Science of Sunsets | Have Bag, Will Travel

It had been a mixed-bag sort of day to day in Lincolnshire. The day started badly with grey skies and a lot of rain and by lunchtime I wrote the day off as a wash out, cancelled golf and prepared for a long afternoon. Then quite unexpectedly the weather changed and the skies cleared and I realised I had been a bit hasty but at least it gave me an afternoon in the garden to spend time reading a book about Thomas Cromwell that I have been struggling with and enjoying the magnificent skies.

Lincolnshire is a rather old fashioned and traditional County situated on the far east of England where the landscape is mostly flat and some of it almost entirely below sea level. One of the things that I had to get used to when I moved here was the absence of roadside hedgerows which for one thing makes driving a different two dimensional experience without trees and hedges to define the boundaries of the road.

What this means is that there are very big skies and everywhere you look it accounts for about 80% of the landscape. These are the skies of the Dam Busters and the Red Arrows, huge, majestic and endlessly blue.

Famous Lincolnshire people who have also looked at these skies include Henry IV (born at Bolingbroke Castle), Isaac Newton (the scientist), Matthew Flinders (the explorer who named Australia; not many people know that), Margaret Thatcher (the first woman Prime Minister, or whatever else you want to call her), Nicholas Parsons (Television host of Sale of the Century) and Geoff Capes (the Olympic athlete).

This day was exceptionally blue and the clouds were magnificent, rising high like great cathedrals in the sky and if you have ever wondered why the sky is blue this is the reason:

Light travels through space in a straight line for as long as nothing disturbs it and as it moves through the atmosphere it continues on its journey until it collides with a bit of dust or a gas molecule and then what happens to the light depends on its wavelength and the size of the thing it hits. Dust particles and water droplets are much larger than the wavelength of visible light and when light hits these large particles, it gets reflected in different directions. Gas molecules however are smaller than the wavelength of visible light and when light hits them, some of it gets absorbed and then the molecule radiates the light in a different direction. The colour that is radiated is the same colour that was absorbed but colours are affected differently because blues are absorbed more easily than reds.

You might need to read that paragraph again!

This process is called Rayleigh scattering and is named after Lord John Rayleigh, an English physicist, who first explained it a hundred and thirty years ago. The blue colour of the sky occurs because the absorbed blue light is radiated in different directions and gets scattered all around the sky and since we see the blue light from everywhere overhead, the sky looks blue. It’s as simple as that!

So what about sunsets? Well, as the sun begins to set, the light must travel farther through the atmosphere before it gets to us and more of the light is reflected and scattered. As less reaches us directly, the sun appears less bright and the colour of the sun appears to change, first to orange and then to red and this is because even more of the short wavelength blues and greens are now scattered and only the longer wavelengths are left in the direct beam that we can see.

What makes it even more dramatic is that the sky around the setting sun takes on a lot of different colours and the most spectacular shows occur when the air contains many small particles of dust or water because these particles reflect light in all directions and then as some of the light heads towards us, different amounts of the shorter wavelength colours are scattered out and we get to see the longer wavelengths and the sky appears red, pink or orange.

And that explains a sunset!

Isaac Newton would be proud of me.