Jim Renda, age 17, of Youngstown, Ohio, for his question:

What is the gegenschein?

The gegenschein is the zodiacal light, alias the counterglow. The zodiac gives a clue to its whereabouts. We expect to see it up in the sky, located among the constellations of the zodiac. But let's expect to observe it only on rare occasions. In our latitudes, we are more likely to see it in early evenings during March and April and again in September and October, this time just before dawn. Its pale light seems to fan up from the sun, which is just below the horizon. It slopes through the zodiac, sometimes to the top of the sky.

The light of the gegenschein is no brighter than a sixth magnitude star, which is the dimmest star visible. It cannot be seen when the sky is illumined by a full or almost full moon. The sky must be clear of clouds and dust and unblurred by city lights. Even then, the pale zodical light is barely visible and lasts but a short while. Astronomers explain it as sunlight reflected from meteoric particles that swarm around on a level with the earth's orbit.