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nautical twilightSee twilight.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006
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.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
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.
An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved