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The two periods of the day immediately before sunrise and after sunset when the sun is less than 12 degrees below the true horizon.
1930s; earliest use found in Nautical Almanac 1937.
雅致网英汉双解词典包含236457条英汉翻译词条，涵盖了常用英语单词及词组短语的翻译及用法，是英语学习的必备工具。