Marc Gates, age 13, of Nashwaaksis, New Brunswick, Canada, for his question:

What causes a lunar nimbus?

Sometimes the golden moon wears a silvery nimbus, like a misty white halo. These ghostly appearances always match certain conditions in the earth's atmosphere. This suggests a relationship between them and the weather. Suppose we keep detailed records of nimbus observations through several seasons, including cloud formations and the particular color of the sky. These records indicate that a lunar numbus is likely to occur with certain weather conditions.

The earth, as seen from the moon, always wears its atmosphere in a cloudy halo. The moon, of course, has no airy atmosphere. Hence, the nimbus we sometimes see around it cannot be hovering above the lunar landscape. Nor is it provided by the vacant space between us and the moon. However, several hundred miles above the earth, this space is occupied by the upper reaches of the earth's atmosphere. These airy outer layers can color the sky with glimmering auras. But they cannot create a nimbus around the moon or the sun.

This artistry requires the moist, dense air of the lower atmosphere. The suitable conditions occur in the weathery troposphere that reaches from five to about 11 miles above the earth's surface. The air must be calm, cool and loaded with icy moisture. These conditions tend to occur in large low pressure cells, most often in winter. Chilly air aloft freezes misty moisture and gaseous molecules of water vapor. The upper troposphere is then a mixture of air and miniature ice crystals. We may not see them, though sometimes they cast a thin milky haze over the blue sky.

These frozen fragments may be visible or invisible    but they must behave like crystals of ice. They refract and reflect beams of white light, somewhat like swarms of miniature glass prisms. These effects reach our eyes at certain angles, in relation to the direction of light beam coming from the moon. Hence, the nimbus does not appear to change, even if we walk several miles. Actually, the hazy halo is built and rebuilt every moment as countless ice crystals bend countless moonbeams in the same angles of refraction.

The most usual lunar nimbus is a milky white circle with a radius of 22 degrees. Sometimes the atmospheric condition bends the refracted light to form a circle 92 degrees wide, large enough to encompass half the height of the sky. Though the minuscule ice crystals act like clouds of tiny glass prisms, the lunar nimbus is usually white. However, on rare occasions it shows some of its rainbow colors. A well developed 22 degree circle may be red on the inside, fading to yellow around the rim.

Sometimes the display is far more elaborate. The lunar light beams may be defracted and scattered to form circles within circles. On rare occasions, the airy ice crystals bisect the circles with wide arcs. However, these arcs are not related to a true lunar bow. This colorful moonbow is created by liquid droplets of atmospheric moisture.