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Heavenly Shades of Nighttime Falling: It's Twilight Time

© Keith C. Heidorn

With the onset of autumn, the shortening hours of daylight allow many of us the opportunity to view both the sunrise and sunset hours. And, often, October skies are clearer than they will be during the winter months. While you might think that clear skies are not conducive to sky watching - unless you look past the atmosphere to the moon, planets and stars - there are subtle changes in the clear sunset/sunrise skies that are worth looking for.

Before we look at the atmospheric effects, I want to define for you what is meant by twilight because it has strict astronomical and legal definitions which are often used in by-laws. The duration of twilight is geometrically dependent on latitude, season and elevation. Astronomers and time-keepers define three twilight periods: civil twilight, nautical twilight and astronomical twilight. When evening twilight ends, the night becomes officially dark until the beginning of morning twilight.

Civil twilight is the period from sunset when the solar disk has just left the horizon until the centre of the sun's disc is 6 degrees below the horizon. If the sky is clear, it is usually practicable to carry out normal outdoor activities, as well as the ability to read normal type, without artificial light during civil twilight. For the sunrise period the sequence is reversed and, civil twilight is from when the sun is 6 degrees below the horizon until its upper limb just touches the horizon. First light marks the beginning of morning civil twilight, and last light, the end of evening civil twilight.

Nautical twilight occurs when the sun is between 6 and 12 degrees below the horizon. The brighter stars become visible during this period, thus providing good conditions for position fixing using manual navigation instruments. Astronomical twilight fills the time interval when the sun is between 12 and 18 degrees below the horizon during this time only the gross outlines of objects can be discerned. When the sun is below 18 degrees, it is officially dark. For regions in the higher latitudes during summer, the sun may set, but no period of official dark occurs as the twilight periods of evening and morning merge together.

The Twilight Sky

Under clear skies, the twilight period can provide subtle sky beauty with its softly changing colours, particularly in the sky regions known as the twilight and anti-twilight arches. When the sun is on the horizon, the sky surrounding the solar disk takes on an orange-yellow glow. The colours in the red/yellow end of the spectrum dominate because the air has scattered out all the blue wave lengths from the sunlight that now reaches our eyes.

The copyright of the article Heavenly Shades of Nighttime Falling: It's Twilight Time in Meteorology is owned by Keith C. Heidorn. Permission to republish Heavenly Shades of Nighttime Falling: It's Twilight Time in print or online must be granted by the author in writing.