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Concept
Dawn
Summary
Dawn is the time that marks the beginning of twilight before sunrise. It is recognized by the appearance of indirect sunlight being scattered in Earth's atmosphere, when the centre of the Sun's disc has reached 18° below the observer's horizon. This morning twilight period will last until sunrise (when the Sun's upper limb breaks the horizon), when direct sunlight outshines the diffused light.
"Dawn" derives from the Old English verb dagian, "to become day".
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. These categories are astronomical, nautical, and civil dawn.
Twilight#Astronomical twilight
Astronomical dawn begins when the Sun is 18 degrees below the horizon in the morning. Astronomical twilight follows instantly until the Sun is 12 degrees below the horizon. At this point a very small portion of the Sun's rays illuminate the sky and the fainter stars begin to disappear. Astronomical dawn is often indistinguishable from night, especially in areas with light pollution. Astronomical dawn marks the beginning of astronomical twilight, which lasts until nautical dawn.
Twilight#Nautical twilight
Nautical twilight begins when there is enough illumination for sailors to distinguish the horizon at sea but the sky being too dark to perform outdoor activities. Formally, it begins when the Sun is 12 degrees below the horizon in the morning. The sky becomes light enough to clearly distinguish it from land and water. Nautical dawn marks the start of nautical twilight, which lasts until civil dawn.
Twilight#Civil twilight
Civil dawn begins when there is enough light for most objects to be distinguishable, so that some outdoor activities can commence. Formally, it occurs when the Sun is 6 degrees below the horizon in the morning.
Official source
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