How we measure
Everything below fits on the laminated card our volunteers carry. If you can read a kitchen scale, you can do this.
Sky in this banner: 17.64 mag/arcsec², the Kalsa, Palermo
The instrument
We use pocket photometers: boxes about the size of a bar of soap, with a light sensor behind a small lens, a temperature sensor and a nine-volt battery. Point one at the sky, press the button, and after a few seconds it shows a number. In a very dark place it may take up to a minute, because the sensor is waiting for enough light to arrive.
The meter looks at a cone of sky about 20° wide. That is roughly the width of your two fists held side by side at arm’s length. It doesn’t see stars one by one. It measures the overall glow in that cone, which is the point: the glow is what hides the stars.
The project owns a dozen meters, numbered and logged. Volunteers borrow them.
When a night counts
A reading is only comparable if the sky itself is the only thing that differs. All of these must hold.
- Sun
- At least 18° below the horizon. In Palermo in June that means no earlier than about 22:30.
- Moon
- Below the horizon for the whole session. This leaves about ten to fourteen usable nights around each new moon.
- Cloud
- None within about 30° of the zenith. Cloud reflects city light and makes a town look brighter than it is.
- Air
- No rain or fog. Haze and Saharan dust are allowed, but they must be noted, and those sessions are flagged in the tables.
- Place
- Within five metres of the fixed point, with no lamp shining on the meter and none inside its field of view.
- Meter
- At outdoor temperature, which takes about ten minutes out of the bag.
The protocol
Version 2, in use since April 2025.
- Reach the point at least 20 minutes before you plan to start, and take the meter out of its bag.
- Check the sky and note what you see: clear, haze or dust. If there is cloud near the zenith, wait or go home.
- Hold the meter flat above your head at arm’s length, pointing straight up. Stay clear of pale walls, and keep your phone in your pocket.
- Take one reading and throw it away. The first reading after switching on tends to run a little high.
- Take five readings, one minute apart.
- Write down all five, with the time in UTC, the temperature shown by the meter and your note on the sky.
- Enter the session in the volunteer portal within 48 hours. A coordinator checks it, and the median of the five goes into our tables.
The scale, in plain language
Readings are in magnitudes per square arcsecond, written mag/arcsec². Higher means darker.
The numbers run backwards because the scale comes from the way astronomers rank stars: the fainter the star, the higher its magnitude. A square arcsecond is a tiny patch of sky, about the size of a coin seen from four kilometres away. A reading of 21 means that each such patch glows as faintly as a star of magnitude 21.
A moonless sky with no artificial light at all reads about 22. Nothing on Earth reads much higher.
16–17Floodlit squares and shopping streets
The Moon, the planets and a handful of the brightest stars. We have measured this only on hazy nights at Politeama.
17–18City centres
The outlines of the brightest constellations, with gaps. The Kalsa reads 17.64.
18–19Inner suburbs and large towns
Orion and the Plough complete, nothing faint. Monreale reads 18.92.
19–20Outer suburbs and small coastal towns
Most constellations. The Milky Way is only suspected. Cefalù reads 19.63.
20–21Hill towns and open country
The Milky Way is plain overhead in summer. Polizzi Generosa reads 20.71.
21–22Mountains, far from any town
The Milky Way shows structure and dark lanes. Piano Battaglia reads 21.58.
Small differences are big
The scale is logarithmic, so a gap between two readings stands for a ratio of brightness.
| Difference | One sky is brighter by |
|---|---|
| 0.1 | 1.1× |
| 0.5 | 1.6× |
| 1.0 | 2.5× |
| 2.5 | 10× |
| 4.0 | 40× |
| 5.0 | 100× |
This is why we quote two decimals, and why we are careful with them.
How sure are we?
A single session is good to about ±0.1 mag/arcsec², or roughly 10% in brightness. We don’t report any change smaller than that as a change, however tempting it looks.
Meters drift apart with age and knocks. Every spring and autumn all of them travel to the same spot at Piano Battaglia and are read side by side. Each meter’s offset from the group median is recorded and applied to its readings.
There are also things a meter cannot tell you. It reads the zenith only, so it says little about the glow along the horizon. It has one sensor and does not see colour as the eye does, so it can under-report some blue-rich white light. We say so wherever it matters.