Sky darkness

How dark is your sky, and what does that mean for what you can see? Type any one measure and the others follow:

Bortle class — John Bortle’s nine-step scale (Sky & Telescope, February 2001), from 1 (an excellent dark site) to 9 (an inner-city sky). Choosing a class uses the middle of its brightness range.
Sky brightness in magnitudes per square arcsecond at the zenith — what a Unihedron Sky Quality Meter (SQM) reads, and what light-pollution maps give. Bigger is darker; the natural moonless sky is about 22.0.
Naked-eye limiting magnitude (NELM) — the faintest star a typical observer sees overhead.
Artificial ÷ natural — how much light pollution adds, as a multiple of the natural sky (the ratio used by the World Atlas of artificial night sky brightness, Falchi et al. 2016). 0.33 already brightens the sky by a third; a city centre is 20 to 100 times natural.
Luminance in microcandela per square metre (171 µcd/m² is the natural sky).

How to use it

Pick a class on the coloured scale or type an SQM reading (from a meter, an app, or lightpollutionmap.info). What class N shows lists what you can expect to see — the Milky Way’s structure, the zodiacal light, the Triangulum Galaxy (M33) and Andromeda (M31) — after Bortle’s descriptions; tap a row of the table to switch class.

What the sky looks like draws the stars that would be visible at that darkness, in a field you choose (Orion, the Summer Triangle, Sagittarius, the Plough, the Southern Cross, Andromeda), or Overhead — the whole sky above the place chosen below, now, with the limiting magnitude falling and the sky glow rising toward the horizon. The Milky Way fades out in stages as the sky brightens. A screen can’t reproduce real darkness; use it to compare classes.

Tonight, with the Moon takes your sky’s moonless zenith brightness from above and the place (Where — shared with the site’s other astronomy tools) and a time (blank means the middle of tonight’s dark hours) and gives the sky brightness and limiting magnitude at altitudes from the zenith down to 10°, looking away from the Moon and toward it, and a curve of the zenith limiting magnitude through the night with the Moon-up hours shaded. Air clarity is the extinction coefficient: how many magnitudes a star loses per airmass.

For a telescope’s limiting magnitude at a given aperture, see telescope optics; for a chart of what is up, sky tonight; for twilight and moon times, sun and moon; for moonless nights around meteor showers, eclipses and meteor showers; and for what the planets are doing, planet events.

How it is calculated

SQM ↔ NELM: NELM = 7.93 − 5 log₁₀(10^(4.316 − SQM/5) + 1), a widely used fit to Bradley Schaefer’s model of visual thresholds. It describes a typical observer; Bortle’s own NELM ranges are about a magnitude more optimistic (they assume an experienced, fully dark-adapted observer), so both are shown.
Bortle ↔ SQM: the SQM ranges commonly paired with Bortle’s classes (they are not part of his definition, which is descriptive): 1 ≥ 21.99, 2 21.89–21.99, 3 21.69–21.89, 4 20.49–21.69, 5 19.50–20.49, 6 18.94–19.50, 7 18.38–18.94, 8 17.80–18.38, 9 below that. Treat the class as a guide.
Luminance: L = 10.8 × 10⁴ × 10^(−0.4 × SQM) cd/m².
The Moon: Krisciunas & Schaefer, "A model of the brightness of moonlight", PASP 103, 1033 (1991): the Moon’s brightness for its phase, Rayleigh and Mie scattering at the angle between the Moon and where you look, and extinction along both paths. Positions from the site’s astronomy core.
Toward the horizon: the dark sky brightens as airmass grows (less the light the extra air absorbs), and stars lose k magnitudes per extra airmass.

The model ignores twilight (it says so when the Sun is above −18°), airglow changes over the solar cycle, the Milky Way’s own light, and light domes in particular directions — a real sky is usually brighter toward the nearest town.

Data

Stars: the Yale Bright Star Catalogue (public domain) to magnitude 6.5 — so at the very darkest settings the simulation runs out of stars before your eye would. Milky Way outline: d3-celestial by Olaf Frohn (BSD licence).

Privacy

Everything is calculated in your browser. If you choose Here your position stays in this browser and is never put in a share link.