This page says where the scale comes from, how to estimate yours, what each class takes away, and what it leaves: the Moon, the planets, double stars.
Where the scale comes from
John E. Bortle, an American observer, published it in Sky & Telescope in February 2001 to give amateurs a common way to describe their sky. It is defined by what you see: the zodiacal light, the Triangulum Galaxy with the naked eye, the structure of the Milky Way, the light domes on the horizon, the faintest star visible.
The nine classes, briefly
- Classes 1 and 2: an excellent sky. The zodiacal light is obvious, the Triangulum Galaxy is visible to the naked eye, the Milky Way almost casts shadows. Faintest naked-eye star, in the site's table: 7.8 and 7.3.
- Classes 3 and 4: the countryside, then the transition to the suburbs. The Milky Way is still rich overhead; light domes rise on the horizon. Magnitudes 6.9 and 6.5.
- Classes 5 and 6: the suburbs. The Milky Way is faint, then visible only overhead; the sky is light grey near the horizon. Magnitudes 6.0 and 5.5.
- Classes 7 and 8: the outer city and the city. The Milky Way is gone; only the bright constellations are recognisable. Magnitudes 5.0 and 4.5.
- Class 9: the centre of a big city. The Moon, the planets and a handful of stars. Magnitude 4.0.
Bortle gives a range of magnitudes for each class; the site keeps one value per class for its arithmetic. Those are the values the simulator's sky slider drives.
What a bright sky costs in aperture
A telescope shows fainter stars than the naked eye, the more so the larger its aperture: about 5 × log₁₀(aperture ÷ 7 mm) magnitudes beyond the naked-eye limit. When the sky costs the naked eye some magnitudes, it costs the telescope the same, and it takes aperture to buy them back.
- Class 5: about 1.26 times the aperture, so 164 mm to do what a 130 mm does at class 4.
- Class 6: about 1.58 times, so 206 mm.
- Class 7: twice, so 260 mm.
- Class 8: about 2.5 times, so 330 mm.
This model counts stars. For diffuse objects, galaxies and nebulae, Bortle notes that light pollution degrades them far more than stars; no source puts a number on the difference, and the site does not invent one. The real gap is therefore, if anything, worse than this table.
What the city does not take
Light pollution does almost nothing to the Moon, the planets, double stars or bright clusters: they are far brighter than the sky background. Saturn's ring looks the same from a city-centre balcony as from a mountain top. What a bright sky takes away is faint extended things, and it takes those almost completely.
Estimating your sky
- By eye: give your eyes twenty minutes to adapt, away from lights, and see whether the Milky Way shows, and where. It is the simplest test on the scale.
- With a chart: count the stars visible in a familiar constellation and compare with their magnitudes on a star chart.
- With the site's Tonight page: for a commune in metropolitan France it estimates the class from satellite images. It is an estimate, to about half a magnitude for a place that was not used to calibrate it, and it reads about a class too bright around Paris.
What to do under a city sky
ExperienceOpinion, not physics — shared by nearly everyone who has helped a beginner.
A bigger telescope to beat light pollution
At class 8 the model asks for 2.5 times the aperture to get back to a class 4 sky, and nebulae suffer more still. A heavy instrument also goes out less often.
- Instead
- The same telescope, taken now and then under a class 4 sky or better.
- What changes
- A drive to the country costs less than a 300 mm tube, and the sky it buys benefits everything you already own.
'Light pollution' filters for everything
A UHC filter helps on emission nebulae such as Orion's; it does nothing for galaxies, clusters or planets.
- Instead
- A programme that fits the sky: the Moon, planets, doubles and bright clusters in town, deep sky in the country.
Questions people ask
What is the Bortle scale?A 1-to-9 scale rating how dark the sky is by what the eye sees, published by John E.
Bortle in Sky & Telescope in 2001. 1 is the darkest sky, 9 the centre of a big city.
How do I find my sky's Bortle class?See whether the Milky Way shows and where, or count the stars in a constellation.
For a commune in metropolitan France, the site's Tonight page gives an estimate from satellite images.
Can you observe from a city, at Bortle 8?Yes: the Moon, the planets, double stars and bright clusters lose almost nothing.
It is the galaxies and nebulae that disappear.
Does a bigger telescope make up for light pollution?Partly, and expensively: in the site's model it takes about 2.5 times the aperture at class 8 to get back a class 4 sky's stars.
A darker sky is almost always the better investment.
Do light pollution filters work?On emission nebulae, a UHC filter helps.
On galaxies, clusters and planets, no.
Method and changes
What this page rests on and when it changed, computed from the catalogue — no date below is typed by hand. There is no price history yet, so this log says what was read and when, never whether it went up or down.
- First published
One price per model: the cheapest observed offer, with its date. Models are ordered by that price, never by what a link pays. How we choose.
Written and maintained by the person behind About.