The sky — astronomy

The Perseids: The August Meteor Shower

The Perseids are an annual meteor shower active from mid-July to late August, usually peaking around 12–13 August. They are best watched after midnight from a dark site, away from city lights and, ideally, when the Moon is not bright. No equipment is needed.

What the Perseids are

The Perseids are a meteor shower that returns every summer. Each “shooting star” is the glowing trail of a tiny dust grain, often no bigger than a grain of sand, that hits Earth’s atmosphere at high speed and heats the air around it until it shines. The light appears roughly 100 kilometres above the ground. For the general picture, see our article on meteor showers.

The shower is one of the most popular in the Northern Hemisphere, largely because it peaks in warm summer nights, when many people are outdoors and skies are often clear.

Comet Swift–Tuttle

The source of the Perseids is comet 109P/Swift–Tuttle, discovered in 1862 independently by the American observers Lewis Swift and Horace Tuttle. A few years later the Italian astronomer Giovanni Schiaparelli showed that the comet’s orbit matches that of the shower’s particles, one of the first clear links between a comet and a meteor shower.

The comet orbits the Sun about every 133 years. Its last closest approach to the Sun was in 1992, and the next is expected in 2126. Each time, a comet leaves a stream of fine debris along its path. Every August Earth crosses that stream, and the particles enter our atmosphere at about 59 kilometres per second. The Perseids you see are leftovers from the comet, not the comet itself.

The radiant in Perseus

Perseid meteors seem to fan out from one patch of sky called the radiant (the point from which the meteors appear to come). It lies in the constellation Perseus, which gives the shower its name. This is an effect of perspective, like railway tracks that seem to meet at the horizon: the particles travel along nearly parallel paths.

Perseus sits in the northern sky, near Cassiopeia. In the late evening it is low in the north-east and climbs higher as the night goes on. That is why more meteors are seen after midnight: the radiant is higher, and Earth’s motion around the Sun turns the observer toward the incoming particles.

You do not need to look straight at the radiant. Meteors appear all over the sky, and the longest trails are often seen 30–40 degrees away from it. Trace one backwards and it will lead you to Perseus.

When is the peak?

According to the International Meteor Organization (IMO), the shower is active from about mid-July to late August, with the maximum usually around 12–13 August. The exact peak time shifts slightly from year to year, so check a current calendar from the IMO or another scientific organisation.

Under ideal conditions (very dark sky, radiant high up) an observer could see up to about 100 meteors per hour. This figure is called the ZHR (zenithal hourly rate, a theoretical count for perfect skies). In practice you will see fewer: light pollution, clouds, a low radiant and moonlight all reduce the count. A few dozen per hour at a good site is a realistic expectation, and far fewer in a city.

Meteors also appear on the nights before and after the peak, so a cloudy night need not spoil the plan. Try the night before or after.

How the Moon affects visibility

A bright Moon lights up the sky and hides the fainter meteors. Years when the peak falls near new Moon or a thin crescent are the most favourable. If the Moon is close to full, you will mostly see the brighter meteors only.

Check the Moon phase for your chosen night. If the Moon is bright, keep it behind a building, tree or hill, or face away from it. Sometimes the Moon rises late and the first part of the night stays dark. You can read more about the Moon in its own article.

Practical tips

• Choose a dark place. Light pollution makes the biggest difference. Head out of town to a field, a mountain or a village with an open horizon.
• Let your eyes adapt. Dark adaptation takes about 20–30 minutes. Avoid your phone screen, or set it to the dimmest red light.
• Skip the equipment. Binoculars and telescopes narrow your field of view. The naked eye and a wide view of the sky work best.
• Lie back comfortably. A reclining chair, mat or sleeping bag lets you look up for a long time without straining your neck.
• Dress warmly. Even after a hot day, nights can be cool, especially in the mountains.
• Be patient. Meteors come in bursts: sometimes several in a minute, sometimes a gap of ten minutes. Plan on at least an hour or two.
• Watch after midnight. The best hours are usually between midnight and dawn.

General night-sky advice is in How to watch the sky. If the Perseids win you over, another popular shower awaits in December: the Geminids.

In short

The Perseids are debris from comet Swift–Tuttle, with a radiant in Perseus. They peak around 12–13 August, and conditions are best after midnight, under a dark sky and without a bright Moon. You need no equipment, only patience, warm clothes and a wide view of the sky.

Positions in Astronativa are astronomically exact. The interpretation is for inspiration. · Glossary · Terms · Privacy