Many a beach vacation cocktail hour is spent gazing at the sunset in hopes of catching the elusive green flash that occurs just before the last bit of sun disappears below the horizon. Now, a photographer at the European Southern Observatory has caught the green flash from the moon.

The series of images was made by Gerhard Hüdepohl, a photo ambassador at the observatory in Chile's Atacama Desert. An image advisory describes them "as very probably the best ever taken of the moon's green flash."

The effect is due to how the Earth's atmosphere bends, or refracts, light, an effect that is greater in the lower, denser layers of the atmosphere.

ESO explains:

"Shorter wavelengths of light are bent more than longer wavelengths, so that the green light from the Sun or moon appears to be coming from a slightly higher position than the orange and red light, from the point of view of an observer. When the conditions are just right, with an additional mirage effect due to the temperature gradient in the atmosphere, the elusive green flash is briefly visible at the upper edge of the solar or lunar disc when it is close to the horizon."

For more details on the physics and history of the green flash, check out this explanation on the Hyperphysics website and then schedule a beach vacation, relax and look for it yourself.