Twilight. Twilight on Earth is the illumination of the lower atmosphere when the Sun itself is not directly visible because it is below the horizon. Twilight is produced by sunlight scattering in the upper atmosphere, illuminating the lower atmosphere so that Earth's surface is neither completely lit nor completely dark. The word twilight is also used to denote the periods of time when this illumination occurs. The farther the Sun is below the horizon, the dimmer the twilight. When the Sun reaches 18° below the horizon, the twilight's brightness is nearly zero, and evening twilight becomes nighttime. When the Sun again reaches 18° below the horizon, nighttime becomes morning twilight. Owing to its distinctive quality, primarily the absence of shadows and the appearance of objects silhouetted against the lit sky, twilight has long been popular with photographers, who sometimes refer to it as sweet light, and painters, who often refer to it as the blue hour, after the French expression l'heure bleue. Twilight should not be confused with auroras, which can have a similar appearance in the night sky at high latitudes.By analogy with evening twilight, the word twilight is also sometimes used metaphorically, to imply that something is losing strength and approaching its end. For example, very old people may be said to be in the twilight of their lives. The collateral adjective for twilight is crepuscular, which may be used to describe the behavior of animals that are most active during this period. 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, nautical twilight, and astronomical twilight. Three subcategories of twilight are established and widely accepted: civil twilight, nautical twilight, and astronomical twilight. Morning civil twilight begins when the geometric center of the sun is 6° below the horizon and ends at sunrise. Evening civil twilight begins at sunset and ends when the geometric center of the sun reaches 6° below the horizon. In the United States' military, the initialisms BMCT and EECT are used to refer to the start of morning civil twilight and the end of evening civil twilight, respectively. Civil dawn is preceded by morning nautical twilight and civil dusk is followed by evening nautical twilight. Under clear weather conditions, civil twilight approximates the limit at which solar illumination suffices for the human eye to clearly distinguish terrestrial objects. Enough illumination renders artificial sources unnecessary for most outdoor activities. At civil dawn and at civil dusk sunlight clearly defines the horizon while the brightest stars and planets can appear. As observed from the Earth, sky-gazers know Venus, the brightest planet, as the morning star or evening star because they can see it during civil twilight. Lawmakers have enshrined the concept of civil twilight. Such statutes typically use a fixed period after sunset or before sunrise, rather than how many degrees the sun is below the horizon. Examples include the following periods: when drivers of automobiles must turn on their headlights; when hunting is restricted; when the crime of burglary is to be treated as nighttime burglary, which carries stiffer penalties in some jurisdictions. The period may affect when extra equipment, such as anti-collision lights, is required for aircraft to operate. In the US, civil twilight for aviation is defined in Part 1.1 of the Federal Aviation Regulations as the time listed in the American Air Almanac. Morning nautical twilight begins when the geometric center of the sun is 12 degrees below the horizon in the morning and ends when the geometric center of the sun is 6 degrees below the horizon in the morning. Evening nautical twilight begins when the geometric center of the sun is 6 degrees below the horizon in the evening and ends when the geometric center of the sun is 12 degrees below the horizon in the evening. Nautical dawn is the moment when the geometric center of the Sun is 12 degrees below the horizon in the morning. It is preceded by morning astronomical twilight and followed by morning nautical twilight.