Wednesday, 15 June 2011

Lunar Eclipse

Tonight we'll have the good fortune to see a beautiful crimson moon in the sky thanks to the lunar eclipse which is taking place. Depending on the amount of light scattering it could look something like this:

I can't help thinking of fruit when I see this picture. It looks delicious.

In solar eclipses the moon appears to block the Sun for a few minutes to an observer on Earth. During a lunar eclipse the order is changed around a bit - the Earth blocks the Sun from the view of an observer on the moon. This means that the light from the Sun has to travel through Earth's atmosphere to reach the moon for a few hours with some interesting effects.

The red colour is caused by a scattering of light travelling at smaller wavelengths, exactly the same effect as you see in a good sunset. Here's a chart about electromagnetic radiation for anyone who is a bit rusty on their physics:

Basically all you need to know is that blue light travels with a higher frequency than red light and it will be scattered at a greater angle than red light, like you see in prisms or on certain Pink Floyd album covers. This means that red light will converge at a point behind the Earth like this:

And it also explains how the moon can appear a lovely pastel blue colour sometimes. Our solar system is certainly a picturesque example of a chance accident if nothing else.

The solar eclipse happening this evening will definitely be one to watch - the amount of refraction the light undergoes depends on the amount of dust in our planet's atmosphere. We've had one or two notable volcanic eruptions recently so the ash clouds from those will intensify the colour seen. If you're in an area affected by the eruptions of Chilean and Icelandic volcanoes or a sandy desert area you may have noticed an intense red appearing in recent sunsets. By the same principle we can expect to see a deep vermillion hue to the moon tonight because of the increased amount of dust in our atmosphere.

Beautiful. I hope these damn rainclouds clear away soon so I can see it too.

(In other less science-y news, I managed to complete a calculus practise paper without completely freaking out today. Maybe things will improve this week after all.)


  1. Tut, tut, tut. There's no such thing as "blue" or "red" light. Haven't I taught you anything? :-)

  2. Think of it as shorthand for EM radiation of a specific wavelength then. Entirely laziness on my part. ;)