Nautical twilight Evening nautical twilight on Lake Ontario, Canada In the US, civil twilight for aviation is defined in Part 1.1 of the Federal Aviation Regulations (FARs) as the time listed in the American Air Almanac. The period may affect when extra equipment, such as anti-collision lights, is required for aircraft to operate. Examples include when drivers of automobiles must turn on their headlights (called lighting-up time in the UK), when hunting is restricted, or when the crime of burglary is to be treated as nighttime burglary, which carries stiffer penalties in some jurisdictions. Such statutes typically use a fixed period after sunset or before sunrise (most commonly 20–30 minutes), rather than how many degrees the Sun is below the horizon. Lawmakers have enshrined the concept of civil twilight. As observed from the Earth (see apparent magnitude), sky-gazers know Venus, the brightest planet, as the "morning star" or "evening star" because they can see it during civil twilight. At civil dawn and at civil dusk sunlight clearly defines the horizon while the brightest stars and planets can appear. Enough illumination renders artificial sources unnecessary for most outdoor activities. Under clear weather conditions, civil twilight approximates the limit at which solar illumination suffices for the human eye to clearly distinguish terrestrial objects. Civil twilight in a small town in the Mojave Desert Civil dawn is preceded by morning nautical twilight and civil dusk is followed by evening nautical twilight. In the United States' military, the initialisms BMCT (begin morning civil twilight, i.e., civil dawn) and EECT (end evening civil twilight, i.e., civil dusk) are used to refer to the start of morning civil twilight and the end of evening civil twilight, respectively. Ĭivil twilight is the period when enough natural light remains that artificial light in towns and cities is not needed. Midtown Manhattan during civil twilight, demonstrating blue hourĬivil twilight is defined as when the geometric center of the Sun is between 6° below the horizon and the horizon itself. For other uses, see Civil twilight (disambiguation). The collateral adjective for twilight is crepuscular, which may be used to describe the behavior of animals that are most active during this period. For example, very old people may be said to be "in the twilight of their lives". 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 and painters, who often refer to it as the blue hour, after the French expression l'heure bleue.īy analogy with evening twilight, the word twilight is also sometimes used metaphorically, to imply that something is losing strength and approaching its end. When the Sun again reaches 18° below the horizon, nighttime becomes morning twilight. When the Sun reaches 18° below the horizon, the twilight's brightness is nearly zero, and evening twilight becomes nighttime. The lower the Sun is beneath the horizon, the dimmer the twilight (other factors such as atmospheric conditions being equal). The word twilight can also refer to the periods of time when this illumination occurs. Twilight is light produced by sunlight scattering in the upper atmosphere, when the Sun is below the horizon, which illuminates the lower atmosphere and the Earth's surface. Evening twilight: civil, nautical, and astronomical stages at dusk. The apparent disk of the Sun is shown to scale. Morning twilight: astronomical, nautical, and civil stages at dawn. Twilight is the time period between dawn and sunrise, or between sunset and dusk. For other uses, see Twilight (disambiguation).
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