April Sky: Halos and Dogs

Clouds and a full (or nearly so) moon are required for a sight almost as rare as a werewolf: the elusive moon dog.

By Corey S. Powell

From the April Issue of Discover Magazine

 

 

For professional astronomers, cloudy weather typically means lost work. For urban skygazers, it can mean nights of exceptional beauty. Some of the loveliest sights result from the high, wispy cirrus clouds that often precede storms. 

These clouds can contain two types of six-sided ice crystals: platelike, planar ones and elongated, pencil-shaped ones. The crystals reflect and refract light, though at night only the near-full or full moon is bright enough for these effects to be visible. 

Planar crystals, which lie roughly horizontal as they fall, occasionally form delicate “moon pillars,” columns of reflected light extending above or below the moon when it is low in the sky. The pencil crystals refract the light to produce a ring 22 degrees away from the moon in all directions. (The full halo stretches almost exactly one-fourth the way across the sky.) Such halos occur with surprising frequency. 

Less often, fuzzy echoes of the moon—known as “moon dogs” or paraselenae—also appear 22 degrees to the right and left of the halo, the result of moonlight refracted sideways through the planar crystals. Like prisms, the crystals spread red light apart from blue, lending moon dogs a faint rainbow sheen. Alas, human night vision is not sensitive to color, so most people cannot see it. Can you? Scan the eastern sky around the full moon on evenings starting around April 25 to find out.