The New Student's Reference Work/Twilight
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Twi′light is the faint light before sunrise and after sunset, which is produced by the reflection by the atmosphere of the sun's rays when it is still below the horizon. If we had no atmosphere, we should have no light until the exact moment when the sun would rise and no light after the sun had set. The sun must be approximately 18° below the horizon at any place before the twilight will entirely cease; so in the polar region where the sun is nearer the horizon than 18° there will be twilight all night, or rather there will be no complete night. Just what there is in the earth's atmosphere that reflects the sunlight is yet unknown. It may be dust-particles; it may be minute crystals of ice; it may be pure air itself.