Limiting magnitude
The brightness of the faintest stars you can see, which shows how dark and clear your sky is.
The limiting magnitude is the faintest star visible in a given patch of sky, either with the naked eye or through an instrument. Under a pristine, moonless sky with fully dark-adapted eyes, many observers reach about magnitude 6.5, sometimes better. In a suburb the limit may be 4 or 5, and in a city centre only 2 or 3, when just a few dozen stars are visible.
For meteor observers this number is essential. Most meteors are faint, so each step brighter in your limiting magnitude means many fewer meteors seen. That is why reported counts are corrected to a standard limit of 6.5 when calculating the zenithal hourly rate. Without an estimate of sky quality, a count of meteors is hard to interpret.
A common way to measure it is to choose a well-known triangle or other area of stars, count the stars you can see inside it and compare with a chart; alternatively, identify the faintest star you can hold steadily and look up its magnitude. Repeat at intervals, because haze, the Moon rising or your eyes tiring can change it. Telescopes and binoculars have their own, much fainter limits, depending on aperture.
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