Dawn

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dawn – заря, рассвет
[dɔːn]
astronomical dawn -
[æstrəˈnɒmɪkəl dɔːn]
астрономический
рассвет
nautical dawn морской рассвет
[ˈnɔːtɪkəl dɔːn]

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civil dawn – рассвет
над городом
[sɪvl dɔːn]

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Dawn

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Astronomical dawn

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Nautical dawn

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Civil dawn

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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. 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. This morning twilight before sunrise is divided into three
categories depending on the amount of sunlight that is present in the sky, which is
determined by the angular distance of the centre of the Sun (degrees below the
horizon) in the morning. These categories are astronomical, nautical, and civil dawn.
Astronomical dawn begins when the sun is 18 degrees below the horizon in the
morning. Astronomical twilight follows instantly until the sun is 12 degrees below
the horizon. 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.

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Nautical twilight begins when there is enough illumination for sailors to distinguish
the horizon at sea but the sky is too dark to perform outdoor activities. Formally, it
begins when the Sun is 12 degrees below the horizon in the morning. The sky
becomes light enough to clearly distinguish it from land and water. Nautical dawn
marks the start of nautical twilight, which lasts until civil dawn.
Civil twilight begins when there is enough light for most objects to be
distinguishable, so that some outdoor activities can commence. Formally, it occurs
when the Sun is 6 degrees below the horizon in the morning.
If the sky is clear, it is blue colored, and if there is some cloud or haze, there can be
bronze, orange and yellow colours. Some bright stars and planets such as Venus and
Jupiter are visible to the naked eye at civil dawn. This moment marks the start of civil
twilight, which lasts until sunrise.
The duration of the twilight period (e.g. 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.

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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.
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.
The polar circle (at 66°34′ north or south) is defined as the lowest latitude at which the
Sun does not set at the summer solstice. Therefore, the angular radius of the polar
circle is equal to the angle between Earth's equatorial plane and the ecliptic plane.
This period of time with no sunset lengthens closer to the pole.
Near the summer solstice, latitudes higher than 54°34′ get no darker than nautical
twilight; the "darkness of the night" varies greatly at these latitudes.
At latitudes higher than about 60°34, summer nights get no darker than civil twilight.
This period of "bright nights" is longer at higher latitudes.

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Around the summer solstice, Glasgow, Scotland at 55°51′ N, and Copenhagen,
Denmark at 55°40′ N, get a few hours of "night feeling". Oslo, Norway at 59°56′ N, and
Stockholm, Sweden at 59°19′ N, seem very bright when the Sun is below the horizon.
When the sun gets 9.0 to 9.5 degrees below the horizon (at summer solstice this is at
latitudes 57°30′–57°00′), the zenith gets dark even on cloud-free nights (if there is no
full moon), and the brightest stars are clearly visible in a large majority of the sky.