Astronomy Blog

Moon halo

Tonight isn't the clearest night of the year despite being very chilly. It isn't good for observing as there is quite a lot of high-level wispy cloud and plenty of airplane contrails. However, I had just got on my bicycle to go home from work when I saw a pretty amazing sight in the sky. Luckily I wasn't going fast, so I managed to stop before I went into a nearby hedge. That'll teach me to look up at the night sky whilst cycling. But what did I see? There was the Moon in the east with Mars about 20 degrees or so away around to the right. Near Mars was something that looked a bit like a curved vapour trail. Looking more carefully I realised that it made a full loop of the Moon. This was no vapour trail but a Moon halo.

Just as rainbows are caused when light from the Sun is refracted by raindrops, crystals of ice high up in the atmosphere can refract moonlight to create bows and dogs. Whereas rainbows require raindrops in front of you and the Sun behind your head, halos are seen in 22° arcs around a bright object such as the Sun or around the almost full Moon whenever there are hazy cirrus clouds about. As the light is refracted, different colours are bent through different angles just as in a prism. Although this happens with a Moon halo, the light is very weak so is difficult to pick up by eye. Hopefully, in the image above you can see the colours. Frankly, I am amazed that this came out on my camera.

The other feature of the photograph is a airplane contrail that was created as I was setting up my camera. If you look carefully, you can see that it even cast a shadow on the clouds below!

For great atmospheric effects such as 'upside down rainbows', supralateral arcs and pillars check out Les Cowley's site.