Heavenly Shades of Nighttime Falling: It's Twilight Time
As autumn flows toward winter, 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 fills 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 is 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.
In the following discussion, I detail the sunset period, but remember that the same sequences take place in reverse (in time and sky position) during the pre-sunrise twilight.
Colours of the twilight arch
As the orange-yellow sun sets, the sky above it glows a pale yellow with yellow-orange patches to either side of the solar disk and a topping blue-white arch — the twilight arch. As twilight progresses, this twilight arch becomes pink with yellow and orange below. The twilight arch is formed by sunlight scattered by the atmosphere and usually begins encircling the sun like an aureole. When the sun drops below the horizon, the red wave bands are scattered toward us (by Raleigh scattering), often producing a coppery or blood-red twilight arch.
The final glimmers of sunlight on the horizon may be tinted greenish-yellow. On rare occasions, and usually when sunset/rise is viewed over water and the air is free from any form of haze, a quick green flash may be seen on the top of the sun's disc just before it totally disappears from view.
As the twilight arch slowly flattens, the sky above darkens from blue grey to a deep blue, darkening as twilight approaches its end and it becomes officially dark.
The Anti-Twilight Arch
If we turn our backs to the setting sun, we see an opposing effect: the anti-twilight arch and the rising earth shadow. The darkest region in the opposite portion of the sky from the sun is the rising earth shadow. It begins as a thin line stretching nearly 180 degrees in the sky opposite the sun and rises as the sun continues to set. The earth's shadow appears as a large blue-grey arc, sometimes tinged with violet. The shadow of the planet is cast onto the atmosphere itself by the setting sun.
Antitwilight Arch from Kitt's Peak Observatory
Photo courtesy National Optical Astronomy Observatory/Association of
Universities for Research in Astronomy/National Science Foundation
The earth shadow's upper edge is often bordered by a pinkish ribbon known as the anti-twilight arch, above which a faint purple or yellow tint is sometimes discernible. It brightness caused by the backscatter of some of the sunlight by the relatively thick lower atmosphere through which the rays pass and is most pronounced when the eastern air is hazy. The earth shadow seems to rise rapidly. Soon its edge becomes less distinct and disappears rapidly then the shadow limb reaches about 10-15 degrees above the eastern horizon.
The anti-twilight arch divides the earth's shadow from that part of the sky still lit by direct sunlight. It is highest at the anti-solar point (the point above the horizon equal to the sun's angle below the horizon) and curves down toward the horizon. Initially, the arch boundary is fairly sharp and edged with a reddish band, the counterglow, which becomes diffuse as it rises. As the sun drops below the horizon, the anti-twilight arch rises and becomes less distinct, finally blending smoothly into the dark sky of night.
So ends the daytime and enters the night, a nightfall often coloured by heavenly shades of pastel light.
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Written by Keith C. Heidorn, PhD, THE WEATHER DOCTOR,
November 1, 2005 A version of this material was previously published by Keith Heidorn on Suite 101: Science of the Sky, 2002
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