Observing Skills · 10 of 12
Light Pollution
Why does a city sky show so few stars, and how do you get them back?
A city hides its stars behind its own glow, not its air: the same patch at Bortle 8 and Bortle 2, the limiting magnitude, and what you can do — shield lights, wait, go farther.
You’ll be able to say why a city hides its stars, read your sky’s limit, and win stars back from the glow.
Before this: Understanding Star Brightness (10 min)
Where and which night
Working out when this is best placed from where you are…
Where
Kept for this guide only, until you change it here — your map settings, and every other guide, stay as they are.
Print, play outside or present
Scan with a phone's camera, or open
Sources
- Bortle, "The Bortle Dark-Sky Scale", Sky & Telescope, February 2001 — the nine classes and the naked-eye limit of each
- Falchi et al., Science Advances 2, e1600377 (2016) — the world atlas of artificial night-sky brightness, from satellite measurements; the class each place is given here, and the domes the map draws
- Walker, "The effects of urban lighting on the brightness of the night sky", PASP 89, 405 (1977) — a town's glow falls as the distance to the power 2.5
- DarkSky International and IES, Five Principles for Responsible Outdoor Lighting — shield and aim light down, where it is needed
- Hecht, Haig & Chase, J. Gen. Physiol. 20, 831 (1937) — the dark-adaptation thresholds the unadapted eye is drawn from
- HYG stellar database v4.2 — the stars counted to each limit, and every magnitude on a tag
With a keyboard, focus the sky, aim it with the up and down arrow keys, and press Enter to drop the pin at the centre of the view; or press Show me.