★ Perseid Meteor Shower

The Perseids are the best-known meteor shower of the year, and for many people the easiest to enjoy: they peak on warm August nights, they produce plenty of meteors, and a good share of those are bright. Under a dark sky you can see dozens an hour on the peak night, and some meteors leave glowing trails that hang in the sky for a second or two.

Where the meteors come from

The Perseids are dust from Comet 109P/Swift-Tuttle, a large comet that loops around the Sun every 133 years or so. Each pass leaves a fresh trail of grains along the comet's orbit, and Earth crosses that stream every August. The grains, mostly the size of sand or small pebbles, hit the atmosphere at about 59 km/s (over 210,000 km/h) and burn up roughly 80 to 120 km above the ground. The comet last came near the Sun in 1992 and is next due in 2126.

When to watch

The shower is active from mid-July to late August, but rates are low until early August. The peak comes around August 12–13, and the nights of August 11–12 and 12–13 are usually the best. The table above gives the expected peak time for the next two years and how much moonlight to expect. From mid-northern latitudes the radiant never sets, but it is low in the north early in the evening. Meteors pick up once it has climbed higher in the late evening, and numbers keep rising through the night: the hours before dawn are best, when Perseus is high in the northeast.

How to find the radiant

The radiant lies at the northern edge of Perseus, where it meets Cassiopeia and Camelopardalis, between Cassiopeia's "W" and the bright star Capella. Find the W of Cassiopeia in the northeast after dark, then look about a fist's width below it: the radiant is not far from the star cluster known as the Double Cluster. You do not need to look straight at it. Perseids can appear anywhere in the sky, and the longest, most impressive ones are often 40 to 90 degrees away from the radiant. Trace a meteor's path backwards: if it leads to Perseus, it is a Perseid.

What to expect

The peak rate (ZHR) is about 100 meteors an hour, but that assumes a perfectly dark sky with the radiant overhead. From a dark rural site, 50 to 80 an hour before dawn on the peak night is a good result; from the suburbs, expect 15 to 30. Perseids are fast and often bright, with a higher than average share of fireballs (meteors brighter than Venus) and lingering trains. A few days before or after the peak you may still see 10 to 20 an hour from a dark site.

Viewing tips

• Get away from city lights. Even a short drive to a darker place can double what you see.
• Lie back on a reclining chair or a blanket and look about halfway up the sky, toward whichever part is darkest.
• Give your eyes 20 to 30 minutes to adapt to the dark, and keep phone screens dim or use a red light.
• Plan to watch for at least an hour. Meteors come in bursts, with quiet spells in between.
• No telescope or binoculars needed: they show too little sky. Your eyes are the best instrument.
• For photos, use a wide-angle lens on a tripod and take long series of 15–30 second exposures.

History and notable displays

People have watched the August meteors for about 2,000 years; the oldest known record, from China, dates from AD 36. In Europe they were nicknamed the "Tears of St. Lawrence", after the saint's feast day on August 10. In 1866 the Italian astronomer Giovanni Schiaparelli showed that the Perseids follow the same orbit as Comet Swift-Tuttle, the first link ever made between a meteor shower and a comet. Around the comet's return in 1992 the shower produced short, strong outbursts above its usual peak, and it stayed strong through the 1990s. Further outbursts, when Earth meets a denser strand of the stream, have happened since, most recently in 2016.