Everyone knows that you can't see an aurora during the day time. The faint light produced by these displays an never compete with the intensity of the daytime light. They can only be seen by those prepared to stay up late into the wee small hours of the night.

But what if you were sitting in the late afternoon one July, looking out towards the headlands of the Giant's Causeway and you saw this? Bands of orange and red over green, just like in an aurora, with what appears to be structure and maybe even pillars in it? Would you think that, mysteriously, the laws of physics had suddenly changed and you were bearing witness to the world's first day time aurora? Or would you suspect something else...?

Of course, it's not an aurora. After all, everyone knows you can't seem them during the day time! So, what is it then? If you saw this, you would in fact be looking at a rainbow. Granted, not the usual form we expect them to take. But a rainbow nonetheless. So what's going on here? How do rainbows take this form? The answer, apparently straight from a Douglas Adams book, has got to do with the number 42...

How rainbows form

To understand this, we need to cover a bit of basic explanation of how a rainbow forms. Everyone knows they are produced by the refraction of light through droplets of water - different wavelengths of light are refracted to differing degrees, hence the full range of Richard Of York Gave Battle In Vain colours appear visible.

Rainbow shapes

But what shape are rainbows? That's easy, you might say. They're bow shaped. The clue is, after all, in the name. Well, yes, but the actual shape of rainbows is circular. The bow we see is only a part of the full display. Here's where the number 42 comes in: rainbows have a radius of 42° and their centre point is directly opposite to the position of the sun in the sky (a point known as the anti-solar point). Early in the morning, or later on in the evening, when the sun is low in the sky, the anti-solar point comes closer to the horizon, and we end up seeing the classic rainbow shape, with much of the semicircle being visible above the horizon.

However, as we move closer to the middle of the day, and especially in the summer months, the sun gets higher and higher in the sky, and so the anti-solar point falls lower and lower below the horizon. As a result, the section of the rainbow circle we can see gets smaller and smaller. In the middle of the day during the summer, it falls below the horizon, and the rainbow is no longer visible from sea level. 

As we move towards the late afternoon and evening, the sun gets lower again in the sky, and the anti-solar point begins to rise again. And yesterday, at around 4.30 pm, the anti-solar point had risen high enough for the top of the rainbow to be visible along the horizon once more - at just the same time that a rain shower to my west was lit from the sun from behind me to form a rainbow! 

The sun at this stage must have been just slightly less than 42°  high, and the top of the rainbow began to emerge as a band of colour on the horizon. In fact, if you look at the first image I shot, you'll see that only the "Richard Of York Gave" colours are visible - hence the decidedly aurora-like colour of green below red in the sky. 

However, over the next 10 minutes or so while the rainbow was visible, the sun continued to drop in the sky, the anti-solar point rose, and the other "Battle In Vain" colours became visible too!

If you compare the first photo I took with the last one, you'll see how much the rainbow has risen in the sky during these 10 minutes. Rainbows move up/down all the time when we're viewing them normally as the sun moves in the sky. But, as we have the horizon as a frame of reference here, it's much easier to observe this happening here.

Click/tap on the arrows to compare the start and end of the display. Can you see how many more colours appeared as the rainbow moved higher in the sky?

So, not a daytime aurora then. That's impossible. But perhaps, like me, you'll still find this a fascinating natural phenomenon, and yet another reason to find beauty and wonder in the natural processes going on all around us!

The low bow appearing on BBC TV. Thanks again to Chris Ibottson for taking this picture for me.

The low bow appearing on BBC TV. Thanks again to Chris Ibottson for taking this picture for me.


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