Define Dawn And Twilight
Giorgina Makara
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.[1] 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 begins with the first sight of lightness in the morning, and continues until the Sun breaks the horizon. The morning twilight is divided in three phases, which are determined by the angular distance of the centre of the Sun (degrees below the horizon) in the morning. These are astronomical, nautical and civil twilight.
Astronomical dawn begins when the center of the Sun is 18 degrees below the horizon in the morning. Astronomical twilight follows instantly until the center of the Sun is 12 degrees below the horizon.[5] 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.[6]
Nautical twilight begins when there is enough light for sailors to distinguish the horizon at sea, but the sky is still too dark to perform outdoor activities. It begins when the center of the Sun is 12 degrees below the horizon in the morning. Nautical dawn marks the start of nautical twilight, which lasts until civil dawn.[6][5]
Civil dawn begins when there is enough light for most objects to be distinguishable, so that some outdoor activities can commence. It occurs when the center of the Sun is 6 degrees below the horizon in the morning.[5]
When the sky is clear, it is blue colored, and if there are clouds or haze, bronze, orange and yellow colors are seen. Some bright stars and planets such as Venus and Jupiter are still visible to the naked eye at civil dawn. This moment marks the start of civil twilight, which lasts until sunrise.[6]
The duration of the morning twilight (i.e. between astronomical dawn and sunrise) varies greatly depending on the observer's latitude: from a little over 70 minutes at the Equator, to many hours in the polar regions.[7][8]
The period of twilight is shortest at the Equator, where the equinox Sun rises due east and sets due west, at a right angle to the horizon. Each stage of twilight (civil, nautical, and astronomical) lasts only 24 minutes. From anywhere on Earth, the twilight period is shortest around the equinoxes and longest on the solstices.[citation needed]
Daytime becomes longer as the summer solstice approaches, while nighttime gets longer as the winter solstice approaches. This can have a potential impact on the times and durations of dawn and dusk. This effect is more pronounced closer to the poles, where the Sun rises at the vernal equinox and sets at the autumn equinox, with a long period of twilight, lasting for a few weeks.[citation needed]
In Islam, Zodiacal Light (or "false dawn") is referred to as False Morning (Subhe-Kazeb, Persian صبح کاذب) and Astronomical dawn is called Sahar (سحر) or True Morning (Subhe-Sadeq, Persian صبح صادق), and it is the time of first prayer of the day, and the beginning of the daily fast during Ramadan.[9]
The Hindu dawn deity Ushas is female, whereas Surya, the Sun, and Aruṇa, the Sun's charioteer, are male. Ushas is one of the most prominent Rigvedic deities. The time of dawn is also referred to as the brahmamuhurta (Brahma is the god of creation and muhurta is a Hindu time of the day), and is considered an ideal time to perform spiritual activities, including meditation and yoga. In some parts of India, both Usha and Pratyusha (dusk) are worshipped along with the Sun during the festival of Chhath.[citation needed]
Jesus in the Bible is often symbolized by dawn in the morning, also when Jesus rose on the third day it happened during the morning.[10] Prime is the fixed time of prayer of the traditional Divine Office (Canonical Hours) in Christian liturgy, said at the first hour of daylight. Associated with Jesus, in Christianity, Christian burials take place in the direction of dawn.[citation needed]
In Judaism, the question of how to calculate dawn (Hebrew Alos/Alot HaShachar, or Alos/Alot) is posed by the Talmud,[11] as it has many ramifications for Jewish law (such as the possible start time for certain daytime commandments, like prayer). The simple reading of the Talmud is that dawn takes place 72 minutes before sunrise. Others, including the Vilna Gaon, have the understanding that the Talmud's timeframe for dawn was referring specifically to an equinox day in Mesopotamia, and is therefore teaching that dawn should be calculated daily as commencing when the Sun is 16.1 degrees below the horizon. The longstanding practice among most Sephardic Jews is to follow the first opinion, while many Ashkenazi Jews follow the latter view.[citation needed]
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. The word twilight can also refer to the periods of time when this illumination occurs.[2]
The lower the Sun is beneath the horizon, the dimmer the twilight (other factors such as atmospheric conditions being equal). 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 and painters, who often refer to it as the blue hour, after the French expression l'heure bleue.
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 θs, 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 (nearest the horizon), nautical twilight, and astronomical twilight (farthest from the horizon).[2]
Civil twilight is the period when enough natural light remains that artificial light in towns and cities is not needed. 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. 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 (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.[6]
The period may affect when extra equipment, such as anti-collision lights, is required for aircraft to operate.[7] In the US, civil twilight for aviation is defined in Part 1.1 of the Federal Aviation Regulations (FARs)[8] as the time listed in the American Air Almanac.[9]
Before nautical dawn and after nautical dusk, sailors cannot navigate via the horizon at sea as they cannot clearly see the horizon.[1][failed verification] At nautical dawn and nautical dusk, the human eye finds it difficult, if not impossible, to discern traces of illumination near the sunset or sunrise point of the horizon (first light after nautical dawn but before civil dawn and nightfall after civil dusk but before nautical dusk).[citation needed]
Sailors can take reliable star sightings of well-known stars, during the stage of nautical twilight when they can distinguish a visible horizon for reference (i.e. after astronomic dawn or before astronomic dusk).
Under good atmospheric conditions with the absence of other illumination, during nautical twilight, the human eye may distinguish general outlines of ground objects but cannot participate in detailed outdoor operations.[11]
Nautical twilight has military considerations as well. The initialisms BMNT (begin morning nautical twilight, i.e. nautical dawn) and EENT (end evening nautical twilight, i.e. nautical dusk) are used and considered when planning military operations. A military unit may treat BMNT and EENT with heightened security, e.g. by "standing to", in which everyone assumes a defensive position.
Astronomical twilight is defined as when the geometric center of the Sun is between 18 and 12 below the horizon.[3][4][2] During astronomical twilight, the sky is dark enough to permit astronomical observation of point sources of light such as stars, except in regions with more intense skyglow due to light pollution, moonlight, auroras, and other sources of light. Some critical observations, such as of faint diffuse items such as nebulae and galaxies, may require observation beyond the limit of astronomical twilight. Theoretically, the faintest stars detectable by the naked eye (those of approximately the sixth magnitude) will become visible in the evening at astronomical dusk, and become invisible at astronomical dawn.[12]
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