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Show Difference Between Dawn and Twilight; Ending and Beginning of Phenomenon Twilight is the diffused illumination of the sky which Immediately precedes sunrise and follows sunset. When the sun sets below our horizon, we are not at once plunged into the darkness of night. There is an intermediate period pe-riod of partial and slowly increasing darkness which we call twilight. It is caused by the reflection of the sunlight by dust and particles of water vapor in the upper atmosphere. The same phenomenon occurs just before sunrise and, to distinguish it from the evening twilight, is called dawn. Dawn begins and twilight ends when the sun is about 18 degrees below the horizon, and consequently their duration varies with the latitude and with the season of the year. The higher the latitude the smaller the angle at which the sun's path meets the horizon, and hence the longer it takes the sun to sink to a distance of 18 degrees below the horizon. In the tropics twilight rarely lasts longer than 30 minutes, while in the latitude of north Scotland it lasts so long that about midsummer there are several nights on which it fills the entire interval between sunset and sunrise. |