Thursday, October 28, 2010

Twilight - the space between day and night (without vampires)

How many painters have attempted to capture the gradiated hues of a sunset in pigments? It takes true mastery to get translucent fleeting colours from flat pigments; some artists do it exceptionally well, but most don't. Detailed observations of actual sunsets is the key: what colours go where? How do a few clouds change things? Cameras can capture some aspects of a sunset, but often miss the nuances. With my digital camera, I took this picture near the end of my drive to Winnipeg last summer – the sunset was much more stunning in person. However, nothing beats sitting on a patio somewhere with a view (perhaps with an accompanying beverage) and watching day turn to night.

Sunsets are a spectacular end to the day – however the entire process of shifting from day to night is called twilight. According to a book published in 1966 by Georgii Rozenberg (called 'Twilight' without a single mention of vampires – I like to read old science books): The term twilight refers to the entire complex of optical phenomenon that take place in the atmosphere when the sun is near the horizon. It occupies the interval separating daytime conditions of illumination from night.

I live far enough north to get reasonably extended twilights. The downside is that I live far enough north that twilight can start in the late afternoon on the shorter days of the year. Every twilight is unique and the shift from day time brilliance to more subdued hues feels almost magical. During twilight, the illumination at the ground decreases by a factor of a billion. If seen from space, twilight covers a global swath separating day from night. Twilight happens because the earth is rotating – so it will occur on every rotating planet with an atmosphere.

Looking up at the sky has been a pass-time for eons. However, early in the 21st century, before spaceflight was common, a keen interest in studying twilight emerged to provide details about the composition of the atmosphere – useful to know if you are trying to communicate by radio.

Many interesting phenomenon occur each twilight (I'll write more in other posts), however sunsets are the most obvious. Atmospheric optical properties are responsible for the vivid colours of sunset. Specifically, the amount of water vapour and dust play a huge role. In 1863, atmospheric scattering and attenuation of light were shown to produce the sunset colours. Since entire books have been written on sunsets, so my description will be brief.

When the sun drops towards the horizon, the sunlight must pass through more atmosphere. Since shorter wavelengths of light are scattered preferentially (see Rayleigh scattering post), the sun appears in redder tones (red is at the long end of the colour spectrum) and the near-by sky takes on yellow and orange hues. When the sun is about 5 degrees below the horizon (like in my picture above), it is out of sight to an observer on the ground. The sky above the horizon remains brightly coloured in deep reds while mountain tops and clouds are bathed with crimson and purple light.

A cool home experiment for generating a sunset in a glass can be found here.

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