You may have heard about the green flash you can see when the sun sets. Now a photographer at the European Space Agency has captured a green flash emanating from the top of the Moon. What causes that green light?

The Atacama desert , stretching between Peru and Chile, is possibly the driest place on earth. It has never seen a drop of rain since formal record-keeping began. It has seen, however, a lot of scientists. Astronomers, in particular, find the ultra-dry climate of the desert welcoming. On top of 8500-foot Cerro Paranal, the Very Large Telescope makes use of the lack of cloud cover and atmospheric moisture to get crystal-clear pictures of the sky. Workers at the VLT often see the green flash that accompanies the setting sun on clear days, but ESO Photo Ambassador Gerhard Hüdepohl captured a picture of a phenomenon that's virtually unknown - the green flash of the moon.


Why is the moon flashing green? Technically, it's not. The flash is a trick of the light, made possible by earth's atmosphere. Light often encounters substances that it can move through, like glass or water, but as it does, it's bent slightly. This slight bending is more pronounced the denser the material it is moving through. It is also more pronounced the shorter the wavelength of light. Prisms are extreme examples of this, separating white light into many different colors, widely space, over a short distance. Although it's more subtle, our atmosphere does the same.

The shorter the wavelength of light, the more it gets bent. This separates shorter wavelengths out from longer ones. As it crosses the dense air at the horizon, the separation is made even more dramatic. The green wavelengths of light move along an entirely different path than the longer yellow wavelengths, and people see them as a green flash of light separate from the main body of the astronomical object. Although there are shorter visible-light wavelength than green, they tend to be scattered by the atmosphere itself, and so are not present in the directed stream of light that reaches people on the surface of the earth.


This may be the best picture we have of the moon's green flash.

Credit: ESO/G.Hüdepohl

via the ESO and National Geographic.