ZHR versus what you see
Meteor shower forecasts quote a zenithal hourly rate (ZHR): the number of meteors one observer would see in an hour under a perfect sky, with the radiant directly overhead. Real conditions are always worse, so the count you actually see is usually much lower.
The calculator applies the standard correction used by the International Meteor Organization:
Visible rate = ZHR × sin(radiant height) × (1 − share of sky hidden) ÷ r^(6.5 − limiting magnitude)
- Radiant height. Meteors come from the radiant. When it is low, half of them are below the horizon.
- Limiting magnitude. The faintest star you can see. A dark rural sky reaches 6.5; a suburban sky about 5.5; a city 4 or less.
- Population index r. How quickly the number of meteors grows toward faint ones. Most showers are between 2.0 and 3.0. With r = 2.2, each magnitude of sky brightness you lose cuts the count by more than half.
- Clouds or obstacles. Trees and buildings count as lost sky.
Example
The Perseids have a ZHR around 100 and r = 2.2. With the radiant 50° up and a suburban limiting magnitude of 5.5, you can expect about 35 meteors per hour, roughly one every 1 minute 43 seconds. Under a 6.5 sky you would see about 77.
Main annual showers
| Shower | Peak | Typical ZHR |
|---|---|---|
| Quadrantids | January 3–4 | 80–120 |
| Lyrids | April 22 | 18 |
| Eta Aquariids | May 6 | 50 (better south) |
| Perseids | August 12–13 | 100 |
| Orionids | October 21 | 20 |
| Leonids | November 17 | 15 |
| Geminids | December 13–14 | 150 |
Tips for more meteors
- Watch after midnight, when the radiant is higher and your side of Earth faces forward along its orbit.
- Check the moon phase: a bright moon can steal two magnitudes of sky.
- Give your eyes 20 minutes to adapt, avoid phone screens, and look about 45° up and away from the radiant.
- No telescope is needed: a reclining chair and a wide view of the sky work best.