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Dawn, from an Old English verb dagian: "to become day", is the time that marks the beginning of twilight before sunrise. It is recognized by the appearance of indirect sunlight being scattered in the atmosphere, when the centre of the Sun's disc reaches 18° below the horizon.[1] This dawn twilight period will last until sunrise (when the Sun's upper limb breaks the horizon), as the diffused light becomes direct sunlight. Dawn begins with the first sight of lightness in the morning, and continues until the sun breaks the horizon. This morning twilight before sunrise, is divided into three categories depending on the amount of sunlight that is present in the sky, which is determined by the angular distance of the centre of the Sun (degrees below the horizon) in the morning. The categories are named: astronomical, nautical, and civil dawn.