How these forecasts are made
Every number on this site is computed, not copied. The same deterministic engine powers the website and our mobile app — here is exactly what goes in and what the numbers promise.
The source data
The eleven major annual showers are seeded from International Meteor Organization (IMO) and IAU Meteor Data Center records: radiant coordinates (RA/Dec), activity windows, peak solar longitude, ZHR, entry velocity and parent body. These are stable, peer-reviewed astronomical facts — they change when science does, and we update the seed accordingly.
The astronomy engine
For every place and date we compute the sky directly with a professional-grade astronomy library (VSOP87-class ephemerides): where the radiant sits hour by hour, when the sun is far enough below the horizon for real darkness, where the moon is and how lit it is. Meteor visibility is fully deterministic from latitude, longitude and date — that is why our forecasts can be computed ahead of time and don’t need to guess.
From ZHR to your sky
ZHR is a laboratory number: one observer, a perfectly dark sky, radiant overhead. We convert it into an honest range for where you stand using the standard observed-rate model — rate × sin(radiant altitude) ÷ r^(6.5 − limiting magnitude) — with the limiting magnitude set by your sky class (city / suburbs / dark) and pulled down further by moonlight. When the number is small, we say so: a forecast that flatters you costs you a cold night.
The weather layer
Cloud cover and precipitation come from Open-Meteo, fetched live in your browser for the night you are looking at. Weather is the one non-deterministic input, so it is layered on top of the astronomy and clearly separated from it — an astronomy verdict never silently hides behind a cloud forecast.
What the numbers promise
A range like “8–18/hr” means: a patient observer, dark-adapted eyes, the stated sky class, watching through the best window. It does not count meteors below your horizon, ones the moon washes out, or the seconds you look at your phone. We would rather under-promise than train you to distrust the forecast.
Reading the numbers
What is ZHR?
Zenithal Hourly Rate — the standard yardstick for a shower's strength. It's how many meteors one observer would count per hour under a perfectly dark sky with the radiant directly overhead. A lab condition, not a promise: your real count is always lower.
Why will I see fewer than the ZHR?
Three honest discounts. The radiant is rarely overhead — at 30° altitude half the meteors never clear your horizon. The sky is rarely perfectly dark — city glow and moonlight wash out the faint majority. And clouds take their cut. That's why we quote a realistic range for your sky instead of the headline number.
What is the radiant?
The point the meteors appear to stream away from — pure perspective, like snowflakes fanning out from ahead when you drive through snow at night. Don't stare at it: trails look longest 40–60° away from it. What matters is its height — the higher the radiant, the more meteors above your horizon.
What does the speed (km/s) change?
The look of the show. Fast showers — the Leonids hit the atmosphere at 71 km/s — draw thin, quick streaks that often leave glowing trains. Slow ones, like the 27 km/s Taurids, roll across the sky as long, bright, sometimes orange fireballs.
Dark window vs. best window
The dark window is pure astronomy: the stretch between dusk and dawn when the sun sits far enough below the horizon. The best window is the practical answer — the hours holding at least ~70% of the night's top rate, when it's genuinely worth standing outside.
What is a parent body?
Every shower is the dust trail of one comet or asteroid. When Earth crosses that trail on its yearly lap, the grains burn up as meteors — the Perseids are crumbs of comet Swift–Tuttle, the Geminids of asteroid 3200 Phaethon.