Moonrise with Green Rim

May 14, 2010

Sounion moonrise and greenrim

Photographer: Chris Kotsiopoulos
Summary Author: Chris Kotsiopoulos; Jim Foster

May 2010 Earth Science Picture of the Day Viewer's Choice

The photo above shows a glorious full Moon on the rise, looking like the Sun, as viewed from the Temple of Poseidon at Sounio, Greece. I was positioned approximately 1.25 mi (2,000 m) to the southwest of the Temple as this golden Moon eased above the eastern horizon. A close inspection along the top edge of the Moon reveals a tinge of green. At moonrise, and more frequently at sunset, a sharp eyed observer can sometimes detect a ray of pure green light atop the lunar rim -- magnification is usually needed. Green flashes and or rims occur as a result of vertical dispersion in the atmosphere. The amount of atmospheric refraction is wavelength dependent and so the blue image of the rising Moon is ever so slightly higher than the green and red images. Each is just one arc minute displaced from a non dispersed image. As the Moon rises (or Sun sets), the green image briefly comes into view before giving way to the red image. Of course, green flashes can be seen at sunrise or moonset as well. Photo taken on April 28, 2010.
Photo details: Canon Xti camera; ISO 200; 1/2 second exposure; SW ED 80; 20:39 local time.