10 things you maybe didn’t know about rainbows

I am currently reading the most excellent “The Sun’s Heartbeat” by astronomy writer Bob Berman. It’s the kind of book I’d love to be able to write, informative, entertaining, engaging and witty. In it, Berman explains the solar things you may always have wondered about from total eclipses (far more astounding than any partial) and sun spots to the inner sun and the secrets of rainbows. Rainbows you will remember are formed by the refraction of sunlight through water droplets in the sky, what is more they don’t exist…without you. Every rainbow that ever there was was there because someone glanced up and saw it. But, rainbows are even more strange and weird than that cynical glance might suggest. Here are Berman’s 10 things you probably didn’t know about rainbows:

1 Rainbows are seasonal, being observed late on summer afternoons and extremely rarely in winter

2 You will never see a rainbow if the sun is more than half way up the sky, so you won’t catch sight of one between 9am and 4pm in early summer

3 If the sun is low, the rainbow will be brighter and the colours deeper

4 The sky above a rainbow is much darker than the sky within its arc

5 The top of a rainbow can never be higher than 42 degrees from ground level, such high rainbows seen at sunrise or sunset lack depth in the blue part

6 The ends of a rainbow stop at the ground only because that’s where the rain stops. A rainbow at a waterfall can form a full circle 82 degrees in extent

7 Double rainbows are not that rare. The second bow appears 9 degrees outside the main bow and the colours are reversed. A third bow within the first is never formed but you might see alternating green and pink fringes (non-spectral colours) known as supernmumerary arcs

8 The gap between a primary and secondary rainbow will be darker than the rest of the sky, this is Alexander’s dark band, named after Alexander of Aphrodisias (a great factoid for taking on a date, perhaps?)

9 Rainbows are like vampires, they have no reflection and cast no shadow. If you look at a rainbow in a mirror, it’s a different rainbow

10 Every rainbow is an arc, part of a circle and at the centre of the circle is no pot of gold, but something quite shocking. Think about it. The sun is directly behind you, the rainbow lies on an arc parallel to the base of a cone the apex of which is the point at which you stand. So, what’s at the centre of a rainbow? The shadow of your head, of course!

The Kindle version of Bob’s book can be found here: The Sun’s Heartbeat: And Other Stories from the Life of the Star That Powers Our Planet

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4 thoughts on “10 things you maybe didn’t know about rainbows

  1. @Alan Exactly. Rainbows are not things. They don’t exist unless someone is looking and that’s not just a philosophical sound of a falling tree in a desserted forest scenario. A rainbow requires three things: light, a refractive medium, an observer.

  2. @Matt That circle around the sun is known as a halo. It wasn’t a rainbow. Similar but not the same. Halos, glories and sundogs was to be the subject of a follow-up post.

  3. I have to disagree with # 2. This Spring I was by a rocky coast and there were large waves and an on-shore breeze around noon. The sun, almost directly overhead, was haloed by a broad, completely circular rainbow. I presumed that there were enough droplets of water suspended as an aerosol above me to create that effect.

  4. Ref Point 6. ‘The ends of a rainbow stop at the ground only because that’s where the rain stops. A rainbow at a waterfall can form a full circle 82 degrees in extent’. I remember reading once that ALL rainbows are full circles, and they showed a photo taken from space to illustrate this. But I recall thinking at the time, ‘How do we know that the rainbow seen from space is the same as the rainbow seen from Earth? (As pointed out elsewhere, rainbows are ephemeral and dependent on there being an observer. The observer in space is different to the one on the ground.(See also point 9:”If you look at a rainbow in a mirror, it’s a different rainbow”

    Ref Point 7 “Double rainbows are not that rare”. In fact I think they’re quite common. In fact, I can see a double rainbow with every rainbow. This has led me to wonder whether yhey’re becoming more common. (Could there be a link with Global Warming”)

    Ref Point 10 10 Every rainbow is an arc, part of a circle and at the centre of the circle is no pot of gold, but something quite shocking…” As I said in Point 8, I find it difficult to believe that ALL rainbows are complete circles. In fact, there is a tremendous glare where some rainbows appear to touch the ground, leading to me believing that they do ‘touch the ground’ – but the point at which they apparently touch the ground moves according to the observer’s positon. Hence there can never be a unique place where the pot of gold could be :-(

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