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Twilight (dawn and dusk)

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  • Twilight is the time between day and night when there is light outside, but the Sun is below the horizon.
  • The diffused light that occurs before the sun rise and sun set gives valuable working hours for humans.
  • Light that is scattered by the gas molecules and reflected by water vapour and dust particles cause illumination of atmosphere.
  • Such effects can be enhanced due to the presence of pollution and other suspended particles as those in volcanic eruptions and forest fires etc.
  • In the morning, twilight begins with dawn, while in the evening it ends with dusk.
  • A number of atmospheric phenomena and colors can be seen during twilight.
  • Astronomers define the three stages of twilight – civil, nautical, and astronomical – on the basis of the Sun’s elevation which is the angle that the geometric center of the Sun makes with the horizon.

Civil Twilight

Civil twilight occurs when the Sun is less than 6 degrees below the horizon. In the morning, civil twilight begins when the Sun is 6 degrees below the horizon and ends at sunrise. In the evening, it begins at sunset and ends when the Sun reaches 6 degrees below the horizon.

Civil dawn is the moment when the geometric center of the Sun is 6 degrees below the horizon in the morning.

Civil dusk is the moment when the geometrical center of the Sun is 6 degrees below the horizon in the evening.

  • Civil twilight is the brightest form of twilight.
  • There is enough natural sunlight during this period that artificial light may not be required to carry out outdoor activities.
  • Only the brightest celestial objects can be observed by the naked eye during this time.
  • Several countries use this definition of civil twilight to make laws related to aviation, hunting, and the usage of headlights and street lamps.

Nautical Twilight, Dawn, and Dusk

Nautical twilight occurs when the geometrical center of the Sun is between 6 degrees and 12 degrees below the horizon. This twilight period is less bright than civil twilight and artificial light is generally required for outdoor activities.

Nautical dawn occurs when the Sun is 12 degrees below the horizon during the morning. Nautical dusk occurs when the Sun goes 12 degrees below the horizon in the evening.

  • The term, nautical twilight, dates back to the time when sailors used the stars to navigate the seas.
  • During this time, most stars can be easily seen with naked eyes.
  • In addition to being important to navigation on the seas, nautical twilight also has military implications.
  • For example, the United States’ military uses nautical twilight, called begin morning nautical twilight (BMNT) and end of evening nautical twilight (EENT), to plan tactical operations.

Astronomical Twilight, Dawn, and Dusk

Astronomical twilight occurs when the Sun is between 12 degrees and 18 degrees below the horizon.

Astronomical dawn is the time when the geometric center of the Sun is at 18 degrees below the horizon. Before this time, the sky is absolutely dark.

Astronomical dusk is the instant when the geographical center of the Sun is at 18 degrees below the horizon. After this point, the sky is no longer illuminated.

  • The duration of dawn and twilight is a function of latitude because the angle of sun above horizon determines the distance travelled by the light in the atmosphere.
  • Lower angle produces longer dawn and twilight periods.
  • At the equator, the light is almost perpendicular hence the dawn and twilight is 30-45 min long while at poles there is about 7 weeks of dawn and 7 weeks of twilight leaving only 2.5 months of near darkness

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