Astronomy Blog

Orange Moon

So much for green cheese. Last night, just a couple of days short of full moon, the Moon was hanging serenely over the houses that I see from my bedroom window. It was looking decidedly orange.

The Moon isn't actually orange, or made of green cheese for that matter, but sometimes it can look so. The reason is the Earth, or rather the Earth's atmosphere. Imagine you were stood on the Moon looking towards the Sun. Obviously, I don't advise that you normally look directly at the Sun, but this is our imagination so go with it.

At full moon, the Moon is almost on the opposite site of the Earth to the Sun although not quite all lined up. That means that to get to the Moon, light from the Sun has to skim through our atmosphere. It has to go through a lot of atmosphere and, for the same reason that sunsets look reddy-orange, the light which gets to the Moon is slightly orange tinged; some of the blue part of the spectrum has been filtered out by Earth's atmosphere. [Edit 20/8/2005] The light has to come back, through a lot of atmosphere, to your eye. It is this last trip which will do the rest of the reddening. That way, the amount of reddening will depend on your local pollution levels - different countries will see different colours. The Moon should also get less red as it gets higher above the horizon as there is less atmosphere for the light to travel through.