The Perseid meteor shower

Amateur astronomer and author Paul Sutherland reveals everything you need to know about summer’s most famous display of shooting stars, the Perseid meteor shower.

What are the Perseids?

The Perseid meteors (Perseids) are a display of shooting stars, or meteor shower, that is particularly strong for northern hemisphere observers during the second week of August. The hourly rate, quoted for a single observer with ideal sky conditions, can vary between 80 and 120 meteors an hour.

This means that with a reasonably good, dark sky, you will be unlucky not to see a few Perseids if you are willing to spend several minutes patiently watching for them.

A bright Perseid flares as it speeds away from the Andromeda Galaxy, M31. Image credit: Paul Sutherland

The meteors can be spectacular, but are actually just small particles of dust, like grains of instant coffee, that flare brilliantly as they enter the Earth’s atmosphere.

The shower is so-called because the Perseid meteors radiate from a point in the constellation of Perseus. However, they can appear in any part of the sky. You can tell if a meteor you see is a Perseid by tracing its path backwards to see if it points to the shower’s radiant in Perseus.

Where do the Perseid meteors come from?

Before they flare up and become ionized in the atmosphere, the Perseid dust grains are known as meteoroids. They are spread in a vast, elliptical stream around the Sun, having been shed by a comet over thousands of years. When they collide with the Earth, they enter the atmosphere at speeds of about 60 km per second (40 miles per second) and blaze fleetingly as “shooting stars”. Some meteors leave a briefly visible glow called a “train” after the initial bright streak.

The comet that gave birth to the Perseid meteor shower is known as 109P/Swift-Tuttle. This comet was discovered separately by astronomers Lewis Swift, of Marathon, New York, and Horace Parnell Tuttle, of Cambridge, Massachusetts, in July 1862. It was independently spotted by several other observers too shortly afterwards, but Swift and Tuttle got the credit! Read more about the comet here.

Meteors can appear anywhere in the sky but shower members can be traced back to a radiant, in this case that for the Perseids. (You will not see several bright meteors at once like this. They are shown together to illustrate how they all radiate from one direction.)

Comet Swift-Tuttle orbits the Sun every 130 years or so, and it was seen again when it returned in 1992, which coincided with some particularly strong displays of the Perseids. The shower may be seen every year though, because the dust has been spread wide around the whole of the comet’s orbit.

Displays of the Perseid meteor shower have been seen throughout history, long before the comet was first discovered, and long before anyone thought to connect meteors with comets. (Both were once thought to be atmospheric phenomena.) The Chinese recorded seeing these meteors in the first century AD, and they were subsequently noted by Korean and Japanese observers too.) To some in the West, they became known as the Tears of St Lawrence.

When can I see the Perseid meteor shower?

The shower reaches a peak on the 12th or 13th of August. But meteors may be seen from late July to the third week of September, with the number visible gradually rising and then tailing off again. The number you will see varies from year to year and is impossible to predict with great accuracy. Astronomers describe the number by means of the ZHR, or Zenithal Hourly Rate. This calculates a theoretical figure that might be seen by a single observer with ideal dark sky conditions and with the radiant overhead.

Some meteors may be seen, which are not from the Perseid stream, throughout the summer, and indeed all year long. These come either from other, lesser showers, or are random particles unattached to any known shower, known as sporadics. However, the number of meteors on view increases notably around the peak period of the Perseids.

Ignore some dubious articles found on the web that promise the brightest meteor shower ever seen in history! These headlines are clickbait which will only lead to disappointment.

A bright Perseid meteor photographed in 2016. Image credit: Paul Sutherland

What is the best way to observe the Perseids?

You do not need a telescope or any other optical aid to see meteors. The only equipment that does help is some warm clothing, because it can get chilly even on summer nights, plus a comfortable chair, such as a deckchair or sun lounger, so you can sit in comfort while gazing at the heavens. You will also need some patience, as you may have to wait several minuted before one appears.

The meteors appear at random, and not at equally spaced intervals, so having seen one, you could see another one or two in quick succession and then face another long wait. Read our article with more advice on how to observe a meteor shower. You can also find some tips on how to photograph a meteor shower.

Related: What’s in the night sky this month?

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