Twilight vs. Light Pollution
A few weeks ago I was waiting for it to get dark so I could go out into my garden and use my telescope. I decided to wait until the end of astronomical twilight (when there is no light from the Sun left in the sky) so that the sky was “properly dark”, but of course given that I live in a city (Glasgow) the light pollution from street lights means that it never actually gets “properly dark”.
So I decided to figure out exactly how long I should wait after sunset before going out to observe, or put another way, at what point does light pollution take over from twilight as the dominant source of light in the sky?
I carried out this little experiment on 28 April 2011, in my back garden on the Southside of Glasgow, under the following conditions:
Sunset Time: 2050 BST
Civil Twilight Ends (Sun 6 degrees below horizon): 2134 BST
Nautical Twilight Ends (Sun 12 degrees below horizon): 2232 BST
Astronomical Twilight Ends (Sun 18 degrees below horizon): 2358 BST
Longitude: 55.866 N
Latitude: 4.257 W
Sky conditions: 100% clear
[Incidentally, sitting out in my garden for four hours as it darkened was an absolute delight: I saw, as well as the emerging stars, many bats, some ducks, two foxes, and two passes of the International Space Station!]
Using a Sky Quality Meter I took readings of the sky brightness at the zenith every two minutes. The SQM-L makes measurements of the sky in magnitudes per square arcsecond, i.e. brightness per unit area in the sky. As a general rule, in city centres you would expect readings of 16-17, while in dark places you can get readings of 21-22. The higher the number the darker the sky. The darkest reading possible under a starry sky is around 22, as at that point the starlight itself becomes the limiting factor.
From experience I know that in my garden the darkest reading possible is around 18.5, so I decided to continue taking readings until I got fifteen in a row that were above 18.4, i.e. for half an hour the sky had not been significantly darkening. I got my first reading of above 18.4 at 2244 and the sky did not appreciably darken over the next 30 minutes.
At 2244 the sun was 13 degrees 07 minutes (13.117 degrees) below the horizon.
I graphed the results to see how they looked, and placed them alongside the projected results if I were under a dark sky free of light pollution i.e. so that the results could get as low as 22.0 at the end of astronomical twilight at 2358 BST
Result: After sunset, and throughout civil twilight (Sun between 0 and 6 degrees below the horizon) and nautical twilight (Sun between 6 and 12 degrees) the dominant source of light in the sky is sunlight.
However shortly after the end of nautical twilight light pollution became the dominant source of light in the sky, when the Sun was a little more than 13 degrees below the horizon.
This means that, while observing from my garden in Glasgow, I shouldn’t wait much later than the end of nautical twilight to go out observing, since beyond that point the sky will not significantly darken.
CAVEAT: It should be noted that this information is really only relevant for my specific circumstances, i.e. the light pollution in your sky may be better of worse than mine, and mean that the point at which it begins to dominate twilight is different for you.