Observing · August 12, 2026

The Bortle Scale: How Many Stars Does Your Sky Actually Let You See?

The Bortle scale numbers skies from 1 to 9, from a pristine site to the middle of a metropolis. It is everywhere, and almost always quoted empty: you are told you are « in class 6 » without being told what class 6 takes away from the sky. It takes away specific, computable things. At Châtelet in central Paris, our atlas gives a sky background brightness of 17.6 mag/arcsec², a naked-eye limiting magnitude of 3.6 and 160 stars visible across the whole sky. On the Millevaches plateau, in central France: 21.9 mag/arcsec², limiting magnitude 6.6, 4,900 stars. Thirty times more, for three magnitudes gained and four hours of driving.

How is the light pollution of a place measured?

The chain starts with satellite measurements: NOAA’s VIIRS radiometer photographs the Earth at night, and David J. Lorenz derives from it a worldwide Light Pollution Atlas, freely published, giving the artificial brightness of the sky at the zenith. We resample it to 0.1°, a cell of about 11 km: the whole grid fits in 342 kB and travels with the app.

From there, four conversions. The atlas zone rank gives the light pollution index, the ratio of artificial to natural luminance, through LPI = 3^((r−1)/2 − 2.75): going up one zone means tripling the stray light. Sky background brightness follows in magnitudes, SQM = 22.0 − 2.5·log₁₀(1 + LPI), with 22.0 the value of a natural sky. The naked-eye limiting magnitude follows an empirical relation from Schaefer. The number of stars, finally, is a real count, interpolated on the cumulative function of the HYG v4.1 catalogue and then halved, since you only ever see one hemisphere at a time.

The drop is brutal in the middle of the scale: between class 4 and class 7 you lose 3,300 stars, 85% of the sky, in three steps. The top of the scale is almost flat, on the other hand: from class 1 to class 2 the limiting magnitude moves by only 0.03. What sets a class 1 sky apart is therefore not the number of stars, but the structure of the Milky Way, the zodiacal light and the atmosphere’s own airglow, none of which the count measures.

What is the Bortle class of your town?

Every row in the table below is one call to the engine, with the coordinates of the place.

Two results are surprising. The Pic du Midi, at 2,877 m on a ridge, comes out in class 3, the same as the Causse Méjean which tops out at 1,000 m: altitude does not protect you from light pollution, only distance from cities does, and the Pic du Midi looks down on the lights of the Tarbes plain. The Roque de los Muchachos, on La Palma, one of the best protected sites in the world, comes out at 21.81: that is the price of an 11 km cell on an island less than 30 km across, whose computation takes in the towns along the coast.

How far do you have to drive to change class?

We computed a transect due south from Châtelet, every ten kilometres. At 10 km you are already in class 8. At 20 km, class 7. At 40 km, class 6. At 50 km, class 5, and the limiting magnitude has gone from 3.6 to 6.0, that is from 160 to 2,700 stars in fifty kilometres. It then takes 140 km to reach class 3, and the gain drops off sharply: from 50 to 140 km you only gain another 0.44 magnitude.

The curve is not monotonic, either. At 190 and 200 km it climbs back to class 5 then 6: that is the halo of Bourges, crossed head-on. An observer’s strategy is therefore not to drive far, it is to drive between towns.

How can you see more stars without moving?

The atlas ignores two things that do not depend on where you are. First, dark adaptation: the pupil opens in a minute, but regenerating the pigment in the rods takes twenty to forty minutes, and a single glance at a white screen resets it to zero. An adapted observer gains a good magnitude over someone who has just got out of their car, the equivalent of two Bortle classes. Second, your position in the landscape: our 11 km cell does not know that a wall, a hedge or the side of a valley is hiding a town’s halo from you. A street light in your field of view costs more than one Bortle step, and sometimes walking a hundred metres is enough to get rid of it. That is why we display the computed class and let the user correct it: out in the field, they are the one who is right.

What does a Bortle class not tell you?

Plenty of sites present this figure as a field measurement. It is not one.

Five limits worth knowing

It is not a measurement on site, it is a conversion. Zenith brightness is measured by satellite and then modelled; the Bortle scale itself is subjective and covers the whole sky, horizon glows included. Lorenz insists that these are two distinct metrics: we give an equivalence, not a reading.

Our 11 km smooths the extremes. In the heart of a large city we slightly underestimate the pollution; deep in the countryside five kilometres from a village, we overestimate it. La Palma is the clearest illustration.

The scale has been recalibrated, and that recalibration is a choice. Lorenz observes, from National Park Service readings, that the « yellow » read as Bortle 4 is more often Bortle 5, and the « orange » read as 5 is more often 6. We apply that shift to the rural and suburban part of the scale; another author could legitimately decline to, and would then quote you one class lower.

The limiting magnitude describes an average observer. The 7.6 to 8.0 sometimes claimed in class 1 assume exceptional eyesight and forty minutes of dark adaptation without a single screen.

None of this is weather. Transparency, humidity, aerosols, the Moon, the altitude of your target and the mere presence of a street light in your field of view weigh, on some nights, more heavily than a Bortle step.

This article was translated from the French edition by AI, then reviewed before publication. The figures are not translated: sky brightness, limiting magnitudes and star counts are computed by SkyChart and are identical in both editions.

• Bortle scale
• Light pollution