Light Pollution Calculator

Bortle Class, named for amateur astronomer John E. Bortle who published the scale in Sky & Telescope in 2001, captures the dominant driver of night-sky brightness: how much artificial light is scattering into the atmosphere from nearby towns and cities. Moving from a Bortle 5 suburban sky to a Bortle 2 rural sky changes the naked-eye limiting magnitude by roughly a full magnitude or more. Altitude's contribution is real -- thinner air scatters and absorbs less starlight -- but the calculator adds only about 0.1 magnitude per 1,000 feet, a small effect that this calculator verifies directly against the underlying formula rather than asserting from general astronomy knowledge.

Moonlight doesn't just illuminate the ground -- it scatters through the atmosphere and raises the overall brightness of the night sky, the same physical effect as light pollution from a city. At 100% illumination (full moon), this calculator subtracts 2 magnitudes from your naked-eye limit as a representative estimate -- published figures for full-moon sky-brightening vary by site and conditions, generally falling somewhere in the range of about 1.5 to 3 magnitudes -- meaning you lose the ability to see many of the fainter stars you could otherwise pick out on a moonless night at the same location.

Telescope Limit is calculated for an 8-inch (200mm) aperture telescope, a common size for amateur astronomers, using the standard aperture-gain formula that compares the telescope's light-gathering area to a dark-adapted human eye's roughly 7mm pupil. A larger telescope gathers more light and would push the limiting magnitude fainter than shown here; a smaller telescope would see less far than this estimate.

SQM Reading is the darkness value a handheld Sky Quality Meter would report in magnitudes per square arcsecond -- a direct instrument measurement rather than a calculated star-visibility estimate. This calculator maps each Bortle class to a representative SQM value (roughly 21.99 at Bortle 1 down to 17.0 at Bortle 9); published Bortle-to-SQM correspondence tables vary somewhat from source to source, so treat these as typical rather than an exact one-to-one conversion. The calculator then applies the same moon-phase penalty used for Naked-Eye Limit, since both describe the same underlying sky brightness from different angles.

The altitude bonus is deliberately small -- about 0.1 magnitude per 1,000 feet -- because thinner air at higher elevation helps, but it's a secondary effect compared to how much artificial light is actually present in the sky. Going from sea level to a 1,000-foot site adds only 0.1 magnitude, well within the resolution most observers can even distinguish by eye; you'd need several thousand feet of elevation change to notice a meaningful difference from altitude alone.