March 06, 2003
The above photo was taken from Cologne Germany on February 17, 2003 and shows an impressive lunar corona. This aureole of light results from diffraction phenomena. Minute water droplets in clouds may interfere with light rays and produce interference rings. The size of the rings depends not only on the wavelength of light but also on the size of the droplets - smaller droplets produce larger rings. A corona is usually bluish closest to the Moon (or Sun) and often terminates in a reddish outer ring, as is the case on this photo. Whereas halos are at least 22 degrees away from the Moon and are caused by refraction in ice crystals, in coronae, the rings of light appear very near the Moon. See the Earth Science Picture of the Day for February 6, 2002.