In July 2010, Sverrir Gudmundsson of Reykjavík, Iceland, decided to search the twilight for naked-eye stars about one and a half hours after sunset. He noticed that the sky surrounding 1st-magnitude Deneb (Alpha [α] Cygni), at an altitude of about 70°, appeared a much darker shade of blue than the sky around similarly bright Altair (Alpha Aquilae), at an altitude of 35°. What Gudmundsson observed is a little-known aspect of the twilight sky — one that artists and nature observers alike have long appreciated.
Stephen James O’Meara’s Secret Sky: Twilight musings
January 2011: If you wait until the dark of night to do your observing, you're missing a lot.
