Perseid Meteor Shower

Comet dust at 59 kilometres per second

Every year, from about mid-July to late August, Earth passes through a stream of dust left behind by comet Swift–Tuttle. The grains hit the atmosphere and burn up as meteors, often called shooting stars. The shower usually peaks around 12 to 13 August, when a dark, clear sky can show dozens of meteors an hour. This meadow sky is calculated for the date, hour and latitude you choose, so the stars and the shower behave as they really would.

Why a grain of dust glows

Perseid meteoroids meet the atmosphere at about 59 km/s, more than 150 times faster than sound in air at ground level. At that speed the air in front of each grain is compressed and heated violently, and the grain’s surface boils away, a process called ablation. Vaporised atoms from the grain and from the air glow as they lose their energy, making a brief streak of light somewhere between about 120 and 80 kilometres up. Most Perseids last well under a second; the brightest can leave a glowing trail, or train, that lingers for a few seconds.

The radiant: a trick of perspective

All the grains in the stream travel in parallel. Seen from the ground, parallel paths seem to spread out from a single point, just as parallel railway lines seem to meet at the horizon. That point is the radiant, and for this shower it lies in the constellation Perseus, which gives the Perseids their name. Meteors can appear anywhere in the sky, but trace any streak backwards and it points to the radiant. Tap a meteor as it streaks to see this for yourself.

How many will you see?

Astronomers describe a shower by its zenithal hourly rate, the number one observer would see in an hour under a perfectly dark sky with the radiant overhead; for the Perseids it is around 100 at the peak. Real counts are lower. When the radiant is low, the stream hits the atmosphere at a slant and fewer meteors reach your patch of sky. Light pollution hides faint meteors, and because faint meteors far outnumber bright ones, a bright city sky can hide most of the shower.

Watching the real thing

You don’t need a telescope or binoculars. Find a dark place, lie back, give your eyes 20 minutes or more to adapt, and look about halfway up the sky, a little away from the radiant.

For the curious

The rate you can expect is roughly HR ≈ ZHR · sin(h) · r^(m−6.5), where h is the radiant’s altitude, m the faintest star you can see and r the population index (about 2.2 for the Perseids). The scene computes the radiant’s altitude from sidereal time and your latitude.