Is the moon ever really blue?

It’s a blue moon tonight, though it’s not really blue or green or made of cheese. It’s just that not so rare, second full moon in a month. I asked my local astronomer in residence if the moon ever appears blue. And guess what? Yes, it can be blue! And it is indeed rare and unpredictable, true to the meaning of the phrase “every once in a blue moon.”

There are several definitions of the term “Blue Moon”. Today we have an example of one of them, “the second full moon in a calendar month”. For me, this occurrence is not very interesting – a statistical artifact of our calender that has absolutely nothing to do with the Moon’s color and isn’t even particularly rare (it occurs every 3 years or so and is entirely predictable). A much rarer, far less predictable, and much more interesting situation can lead to a Blue Moon that is true to its name.

We are all familiar with the fact that the Sun appears red when it is low in the sky, after sunrise or before sunset. This is caused by the Earth’s atmosphere, which removes more blue light than red from the Sun’s rays, so that it appears not only dimmer but also redder than it really is. This happens all the time that the Sun is up, not just when it is near the horizon, but we only really notice it when it is low in the sky because the light passes through more of the atmosphere at that angle. And of course, any astronomical object is affected, not just the Sun – the Moon, for example, also appears dusky red or orange when low in the sky.

What is much less well known is that under certain circumstances the atmosphere can have a very different effect. The way it alters the color of light depends on the size and composition of the particles – aerosols – in the atmosphere that cause the dimming. For example, in a foggy atmosphere the aerosols grow too big to have any coloring effect at all, and just produce a lot of colorless dimming. The smaller particles in a fog-free atmosphere almost always produce the familiar reddening, but just occasionally – especially after forest fires or volcanic eruptions – aerosols that have just the right properties to “blue-en” the light gain the upper hand. But this happens very rarely – only once in a Blue Moon.

Many thanks to Douglas Whittet, my personal astronomer here at FLIFA – the Fletcher Road Institute for Astronomy. 🙂