The University Record, December 12, 1995
December’s cold weather gives Michigan skywatchers an opportunity to see sun dogs, light pillars and halos around the sun and moon. Caused by tiny ice crystals suspended in the air overhead, these light displays usually can be seen several times each week all winter, according to astronomer Richard Teske.
“During the winter, it is common to see a bright ring or halo encircling the sun in daytime or the moon at night,” Teske says. “When the cold air is tranquil and slightly hazy and the sun and moon shine brightly, but their edges seem fuzzy, look for a large, rainbow-like halo surrounding them.
“To see the daytime halo around the sun, stand with your head in the shade of a house or sign and hold your hand up to block out its direct rays. Never look directly at the sun,” Teske warns.
No such precautions are necessary to look for a halo around the moon. “Put out your arm and hold your hand up against the sky. You will find that either kind of halo is about four hand-breadths across from one side to the other.”
Halos are caused by a myriad of tiny ice crystals suspended in the atmosphere above us, Teske explains. Many of them, the size of the smallest bits of beach sand, are six-sided hexagons, shaped like sawed-off lead pencils. In the windless air they float slowly downward. Sunlight or moonlight enters them, passes through, and is bent by the same phenomenon that causes light to be deflected when passing through a slab of glass. “The deflected light seems to come from a position about two hand-breadths from the sun or moon,” Teske says. “This occurs all around the sun or the moon and the total effect is to produce the gauzy halo seen against the sky.”
A related phenomenon called light pillars becomes visible when tiny flat icy flakes float downward in windless air with their flat faces nearly parallel to the ground. “Look for a pillar of light above and below the sun and moon while they are rising or setting. The portion of the pillar above the sun or moon is usually short and stubby. Below the sun or moon, the pillar may stretch down to the horizon.
Pillars appear when sunlight or moonlight reflects off the flat sides of flakes of ice suspended in the atmosphere,” Teske explains. “Sometimes at night, when winds are calm and the air full of ice crystals, one can even see light pillars above or below streetlights and yard lights.”
Sun dogs are the brightest of the winter atmospheric displays. “They usually appear in pairs two hand-breadths on either side of the sun when it rises or sets behind a very thin veil of high clouds composed of ice crystals,” Teske says.
Sun dogs are distinctly red on the side closest to the sun and bluish-white farther from it. They also are caused by hexagonal ice crystals, distant ones this time, floating downward in the air with their flat sides parallel to the ground. “Sunlight enters the thinnest part of the flakes and reflects around inside them two or three times before it emerges to bring you concentrated sunlight from a part of the sky far from where the sun is actually located.”
