Luminous halos around the Moon are nothing unusual, especially in wintertime Finland where the air is so often filled with ice. Crystals of frozen H2O catch the moonlight and bend it into a circular ring of light. A few nights ago, however, Sauli Koski of Muonio, Finland, witnessed a halo that was not circular, but elliptical:
"On Jan. 15th, the weather changed. As the temperature dropped from -7C to -37C, there were all kinds of ice halos to photograph," says Koski. "The best and rarest were these elliptical forms that lasted more than 20 minutes."
Although physicists have been studying ice halos for decades, not all are understood. "Elliptical halos are one of the puzzles," says atmospheric optics expert Les Cowley. " We can simulate them by invoking hexagonal plate-like crystals topped by almost flat pyramid faces. However, the simulations do not fit very well and such crystals are unphysical. Crystal facets like to form along planes where there are lots of atoms or molecules – almost flat pyramids do not fit the bill at all. Perhaps some peculiar distorted snowflake types instead?"
"These mysteries all add to the spice of halo observing, the beautiful, the unexpected, the unexplained, something new!"