Twilight is defined according to the solar elevation angle ?s, which is the position of the geometric center of the sun relative to the horizon. There are three established and widely accepted subcategories of twilight: civil twilight (brightest), nautical twilight, and astronomical twilight (darkest).
Under civil twilight circumstances, the horizon is clearly visible, and terrestrial objects are easily perceptible, without artificial light.For the band Civil Twilight, see Civil Twilight (band). Morning civil twilight begins when the geometric center of the sun is 6° below the horizon (the point of civil dawn), and ends at sunrise. Evening civil twilight begins at sunset and ends when the center of the sun reaches 6° below the horizon (the point of civil dusk). In general, civil twilight is the point where artificial illumination is required to read outside.[2] This concept is sometimes enshrined in laws, for example, when drivers of automobiles must turn on their headlights, when pilots may exercise the rights to fly aircraft, or if the crime of burglary is to be treated as nighttime burglary, which carries stiffer penalties in some jurisdictions. |
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The brightest stars appear during the civil twilight, as well as planets, such as Venus, which is known as the 'morning star' and/or 'evening star'. During this period there is enough light from the sun that artificial sources of light may not be needed to carry on outdoor activities.