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nautical twilightSee twilight.
nautical twilight[′nȯd·ə·kəl ′twī‚līt]
The interval of incomplete darkness between sunrise or sunset and the time at which the center of the sun's disk is 12° below the celestial horizon.
That period before sunrise and after sunset when the upper limb of the sun is below the visible horizon and the center of the sun is not more than 12° below the celestial or sensible horizon. During this interval of incomplete darkness, the degree of illumination is such that general outlines are still discernible, although detailed operations become impossible. All the bright stars are visible at this time. It is the time when the navigational stars are visible and the sea horizon is still sufficiently visible to permit celestial observations with a sextant.