What it does
A shower’s headline number, its zenithal hourly rate (ZHR), is what one observer would see under a perfect sky with the radiant straight overhead. Real nights are never that kind. This planner corrects the ZHR for your place and your sky, hour by hour through the night, so you know when to go out and what to expect. It also shows where the Moon is, the moonless dark hours on the nights either side of the peak, and lists every major shower of the coming year with its best rate from where you are.
How to use it
- Set your place at the top. It is shared with the other astronomy tools.
- Pick a shower. The planner starts on the next one to come.
- Choose your sky: the faintest stars you can see overhead. A dark country sky shows stars to about magnitude 6.5, suburbs about 5, a city about 3.
- Read the result: the best time and rate, a chart through the night, an hour-by-hour table with the radiant’s height and the Moon, and the moonless dark hours on the five nights around the peak.
How it works
For each half hour from sunset to sunrise, the planner finds the radiant’s altitude h and uses the International Meteor Organization’s formula: HR = ZHR × sin(h) / r^(6.5 − LM), where LM is your limiting magnitude and r the shower’s population index, which says how many more faint meteors there are than bright ones. Rates count only once the Sun is 12° below the horizon. Peak dates, ZHRs and population indices are typical values from the IMO’s calendar; the exact peak moves by up to a day from year to year.
Worked example
The Perseids have a ZHR of about 100 and a population index of 2.2. With the radiant 60° up under a rural sky (limiting magnitude 6.0), the rate is 100 × sin 60° / 2.2^0.5 ≈ 58 meteors an hour. From a suburb (5.0) the same hour gives about 27, and from a city (3.0) about 5. That is why getting away from lights matters more than anything else. The radiant climbs all night, so the best hours are usually just before dawn.
Frequently asked questions
Does it count the Moon? It shows whether the Moon is up and how much of it is lit, but the rates assume a moonless sky. A bright Moon can easily halve what you see; plan for the hours after moonset.
Why can I see meteors before the radiant rises? A few “earthgrazers” skim the atmosphere when the radiant is near the horizon. They are rare but long and spectacular.
Is my location sent anywhere? No. Everything is worked out in your browser and works offline.
Last updated 2026-11-08.