Sailors can make reliable observations of known stars at nautical twilight if they can distinguish a visible horizon for reference (i.e. after astronomical dawn or before 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 U.S. military, the acronyms BMCT (begin morning civil twilight, i.e. civil dawn) and EECT (end evening civil twilight, i.e. civilian dusk) are used to refer to the beginning of civil twilight in the morning and civil twilight in the late evening, respectively. Civil dawn is preceded by morning nautical twilight and civil dawn is followed by evening nautical twilight. At high latitudes and around the summer solstice, the sun does not move lower than 18° below the horizon, so twilight can last from sunset to sunrise. Twilight is important in Islam because it determines when certain universally obligatory prayers should be recited. Dawn is the time when morning prayers (Fajr) are performed, while dusk is the time of evening prayers (Maghreb prayer). Also, during Ramadan, Suhoor time (morning meal before fasting) ends at dawn, while fasting ends after sunset.
There is also an important discussion in Islamic jurisprudence between “true dawn” and “false dawn”. Twilight is the light produced by sunlight scattered into the upper atmosphere when the sun is below the horizon, which illuminates the lower atmosphere and Earth`s surface. The word twilight can also refer to the periods during which this lighting occurs.  For the polar regions, the summer sun does not set below 12 degrees below the horizon. Therefore, these regions have nautical twilight all night and never reach astronomical twilight or complete darkness. For mid-latitudes, nautical twilight can last from about 1/2 hour in spring, winter and autumn to about 45 minutes in summer. The morning twilight phases are often called dawn, while the evening twilight phases are called twilight. However, unlike the term twilight, which describes a period of time, the terms dawn and dusk refer to times during transitions between day and night. For mid-latitudes, civil twilight lasts slightly longer in summer and winter and is slightly shorter in spring and autumn. In spring and autumn, the sun rises more directly to the east and west and descends (or rises) and thus reaches the six degree mark in less time. In summer as in winter, the sun arches above the sky and cuts diagonally above the horizon. This angle is more pronounced in summer, which is why civil twilight lasts the longest in summer.
Civil twilight in mid-latitudes can last 1/2 hour on average. The darkest of twilight is astronomical; For an astronomer, it`s not the same as at night, but for the layman, it`s pretty close. Nautical twilight is deep twilight – a faint bluish sky, bright planets visible. At civil twilight, the sky is bright everywhere, although the sun is not visible. The term civil twilight seems to use the word civilian in its sense in relation to time management, suggesting ordinary civilian use as opposed to military or technical usage. (Nautical twilight is named after the practical considerations of sailors, and the name astronomical twilight refers to when most astronomical observations can begin.) Nautical twilight, morning: Half an hour during which it passes from darkness to sky lightening and pale or reddish on the eastern edge. Trees and buildings can be seen abruptly against the sky, but it`s still very dark on the ground – it would be easy to walk into a hole, and pedestrians would probably want a flashlight or headlamp. Cyclists on the road would definitely want the full amount of lights. The term nautical twilight comes from the time when sailors used the stars to navigate the seas. Meanwhile, most stars are clearly visible to the naked eye, and the horizon is usually visible even on a clear day.
Civil twilight, morning: Half an hour when the sky lights up everywhere. At the beginning of civil twilight, the sky is clear blue and the colors of the objects are easier to recognize. In the middle, you can read a newspaper outside in the open. Pedestrians no longer need lights, but will likely want light-colored reflectors or clothing when they are near traffic, especially on cloudy days. Dubai`s skyline at dusk. The sky can take on spectacular colors at dusk. Similarly, higher latitudes may experience a longer period of nautical or civil twilight. In Greenwich, England (51.5°N), civil twilight varies between 33 minutes and 48 minutes, depending on the season. At the equator, civil twilight can last as little as 24 minutes. This is true because the apparent motion of the Sun at low latitudes is perpendicular to the observer`s horizon. But at the poles, civil twilight can last up to 2-3 weeks. In the Arctic and Antarctic, twilight (if any) can last several hours.
There is no astronomical twilight at the poles near the winter solstice (for about 74 days at the North Pole and about 80 days at the South Pole). As one approaches the Arctic and Antarctic circles, the solar disk moves at a lower angle to the observer`s horizon. The observer`s terrestrial location will pass less directly through the different twilight zones and will take longer. The region, which experiences civil twilight around the summer solstice throughout the night, lies between about 60°33′ and arctic circles at 66°33′ north and south. In the Northern Hemisphere, this roughly corresponds to the southern half of the Northwest Territories and Yukon in Canada, including their capital cities and Whitehorse. In Europe, it covers much of central Scandinavia. A twilight at night is also known as a sleepless night. At astronomical twilight, most celestial objects can be observed in the sky. However, the atmosphere still scatters and refracts a small amount of sunlight, which can make it difficult for astronomers to see the faintest objects. The exact timing and duration of civil twilight varies depending on location and season. Technically, the end of civil twilight coincides with sunrise.
However, in much of the Bay Area, the sun is only visible about 10 minutes after the official sunrise time, as the hills are to the east. In Arctic and Antarctic latitudes, the polar night rarely produces complete darkness 24 hours a day in winter. This can only happen in places within 5.5 degrees latitude of the pole and only on dates close to the winter solstice. In all other latitudes and dates, the polar night involves a daily twilight when the sun is not far below the horizon. Around the winter solstice, when the solar declination changes slowly, complete darkness at the pole itself lasts for several weeks, for example from May 11 to July 31 at the Amundsen-Scott station at the South Pole. [a] The North Pole experienced this from November 13 to January 29. In general, the term twilight most often refers to the time after sunset before complete darkness, but can also refer to the time before sunrise. In both cases, civil twilight occurs – this is the time immediately before sunrise and the time immediately after sunset.