Daylight

Here's a very useful link for trip planning when daylight is limited:

http://aa.usno.navy.mil/data/

It even provides "Civil Twilight" data, which is defined to begin in the morning, and to end in the evening when the center of the sun is geometrically 6 degrees below the horizon. This is the limit at which twilight illumination is sufficient, under good weather conditions, for terrestrial objects to be clearly distinguished.

Useful stuff...