Definition
Defined by the sun's centre being between 6 and 12 degrees below the horizon. During this period the brighter navigational stars and planets are visible while the sea horizon remains sharp enough to measure against, which is the only practical time for star sights. The almanac tabulates the beginning of morning and end of evening nautical twilight by latitude and date; the usable window shrinks toward the tropics and lengthens in high latitudes.
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