TWILIGHT

[GordMay]

Twilight is the time before sunrise or after sunset when sunlight scatters in the upper atmosphere illuminates the lower atmosphere and the surface of the Earth.
The length (duration) of twilight, after sunset and before sunrise, is heavily influenced by the latitude of the observer
The sun (appears to) rises and sets because of the Earth's rotation. At lower latitudes (ie: nearer the equator), the sun sets perpendicular to the horizon, while at higher latitudes, the sun can set at a more oblique angle, allowing it to remain close to the horizon after sunset for a longer period of time.

More: “Twilight time explained" ~ by Cameron Bright (an excellent article!)
http://www.oceannavigator.com/article.php?a=1289