Light Pollution Survey

Forums General Discussion CPRE Star Count 2019 Light Pollution Survey

Peter Anderson

Just a few comments from the ‘peanut gallery’ as it were.

In 1978 our Association (Astronomical Association of Queensland) in Brisbane, Queensland Australia (27.5 deg south 153 deg east) conducted a light pollution survey which we repeated in 2018 forty year later.

Needless to say, the membership was generally lethargic, even for something as simple as this involving simple naked eye observing, but we got enough results to show that as the city grew skies deteriorated. The mean limiting magnitude in 1978 was 5.5 and in 2018 it was 4.6. My personal site deteriorated from 5.8 to 5.4. By this statement you will see that it was not a simple ‘star count’ which provides limited information, and in a large area like Orion there can be miscounts and double ups. Besides spanning (from the UK) over 15 degrees in altitude, extinction will play a factor. (See later comments).

However I would like to make my point that we tried to do this as scientifically as we could.  The ideal would be to have a scientific device to properly measure it, and the next best would be to have a roving team who would first meet and test their eyes so that they could add a ‘correction factor’.  Then they would visit key sites and take the readings.  However this sort of exercise is costly and you have to operate with those funds and volunteers that you have.

Anyway, what we did was publish a map of Scorpius with the brightness of the stars marked.  This was to be our reference.

The sightings were to take place within one half hour of 9pm during a specified moonless week in July when Scorpius would be nearly overhead. This way we avoided contamination from shopping centres and stuff near the horizon as well as  extinction at lower altitudes.

The instructions were to get dark adapted, and first identifying stars that were easy to see, then moving to ever fainter ones until they were no longer visible. A bit of ‘averted vision’ might be used but not ‘wishful thinking’. I remember reporting 5.3 okay and 5.4 with a ‘following breeze’.

Now people’s eyes change with age. There is a grey/green cast which develops and reduces contrast and changes the colour balance. (Fixed by cataract surgery.) So without cross-checking, which in an amateur society is probably difficult, you will have to take the results on faith. There is also the factor that some people like to ‘talk up’ how good their site happens to be, but though you know who they are, you can’t reasonably address this.

I just supply all of this for interest. Orion is easy to identify. It straddles the celestial equator so from your locations would at best be under 40 degrees altitude on average and at a reasonably altitude would be in the South East- South- South West quadrant. If sites had bright developments in that direction, it would badly skew results.

For near overhead at your latitude in early evening, at present there is Capella, but the surrounding star patterns are not distinctive.  Might I suggest using Ursa Major in April-May. Nearly overhead early/mid evening and very distinctive. Just print and distribute a map with magnitudes marked, say in your Journal/Newsletter.

I hope my thoughts are helpful.