Meteor Shower Calculator

ZHR assumes an idealized observer under a perfectly dark sky with the radiant directly overhead, conditions almost nobody actually observes under. Radiant altitude below 90°, a limiting magnitude below the darkest-sky value of 6.5, and any light pollution above pristine Bortle 1 skies all reduce the visible rate multiplicatively, so a shower with a ZHR of 100 might realistically show only a fraction of that from a typical suburban backyard.

Meteors from a low-altitude radiant travel through more atmosphere and appear foreshortened near the horizon, so fewer of them are visible or bright enough to notice compared to when the radiant sits high overhead. The correction here uses the sine of radiant altitude, which falls off steeply as the radiant approaches the horizon — this is why meteor showers are typically best watched after their radiant has climbed well above the horizon, often in the pre-dawn hours.

Limiting magnitude is a direct, personal measurement of the faintest star you can actually see that night, which can vary with your eyes' dark adaptation and momentary conditions, while Bortle class is a standardized 1-9 scale describing your location's typical sky darkness independent of any single night. Using both together lets the calculator account for both your site's baseline light pollution and how well your eyes have adapted on a given night.

Even a fully attentive observer under ideal conditions doesn't catch every single meteor that crosses their field of view — some appear at the edge of peripheral vision, some happen during a blink, and sustained attention naturally lapses over an observing session. The 70% efficiency factor is a rough real-world adjustment on top of the physically corrected expected rate, acknowledging that human observation is imperfect even when everything else is accounted for.