Blue Sky

Moon in daytime sky
Moon: Day and Night

Can you see the difference? Moon in the day sky on the left, Moon in night sky at right.

When the Sun is above our horizon, what we usually call day time, the sky is typically blue in colour. We are so used to seeing it that we barely give is a second thought most days, but I hope at some point you may have stopped to consider this and wondered “Why blue? Why not green or red or yellow?” At a simple level it’s the oxygen (and nitrogen) in our atmosphere scattering the light of the Sun in all directions as it comes through the atmosphere. And perhaps we should note that there is a violet element in there too, but our eyes are generally not able to see into the ultraviolet part of the spectrum so we don’t notice it.

John Tyndall, the person to explain the why of this was born on 2nd August 1820. He became the Professor of Natural Philosophy at the Royal Institution in London in 1853 and it was from there he made significant contributions to our knowledge of how the atmosphere works. As well as explaining the process of heat retention we know as the Greenhouse Effect he also showed how light is scattered but impurities in the air.

It’s one thing to read all this in theory, but another to see it in action. One morning I had my telescope out early to catch a comet before dawn. I did this successfully, and after the Sun had risen I noticed the waning Moon up on high, and swung the telescope around for a look before I put it away. Instead of the crisp, black shadows I was used to seeing on this stark world at night I could only get a hazy, faded image in the eyepiece. The sunlight scattered by the atmosphere seemed to thicken the air into a visibly opaque barrier and no amount of focus adjustment was going to give me a clear view. It was a reminder that we really do live in a sea of invisible gas we know as the sky


Moon in daytime sky

The waxing Moon in the daytime sky.

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