Zenithal hourly rate

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In astronomy, the zenithal hourly rate (ZHR) of a meteor shower is the number of meteors a single observer would see in an hour of peak activity if the radiant was at the zenith, assuming the seeing conditions are perfect^[1] (when and where stars with apparent magnitudes up to 6.5 are visible to the naked eye^[2]). The rate that can effectively be seen is nearly always lower and decreases the closer the radiant is to the horizon.