The Suns rays penetrate and brighten the sky before sunrise and after sunset. These are the twilight periods. There are three stages of twilight. Civil Twilight is the period before sunrise and after sunset until the time when the center of the disk of the sun is 6 degrees below the horizon. When you calculate civil twilight using the appropriate table in Air Almanac, you are actually calculating "Beginning-of-Morning Civil Twilight" (BMCT) for the time of civi  twilight preceding sunrise, and "End-of-Evening Civil Twilight" (EECT) for the time of civil twilight following sunset. During the civil twilight period, stars and planets of the first magnitude may be visible. Only the brightest stars and planets will be noticeable during BMCT and EECT. During the civil twilight period, civilians and military troops can carry on all normal daytime activities without being hindered by any loss of natural illumination.

The next stage of twilight is Nautical Twilight. This is the period between civil twilight and the time at which the center of the disk of the sun is 12 degrees below the horizon. The Nautical Twilight tables yield times for "Beginning-of-Morning Nautical Twilight" (BMNT) and "End-of- Evening Nautical Twilight" (EENT). During the period of nautical twilight, stars and planets of the second to fifth magnitude may be visible. Available illumination is described as "approaching conditions expected under full light of day." Civilians and military troops may continue most daytime activities without notice-able hindrance except at the time of EENT and BMNT when the Sun illuminates only a trace of a bright band on the horizon. Most people will need supplemental lighting to carry on normal outdoor activity before BMNT and after EENT. "Beginning-of-morning nautical twilight" and "end-of-evening nautical twilight" are the terms usually being referred to when people use the generic terms twilight, first light, and last light. The last stage of twilight is the period between nautical twilight and the time the Sun reaches a position 18 degrees below the horizon. This period is called Astronomical Twilight. The times of astronomical twilight are referred to as "Beginning-of-Morning Astronomical Twilight" (BMAT) and "End-of-Evening Astronomical Twilight" (EEAT). Stars of the sixth magnitude may be visible during the astronomical twilight period. There is no trace of any bright band of sky on the horizon at the time of BMAT and EEAT. During the nighttime period after EEAT and before BMAT, it is, without question, DARK. Unless lunar illumination is available, movement of either civilians or military troops without the aid of artificial lighting, Night Vision devices, or sight-substitute devices cannot be done.

The Twilight tables are similar to the Sunrise and Sunset tables. See table 6-7-3. The twilight tables are used to obtain the times of first light (morning) and last light (evening) for latitudes 72N through 60S.

Learning Objective: Identify the tables and procedures used to compute sunrise, 72N.

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