Measuring Your Own Sky: Sky Quality Meters and What the Numbers Mean
Light pollution maps describe a region and are years out of date. A twenty-second measurement tells you what your actual garden is doing tonight.
Most people estimate their sky from a light pollution map, get a Bortle class, and stop there. That estimate is regional, several years old, and blind to the tree line, the neighbour's floodlight and the fact that half your local authority converted to LED last spring.
Measuring it yourself takes seconds and changes how you plan sessions, because it turns "my sky is bad" into a number you can compare across sites and across nights.
What is being measured
Sky quality is expressed in magnitudes per square arcsecond, written mag/arcsec². It describes how bright a patch of empty sky is.
The scale is logarithmic and runs backwards, which trips people up: higher numbers mean darker skies. Each whole magnitude is a factor of about 2.5 in brightness.
- 22.0 and above — pristine, Bortle 1. Rare.
- 21.5-21.9 — excellent rural, Bortle 2-3.
- 21.0-21.4 — good rural, Bortle 3-4. Galaxies are comfortable.
- 20.0-20.9 — suburban, Bortle 5-6. Filtered nebulae are strong.
- 19.0-19.9 — bright suburban, Bortle 7.
- 18.9 and below — urban, Bortle 8-9.
The step from 20.5 to 21.5 does not sound like much and represents roughly a factor of 2.5 less sky brightness, which in imaging terms is transformative.[1]
How to measure
A dedicated meter is the accurate option: point it at the zenith, press the button, read the number. Consistent, repeatable and independent of your phone's camera.
Phone apps that estimate sky brightness from the camera vary enormously in quality. Treat them as indicative rather than authoritative, and never compare a reading from one app against a reading from another.
Naked-eye limiting magnitude costs nothing. Count the faintest stars you can see in a known asterism and convert. Crude, but it correlates well and it is free.
Measuring properly
Four rules make readings comparable rather than merely numeric.
- Point at the zenith. Sky brightness varies enormously with altitude and direction. Zenith is the convention, and comparing a zenith reading against someone's horizon reading is meaningless.
- Wait for full darkness. Take readings after astronomical twilight ends, or your number describes the sun rather than your neighbourhood.
- No moon. A first-quarter moon can cost you a magnitude and a half. Record the moon phase alongside the reading or the number is uninterpretable later.
- Watch the Milky Way. If the galactic plane is overhead your reading will be slightly brighter than the same site with the Milky Way low. At the precision most people need this is a footnote, but it explains small discrepancies.
What to do with the numbers
Build a personal site list. Measure your garden, the field ten minutes away, the layby on the hill, the park you drive to. Within a month you will know exactly what each is worth, and the twenty-minute drive that gains a full magnitude becomes an obvious decision rather than a guess.
Track your own sky over time. Readings from the same spot over a year reveal seasonal patterns, the effect of snow cover, and whether local lighting changes have made things worse. This is also the data that makes a case if you ever want to raise lighting with a local authority.
Sanity-check the maps. Where your measurement and the map disagree, trust the measurement. Maps are models built on satellite data that looks down; you are standing underneath looking up, and only one of those is your actual observing condition.
Notes & sources
- Sky brightness in magnitudes per square arcsecond, its logarithmic scale, and typical values across rural to urban sites. DarkSky International, measuring light pollution ↩