United States Naval Observatory
Astronomical Twilight Table 2011
Twilight Definition:
Before sunrise and again after sunset there are intervals of time, twilight, during which there is natural light provided by the upper atmosphere, which does receive direct sunlight and reflects part of it toward the Earth's surface.
Some outdoor activities may be conducted without artificial illumination during these intervals, and it is useful to have some means to set limits beyond which a certain activity should be assisted by artificial lighting.
The major determinants of the amount of natural light during twilight are the state of the atmosphere generally and local weather conditions in particular.
Atmospheric conditions are best determined at the actual time and place of events.
