Meteor Shower Activity Calculator
Estimate how many meteors per hour you will actually see during a meteor shower, accounting for radiant altitude, sky darkness, and moonlight. Use it on the night of a shower to set realistic expectations.
About this calculator
The Zenith Hourly Rate (ZHR) is the theoretical meteor count an ideal observer under perfect skies would see if the radiant were directly overhead. Your actual observed rate (ObsZHR) is always lower and is calculated as: ObsZHR = round(ZHR × sin(h) × 10^((LM − 6.5) × 0.4) × (1 − M / 200)), where h is radiant altitude in degrees, LM is limiting magnitude (sky darkness), and M is moon illumination in percent. The sin(h) term corrects for the radiant not being at the zenith — a radiant at 30° cuts the rate in half. The limiting magnitude factor scales relative to a reference sky of magnitude 6.5; darker skies increase the count exponentially. The moon factor linearly reduces rates as illumination rises, with 100% illumination halving the observable count.
How to use
You are observing the Perseids (ZHR = 100) with the radiant at 50° altitude, a limiting magnitude of 6.0, and 30% moon illumination. Step 1 — altitude term: sin(50°) ≈ 0.766. Step 2 — limiting magnitude term: 10^((6.0 − 6.5) × 0.4) = 10^(−0.2) ≈ 0.631. Step 3 — moon term: 1 − 30/200 = 0.85. Step 4 — multiply: 100 × 0.766 × 0.631 × 0.85 ≈ 41 meteors per hour. So despite a ZHR of 100, real conditions cut your observed rate to about 41 per hour.
Frequently asked questions
What is the difference between ZHR and actual observed meteor rate?
ZHR (Zenith Hourly Rate) is a standardized theoretical figure measured under perfect conditions: the radiant at the zenith, limiting magnitude of 6.5, and no moonlight. In practice, the radiant rises and sets over the course of the night, your local sky may be lighter due to light pollution or haze, and the Moon can wash out faint meteors. The actual observed rate is almost always a fraction of ZHR and can be less than 20% of it under poor conditions. This calculator converts ZHR into a realistic expectation for your specific observing setup.
How does radiant altitude affect the number of meteors I can see?
When the radiant is on the horizon (altitude 0°), you see virtually no meteors because the shower's meteoroids are traveling parallel to Earth's surface — they graze the atmosphere at a shallow angle and most burn up far from your location. As the radiant climbs, the number of meteors entering your sky sector increases proportionally to the sine of the altitude. A radiant at 90° (zenith) gives maximum rates. This is why peak ZHR is usually experienced in the pre-dawn hours when most shower radiants are highest in the sky.
How much does moonlight reduce meteor shower visibility?
The Moon is one of the biggest factors degrading meteor shower observation. Even a 50% illuminated Moon raises the sky background enough to hide meteors fainter than about magnitude 3 or 4, whereas a dark sky reveals meteors down to magnitude 6 or dimmer. Since most shower meteors are faint, moonlight can eliminate 60–80% of what you would otherwise see. The optimal strategy is to observe during the hours between moonset and dawn, or to wait for a year when the shower peak falls near a new Moon.