I recently purchased an iphone app called ‘skyclock’ which gives me a real-time clock face showing the twilight zone at my location. There is now a PC version available with others for the Mac, etc to follow. This is an excellent app for both real-time fliers and simmers as it will give the times that you can fly in the real twilight hours. Skyclock shows twilight accurately ie when the sun is below the horizon and the earth is illuminated by the diffused light from the sky during the early evening or early morning. Features include:
-Built-in Locations–worldwide showing twilight zones:
-Sky Calendar–future twilight times.
-12 and 24 Hour clock with evening and morning twilight hours.
-Next 12 Hour Look Forward
-Data points–Navigational location points.
In other words when it is no longer light or dark and seeing can be incredibly difficult. The periods of twilight depicted by the Skyclock include Solar, Civil, Nautical and Astronomical Twilight. It is available here, and there is an excellent video here:
httpv://www.youtube.com/watch?v=ewy5wdedjeA