Twilight

 Twilight is the period before sunrise when darkness is giving way to daylight, and after sunset when daylight is giving way to darkness.  It is the result of the sun shining on the upper atmosphere of the Earth, but not on the surface of the Earth at the observer’s location.  The atmosphere scatters some of that light down to the surface.  Morning twilight ends at sunrise, the first appearance of the sun’s upper limb above the visible horizon.  Evening twilight begins at sunset, the disappearance of the sun’s upper limb below the horizon.  The navigator takes particular interest in the occurrence of twilight as this is the only time when both the visible horizon is light enough to be clearly defined and the navigational stars and planets are bright enough to be observed and their altitudes measured with a standard marine sextant.  Civil twilight is the period extending from sunrise or sunset at the lighter limit to the time at which the center of the sun is 6° below the celestial horizon at the darker limit.  Nautical twilight is the period extending from sunrise or sunset at the lighter limit to the time at which the center of the sun is 12° below the celestial horizon at the darker limit.  Civil twilight is characterized by a fairly sharp horizon and a light sky, with only the brighter first and second magnitude stars visible.  During the darker stages of nautical twilight, dimmer second and third magnitude stars become visible, but the horizon is more difficult to discern.  Astronomical twilight is the period extending from sunrise or sunset at the lighter limit to the time at which the center of the sun is 18° below the celestial horizon.  When the sun is more than 18° below the horizon, its light no longer reaches the upper atmosphere at the location of the observer.  The length of twilight is heavily influenced by the latitude of the observer.  It is shortest near the equator and longest in the polar regions.