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What Causes Twilight: The Physics of the Blue Hour
Twilight, the period of time just before sunrise and just after sunset, is probably best known for the stunning visual quality it brings to travel guide photos, dramatic movie establishing shots, and romantic walks on the beach. Photographers and filmmakers refer to this period as the blue hour for its dramatic visual effect. However, there is also some very interesting astrophysical science happening during the twilight period as the Sun continues to light the skies from beyond the horizon.
What Gives Twilight Its Colors?
The stunning colors of the twilight sky continue long after sunset - and begin long before the breaking dawn - due to Rayleigh scattering, the same process that makes the daytime sky appear blue. Named for 19th Century physicist John William Strutt, the 3rd Baron Rayleigh (better known as Lord Rayleigh), this refers to the scattering of light by particles smaller than the wavelength of visible light.
When the sun is directly overhead, higher wavelengths of light from 500 to 650 nanometers (green to red) pass directly through the atmosphere. About 25% of the light photons with wavelengths of 450 nanometers and below (blue and violet) are deflected by molecules of nitrogen, oxygen, argon, and other atmospheric gases. These scattered photons bounce around the atmosphere until they reach our eyes, and thus appear to be coming from all directions.
When the sun is near or below the horizon, the light has to travel through a much thicker blanket of atmosphere. Nearly all of the blue, violet, and green wavelengths are scattered away, leaving the remaining red and yellow wavelengths to bounce around the atmosphere overhead. This is what turns the sky shades of red, yellow, and orange during twilight. The reddening effect we see in the sky is the same process that creates a reddish hue on the Moon during a lunar eclipse.
The Stages of Twilight
For clarity, the definitions below will be for evening twilight, after sunset. The same subcategories apply during sunrise as well, but in the reverse order.
Solar twilight begins when the center of the Sun has passed below the horizon. The Sun as viewed from Earth is about 30 arc-minutes, or half of a degree, in diameter. (One degree is approximately the width of a pinky finger when viewed at arms' length.) Once the center of the Sun has set, the upper 15 minutes of the Sun will still be visible for a brief period. This lasts longer when viewed over the ocean, as due to the curvature of the Earth, the Sun is setting below the astronomical horizon - an imaginary plane perpendicular to a line pointing directly above the observer.
When the Sun reaches six degrees below the horizon, the period known as civil twilight begins. Although the Sun is no longer directly illuminating the ground, it is continuing to illuminate the upper atmosphere. During civil twilight, there is still enough illumination in the sky for most outdoor activities without the need for artificial lighting. Bright celestial objects such as Venus and Sirius will become visible. Also visible during this period is the shadow cast by the Earth on the atmosphere, which appears in the sky opposite the setting sun as a purple or dark blue band near the horizon.
Civil twilight is used in many local jurisdictions as the official legal beginning of night for national and local laws on headlight use, aviation safety, streetlight lighting, noise restrictions, and where criminal laws specify stiffer penalties for crimes committed at night.
The period after civil twilight, when the Sun is between six and twelve degrees below the horizon, is known as nautical twilight. During this stage of twilight, the sky has darkened considerably and outdoor activities are no longer possible without artificial lighting. The name "nautical" refers to the fact that at this time, navigation at sea via the horizon is impossible.
Nautical twilight is also critical in the planning of nighttime military operations. The Army Field Manual lists the acronyms BMNT (Begin Morning Nautical Twilight) and EENT (End Evening Nautical Twilight) as key pieces of data to be collected when gathering intelligence about a location.
The final stage of twilight, when the Sun is between twelve and eighteen degrees below the horizon, is known as astronomical twilight. Though skies are generally dark enough during this time to see planets, stars and other bright sources of light, dimmer objects such as nebulae and galaxies are not yet visible. The ideal period for astronomical observation of these objects occurs only when the Sun is greater than eighteen degrees beyond the horizon after sunset or before sunrise.
Visibility of celestial objects during astronomical twilight is also dependent on other factors such as light pollution, humidity, and the phase of the moon. In remote, arid areas during a new moon phase, stars up to 6th magnitude - the dimmest stars that can be seen by the naked eye - are visible during astronomical twilight.
The Duration of Twilight
Twilight can last from several minutes to several hours, depending on the latitude of the observer. In the polar regions, twilight conditions can persist for a couple of weeks before and after the perpetual night of the winter solstice. Near the equator, on the other hand, twilight can pass in about twenty minutes. Web sites such as Weather Underground or the NOAA Solar Calculator can provide twilight times for any location on Earth.
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