Understanding Light Pollution and the Bortle Scale

Of everything that determines what you can see, sky brightness matters more than telescope aperture, more than eyepiece quality and more than experience. Understanding your sky is the highest-return piece of knowledge in the hobby.

The Bortle scale

John Bortle's nine-class scale describes sky brightness in terms of what is actually visible, which is far more useful than a photometric number. Most people live in class 6 to 8 and do not realise how much they are missing until they visit class 3.

Class	Name	Naked-eye limiting magnitude	What it means in practice
1	Excellent dark site	7.6 – 8.0	Milky Way casts shadows; zodiacal light visible; M33 seen directly
2	Typical truly dark site	7.1 – 7.5	M33 easy with averted vision; summer Milky Way highly structured
3	Rural sky	6.6 – 7.0	Milky Way structure obvious; horizon glow slight; clouds appear dark
4	Rural / suburban transition	6.1 – 6.5	Milky Way visible overhead but washed low; local light domes obvious
5	Suburban sky	5.6 – 6.0	Milky Way faint or invisible; clouds brighter than the sky
6	Bright suburban sky	5.1 – 5.5	Milky Way invisible below 35°; M31 barely visible
7	Suburban / urban transition	4.6 – 5.0	Milky Way invisible; M31 not visible without optics
8	City sky	4.1 – 4.5	Only bright constellations recognisable; M31 requires a telescope
9	Inner-city sky	≤ 4.0	Only the Moon, planets and the brightest stars visible

Measuring your own sky

The simplest test costs nothing. From a fully dark-adapted state, count the stars visible inside the bowl of the Little Dipper. Fewer than three means heavy pollution; more than ten means a genuinely dark site. It is a crude measure and it is good enough to make decisions with.

A phone-based sky-quality meter gives a reading in magnitudes per square arcsecond, which is comparable between sites. Around 21.5 mag/arcsec² is pristine; 19.0 is suburban; 18.0 is urban. A difference of one magnitude per square arcsecond is a very large difference.

What each class costs you

Light pollution raises the background brightness, which reduces contrast. Bright objects — the Moon, planets, double stars, open clusters, and the brightest globulars — survive. Faint, low-surface-brightness objects — galaxies, faint nebulae, the outer arms of M31 — disappear first.

Tip

Surface brightness, not total magnitude, predicts visibility. M33 is magnitude 5.7 and enormous, which makes it one of the hardest Messier objects from a suburb. M57 is magnitude 8.8 and tiny, and it is easy from the same suburb.

Working with a suburban sky

Observe objects that survive: the Moon, planets, double stars, open clusters, planetary nebulae and bright globulars.

Use a filter where it applies — UHC and OIII on emission nebulae, and accept that they do nothing for galaxies.

Use higher magnification on small objects. It darkens the sky background while keeping the object's surface brightness, which raises contrast.

Wait for transparency. A clear, dry night after a cold front is worth more than any accessory.

Travel when it matters. Two hours of driving to class 3 will show you more than a year of class 7 evenings.

The LED transition

Municipal lighting is shifting from low-pressure sodium, whose light sits in a few narrow lines that filters can block, to broad-spectrum white LEDs. That is good for energy use and bad for astronomy: broadband light cannot be filtered without also filtering the object.

It also changes the advice. Broadband light-pollution filters were designed around sodium lighting and do much less under LED. If your town has converted, expect those filters to disappoint.

Note

Bortle classifications are descriptive rather than regulatory, and the boundaries are approximate. Local conditions, weather and altitude shift a site by as much as a class.

Being a good neighbour about it

Shield your own outdoor lights so they point downwards only.
Use warm colour temperatures — 2700 K or below — and the lowest brightness that does the job.
Turn lights off when they are not needed, especially at observing sites.
Support dark-sky designation and sensible lighting ordinances where you live.

Dark skies are a commons. They are degraded by thousands of small individual decisions and protected the same way.

Disclaimer

This article is educational reference material. Figures are typical approximations, and no observing result is guaranteed — your equipment, sky and conditions will differ.

Mara Vance

Observing editor. Twenty years of visual observing, mostly from a Bortle 4 backyard and a borrowed field in the Chilterns.