http://aa.usno.navy.mil/faq/docs/RST_defs.html#top


Twilight: 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. Nevertheless, it is possible to establish useful, though necessarily approximate, limits
 applicable to large classes of activities by considering only the position of the Sun below the local horizon.
 Reasonable and convenient definitions have evolved. 

         Civil twilight 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; at the
         beginning of morning civil twilight, or end of evening civil twilight, the horizon is clearly defined
         and the brightest stars are visible under good atmospheric conditions in the absence of moonlight
         or other illumination. In the morning before the beginning of civil twilight and in the evening after
         the end of civil twilight, artificial illumination is normally required to carry on ordinary outdoor
         activities. Complete darkness, however, ends sometime prior to the beginning of morning civil
         twilight and begins sometime after the end of evening civil twilight. 

         Nautical twilight is defined to begin in the morning, and to end in the evening, when the center of
         the sun is geometrically 12 degrees below the horizon. At the beginning or end of nautical twilight,
         under good atmospheric conditions and in the absence of other illumination, general outlines of
         ground objects may be distinguishable, but detailed outdoor operations are not possible, and the
         horizon is indistinct.