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The sun can be a substantial glare source when shining in a driver's eyes when the sun is low on the horizon after sunrise or before sunset, or at virtually any time during daylight hours if the object of concern presents itself at the same altitude and azimuth as the sun. Second, the ambient light levels during the civil twilight period, either before sunrise (dawn) or after sunset (dusk), are brighter than complete darkness, yet are less than normal daylight driving conditions. Methods are presented for calculating the times of sunrise and sunset, the times of civil twilight darker limit, the angular position of the sun, for any time, and date, and for estimating the natural light levels at the time of the incident, to enable an assessment of driver visibility.