Civil dawn in Washington, DC.

"Dawn" refers to the beginning of twilight, which is the period during which sunlight scattering in the upper atmosphere illuminates the lower atmosphere, and the surface of the earth is neither completely lit nor completely dark. The sun itself is not directly visible because it is below the horizon. (Wikipedia, Dawn (as of October 2, 2012)).

There are different defined periods of "twilight". "Astronomical twilight" is when the sun is between 12° and 18° below the horizon. "Nautical twilight" refers to the period when the sun is between 6° and 12° below the horizon. "Civil twilight" refers to the period beginning when the geometric center of the sun is 6° below the horizon (civil dawn) and ends at sunrise. (Wikipedia, Twilight (as of October 2, 2012)).