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Related to astronomical twilight: astronomical year
astronomical twilightSee twilight.
astronomical twilight[‚as·trə′näm·ə·kəl ′twī‚līt]
The period of incomplete darkness when the center of the sun is more than 6° but not more than 18° below the celestial horizon.
The period before sunrise and after sunset when sun's center is between 12° and 18° below sea-level horizon. This may be regarded for all practical purposes as synonymous with complete darkness.