Perseid meteor shower at a glance
| Detail | Value |
|---|---|
| Activity window | about July 17 to August 24 |
| Peak | around August 12–13 |
| Zenithal hourly rate (ZHR) | about 100 |
| Radiant | Perseus |
| Parent body | comet 109P/Swift–Tuttle |
| Entry speed | about 59 km/s |
These dates follow the meteor shower calendar of the International Meteor Organization (IMO). The exact peak shifts by a few hours to about a day from year to year, so check the meteor shower calendar for current dates.
Where the Perseids come from
Each August, Earth passes through a trail of dust and small grains shed by comet 109P/Swift–Tuttle. The comet orbits the Sun about once every 133 years and last passed closest to the Sun in 1992. Over many centuries its debris has spread along the whole orbit, which is why the Perseids return every year rather than only when the comet is nearby.
When a grain no bigger than a sand particle hits the upper atmosphere at about 59 km per second, it heats the air around it and makes a brief streak of light. That streak is the meteor.
What the ZHR really means
A ZHR of about 100 is the number of meteors a single observer would see in an hour under ideal conditions: a perfectly dark sky and the radiant straight overhead. Real conditions are almost never that good. From a suburban site, or with the radiant lower in the sky, you might see a few dozen Perseids an hour at the peak. From a truly dark rural site, 50 to 80 an hour is a good night.
The shower builds slowly from late July and fades quickly after the peak, so the nights just before the maximum are often worth watching too.
Where to look
The radiant is the point the meteors appear to come from. For the Perseids it lies in the constellation Perseus, below the W shape of Cassiopeia. You do not need to stare at the radiant itself. Meteors can appear anywhere, and those far from the radiant tend to have longer trails. Looking about 45 degrees away from the radiant, and fairly high in the sky, is a good choice.
To check whether a meteor is a Perseid, trace its path backward. If it points back to Perseus, it belongs to the shower. Others are sporadic meteors or members of minor showers active at the same time.
How the Moon affects the Perseids
Moonlight is the biggest factor you cannot control. A bright gibbous or full Moon near the peak can wash out all but the brightest meteors and cut the visible rate by half or more. A crescent Moon or new Moon around mid-August gives the best show. Check the moon phases and moonrise and moonset times for your location; sometimes the Moon sets early enough to leave a dark window before dawn.
Viewing tips
- Get away from lights. Even a short drive out of town makes a big difference.
- Give your eyes time. Allow 20 to 30 minutes for full dark adaptation and avoid looking at your phone. If you need light, use a dim red one.
- Lie back. A reclining chair or a blanket lets you take in a wide area of sky without neck strain.
- No equipment needed. Binoculars and telescopes show too little sky. Your eyes are the best instrument.
- Be patient. Meteors come in bursts and lulls. Watch for at least an hour.
- Dress for the night. August evenings can still turn cool and damp after midnight.
For a quick overview of what else is above the horizon, see the sky tonight.