Albert Sheppard, age 11, of Atlanta, Georgia, for his question:

What exactly is the zodiacal light?

From the earth's surface, the zodiacal light can be seen at different angles in different parts of the sky at different seasons. Except on special occasions its pale and ghostly glow is hard to define. But a glimpse of it is well worth the trouble of checking its schedules and either rising before dawn or finding the right place to view its appearance after sunset. Don't expect the zodiacal light to rival a dazzling aurora    but its cause is just as fascinating.

We can see the zodiacal light only when the sky is very clear, free of smog and smoke and clouds. The moon, even a slim small moon, out dazzles and hides its thin, ghostly white glow. But when conditions are just right, we may see this pale light, aide at the horizon and tapering up over the sky. In North America, we may see it in March and April after sunset. Its wide base glimmers above the western horizon and tapers to nothing before it reaches the top of the sky. We can also search for it again through September and October. However, the fall showing appears on moonless mornings above the eastern horizon and we must rise before dawn to see it.

At the equator, the zodiacal light puts on a more noticeable show. It is much brighter and visible throughout the year. When the moonless sky is dark and clear, it may arch high overhead, spanning the heavens from the eastern to the western horizon. Farther from the equator, in the northern and southern hemispheres, we notice that the slanting angle of the zodiacal light follows the angle of the sun's path over the heavens. This path across the celestial sphere is called the ecliptic.

The sun's path seems to move up and down in the sky because the earth's axis is tilted 23 1/2 degrees to the plane of its yearly orbital path around the sun. Therefore, we see the sky and everything in it from a tilted planet. Through the seasons a series of 12 different constellations parade along the sun's path, as well as the moon and the major planets. This belt is eight to ten degrees wide, or 16 to 20 times the width of the full moon. Its angles and the angle of the ecliptic are related to the angle of the earth's axis. And this highway in the sky is the zodiac.

The zodiacal light gets its name from the zodiac because it too angles along the same celestial highway. To the naked eye its light is dim but it shows up well in the telescope. A spectroscope reveals that its pale glimmer is reflected light. What we see is sunlight bouncing from countless trillions of meteors. Some are sizeable bodies several feet wide, but most of them are dusty fragments, measuring perhaps 25 to an inch and maybe as much as five miles apart. This cloud of meteors and meteor dust forms a flat lens around the sun's equator, reaching far out past the inner planets. It is thickest near the sun and most of its fragments are inside the earth's orbit. At certain seasons from certain angles we see reflections from this cloud of meteor fragments, and this is the zodiacal light.

The word zodiac dates back to antiquity when the 12 seasonal constellations were named for sacred animals and symbols. Perhaps it has nothing to do with coming events of the earth's history, but we can visualize its location by extending the earth's orbit out into the celestial sphere. Allow for the tilt of our axis and we can figure out where and probably when to expect the zodiacal light.