And last week in some parts of the world (not mine unfortunately), people were able to watch the moon turn red as it was eclipsed by the Earth's shadow. Why the dramatic change in colour?
Colours of the rainbow
If you think about it, light is a very curious phenomenon. Normal daylight is bright white, yet how weird that it can be split into the many colours of a rainbow by something as commonplace as a rain-shower.
It's all to do with the way that the path of light is changed by particles in its path. As many of us have seen from those prism experiments in Physics lessons at school, white light is actually made up of many different colours which all have different wavelenths. Violet and blue have the shortest wavelengths, red the longest.
When something interrupts the light's path, such as water droplets or particles and molecules in the atmosphere, this causes shorter wavelengths of light travel in different directions to longer wavelengths, so the different colours get separated.
The Green Flash
When the sun is setting, its light is bent (refracted) by the earth's atmosphere. At sunset the light passes at a low angle through the atmosphere, making its path longer and so increasing the refraction effect. As the sun sinks below the horizon, shorter wavelengths are bent most and are the last ones to escape over the horizon. However, the very shortest ones (violet and blue) are scattered by contamination in the atmosphere, so the very last light to be visible as the sun sets can appear as a green flash.But this effect is only usually seen in particular conditions – when there is very good visibility with a distant, uncluttered view of the horizon, and when layering in the atmosphere exaggerates the vertical scale of the separation of colour
The full moon is of course normally bright white. When a lunar eclipse occurs, the Earth is positioned exactly between the Sun and the Moon so that the Earth's shadow falls on the moon. You might expect that the Moon would just turn dark at this time – but in fact it transforms to beautiful glowing copper red. I was lucky enought to be able to observe this magical sight from my home in 2011. This effect is again caused by sunlight being affected by the Earth's atmosphere. If you were to look at the Earth from the Moon during a lunar eclipse you would see a thin ring of atmosphere all around it, and the only sunlight reaching the surface of the moon at this time is refracted light that passes through this layer of atmosphere. All the blue light has been scattered out over Earth (causing our blue skies) – only the longer reddish wavelengths avoid being scattered and come out the other side, falling onto the Moon's surface and making it glow orangey-red. In effect it is a reflection of all the sunrises and sunsets on Earth happening at that time.
It's also possible to see sunlight in the middle of the night in form other than moonlight, though dark skies are a must. The image below shows the ghostly 'Zodiacal Light' . This time Earth's atmosphere is not the culprit - this effect is caused by dust in the plane of the solar system, scattering sunlight in a distinctive pyramid shape pointing up into the sky from the horizon, up to two hours before dawn or after dusk, particularly around the exquinox periods. I haven't been lucky enough to see it myself yet, but will be trying my chances, if I can get up early enough of course!
http://www.mtwilson.edu/vir/parkinglot/greenflash/ (explanation of how green flash is caused)
http://earthsky.org/space/why-does-the-moon-look-red-during-a-total-lunar-eclipse
http://www.skyandtelescope.com/astronomy-news/observing-news/zodiacal-light-dust-thing-beauty101514/4
1. Brocken Inaglory (Creative Commons)
http://commons.wikimedia.org/wiki/File:Big_green_flash.JPG
2. Steev Selby (Creative Commons)
http://upload.wikimedia.org/wikipedia/commons/1/15/Lunar_Eclipse.jpg
3. ESO/Y. Beletsky (Creative Commons)
http://commons.wikimedia.org/wiki/File:Zodiacal_Glow_Lightens_Paranal_Sky.jpg