Twilight

The twilight is the atmospheric gleam present before the rising or after laying down it Sun. The term also indicates the moment of the day when this gleam is visible.

Causes

The twilight is caused by the Réfraction of the sunlight by the high layers of the atmosphere.

Duration

The twilight (but not always) is generally divided into two parts: one which follows to lay down it Sun, the other which precedes its rising, separated by the Nuit.

The duration of the twilight depends on the Latitude of the observer: in the areas Arctic S and the Antarctic S, it can last several hours or not to be present whole, while with the equator, it can disappear in less than 20 minutes. With the average latitudes, the twilight is with shortest with the approach of the equinox S, longer towards the Solstice of winter and even longer towards the summer solstice.

Beyond the polar circles, the Sun does not lie down towards the summer solstice. To the latitudes raised in on this side these circles, the Sun goes down under the horizon but the twilight continues its to sleep with its rising, a phenomenon known under the name of Sleepless night. With the top of approximately 60° of latitude, the civil twilight continues all the night at this period. With the top of 55°, it is the case of the nautical twilight. Lastly, the astronomical twilight can last all during the night during several weeks until 50°.

Divisions

Civil twilight

The civil twilight is the period when the center of the Sun is located under the horizon (a idealized horizon, located at 90° of the zenith) but at less 6° of this one.

During the civil twilight, the Planet S and the most brilliant star S appear and there remains light still sufficient so that the majority of the activities do not require artificial sources of light.

Nautical twilight

The nautical twilight is the period when the center of the Sun is located between 6° and 12° under the horizon.

It is about the moment when the stars of second size become visible; at the same time, at sea, the horizon is always visible thus allowing a navigation stars. At the end of this period (in evening) or at its beginning (in morning), the last or first gleams can be distinguished in the direction of the Sun.

Astronomical twilight

The astronomical twilight is the period when the center of the Sun is located between 12° and 18° under the horizon.

During the astronomical twilight, and in the case of a clear sky from all luminous Pollution, the weakest stars visible with the naked eye (towards the Magnitude connect 6) appears. From the astronomical point of view, there remains light however sufficient so that the diffuse objects like the Nébuleuse S or the Galaxie S cannot be observed under satisfactory conditions, even if this light is unperceivable with the naked eye.

The evening, the end of the astronomical twilight marks the beginning of the complete night; the morning, it is the end of the night, the appearance of the very first gleams of the paddle.

See too

Internal bonds

External bonds

  • '' Twilight time calculator ''
  • Like calculating the duration of the twilight

Beats-smg: Sotemas Simple: Twilight

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