Many of us will be well aware of the old weather saying, "Red sky at night, shepherds' delight; red sky in the morning, shepherds' warning. Long before we had satellite images helping us forecast, long before we had little weather symbols on weather maps (whether computer generated, or with little magnetic strips that sometimes wouldn't work right!), people had to rely on their own observations to try to figure out what the unpredictable beast that is our weather might do next.

One of the ways they did so, as our little rhyme reminds us, is through looking carefully at the colours of the sky. And it turns out they were just right. The colours can tell us a lot about what is going on in the skies above us.

Why is the sky blue - and why are sunsets red?

Anyone who has messed around with a prism in a science class at school will know that the light we see as white can be split into a number of different colours (the good old 'Richard Of Tork Gave Battle In Vain"). When a rainbow forms in the sky, what we are seeing is the white light separated out into all these various elements, arrayed in a beautiful sky band for us to enjoy.

Similarly, when sunlight passes through our atmosphere, the various elements of the white light can be separated out by what is known as Rayleigh Scattering. The two most common gases in our atmosphere - nitrogen and oxygen - scatter some of this light. But they tend to scatter the blue wave lengths better than the reds. And this blue light, scattered all through the sky, gives it its blue colour.

When it comes to sunset, as the sun reaches the horizon, these same processes are occurring. But now, the sun's angle means that we are viewing in through more of the atmosphere. And so the effect is more pronounced. In the direction in which we are looking, more and more of the blue light is scattered, leaving only the red wavelengths to get through.

Red moon setting

Which brings me nicely to a wee stroll I was having the other morning up Cavehill for sunrise. More often than not, this location has been very good to me for light. I've had some amazing views from up there. But, this particular morning, there was a lot of cloud to the east as I walked up Belfast's most famous mountain.

To the west, it was a whole other story, however. The skies were clear, and the red glow from the rising sun far to the east had spread out and reached all the way over to the western skies, giving them a beautiful pinkish/orange glow that gradually gave way to a soft blue close to the horizon.

Sights like this can be beautiful enough. But the whole scene had something that just set it all off: a full moon, sinking slowly and majestically closer to the western horizon as it set. As the skies to the east weren't exciting me too much, I decided to turn my attention to this beautiful orb hangning low in our sky.

As I got high enough to get a clear enough view of it, the moon had now dropped into the soft blue band just above the horizon. But the light travelling from our celestial neighbour was now passing through more and more of our atmosphere. And the blue wavelengths of light were being scattered, causing the moon to take on a beautiful pinkish hue which make it stand out beautifully against its bluer backdrop.

Who stole the moon?!

But we're not quite done with our tour of lunar atmospheric optics! Because, if you look closely at the moon in this picture, you'll see that it's not quite round. It appears to have been squashed a bit and is slightly oval shaped. Indeed, the edge is a bit uneven, with what looks like a few nibbles taken out of the sides (a huge lunar mouse chewing on the cheese, perhaps...?!)

This is another effect of the light passing through our atmosphere. The atmosphere is made up of various layers, each of which have slightly different densities. These act like lenses, refracting the light. When the moon (or sun) is higher in the sky, this effect is negliable. But, near the horizon, again as the light is passing through more of the atmosphere, this effect is more pronounced, giving this optical effect. The lower part of the moon (or sun) is refracted more, lifting it up and making the moon appear squashed.

But the effects of the refractions are even more bizzare than that. In fact, the setting sun or moon that you are gazing at in beautiful wonder as it reaches the horizon line isn't actually really there!

Yes, you heard me correctly! It's disappeared already below the horizon.  What you are seeing is its light refracted and bent around the curvature of the Earth by our atmosphere.

Now, that's a concept I find truly amazing. Next time you see the setting sun over the ocean, feel free to stagger your friends with this fact. "Do you know, that sun actually set 2 minutes ago. You are seeing the light from below the horizon bent around for us to see by density differences in the various laters of the atmosphere, you know..." And prepared for their stunned reaction!

Why is the lunar eclipse red?

While I'm on a roll, let me tie together our red moon and light refraction in one last example. I'm sure you remember our last lunar eclipse in September 2015. We were lucky enough in Northern Ireland to have clear skies for that amazing show. I remember so vividly the experience of watching it. The shadow of the Earth progressing inexorably across the face of the Moon, nibbling away more and more of it's glowing white surface. The moon got darker; the night got darker. Until totality arrived - the Earth's shadow had now totally eclipsed the moon.

Only the moon didn't go completely dark. Instead, it glowed a rich red colour. But, if it was in our shadow, where was the light coming from?

A montage of some of the shots I got during 2015's lunar eclipse, when the moon turned red.

Refraction. Our atmosphere was doing what it does as the sun reaches the horizon - it was refracting light, bending it around the sphere of our planet, and sending it on past towards the Moon. But this light was approaching the atmosphere at a very low angle, passing through a greater thickness of our atmosphere. And Rayleigh scattering was allowing just the longer, redder wavelengths to get through. Our atmosphere was bending red light around the planet, flooding the Moon with the red light that it was then reflecting back down to those of us braving the cold night to view this amazing scene.


So, the colours of the sky are beautiful. Who hasn't gazed up at a stunning sky in awe and wonder of the sheer epic majesty of it all. But, like our weather forecasters of old, we can appreciate not just the visual beauty of the sight laid out before us. We can also see beyond that the a deeper beauty - the beauty of the science that lies in plain sight, colourfully calling out for our appreciation too.

 

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