 20° high, cutting the number of meteors you’ll see each hour to 20 or so. The longer you stay out overnight, the higher the constellation will rise and the more meteors you’ll catch.
What’s happening
Perseid meteors are generated by debris left by Comet 109P/Swift-Tuttle, which swings close to the Sun every 133 years. Its last closest approach to our star was in 1992 and its next will be in about 100 years, in 2125. The comet was discovered in 1862 and linked shortly after to the Perseids in 1865. The shower itself has been popularly known since the 1830s, but likely appeared annually long before that date as well.
Because comets have well-defined orbits around the Sun, the debris they shed tends to stay on that orbit. When Earth’s orbit intersects the path, we see a meteor shower. As dust from Swift-Tuttle hits Earth’s atmosphere at speeds up to 133,000 mph (214,000 km/h), it burns up in a brilliant flare of light — a Perseid meteor. Most meteors burn up high in the atmosphere, more than 50 miles (80 kilometers) in altitude. None of the particles that generate meteor showers are large enough to survive their trip, so meteor showers do not produce meteorites, which are space rocks that make it through the atmosphere to hit the ground.
