Don R. Wiesner, age 16, Of Seattle, Wash., for his question:
What is the zodiacal light?
The sun and its upper gases were studied intensively during the International Geophysical Year. A lot of new information was gathered, and some of our old ideas had to be changed. Some of the thin, solar gases, it seems, reach far out beyond the earth. Our old ideas of the zodiacal light, however, were proved to be more or less correct.
The zodiacal light is a pale glow at the horizon, tapering to a rounded hump half way up the sky. In March and April, it appears in the west just after sunset. In September and october, it appears in the east just before dawn. In our latitudes, it is paler than the dimmest star, and the light of the full moon outshines it, making it invisible. In the tropics, it is Often brighter than the Milky Way.
The zodiac is a wide path over the sky. Inside it travel the sun, moon and most of the planets. Also in this path are the 12 zodiac constellations which change with the seasons. The zodiacal light also glows within the zodiac.
For a long time, astronomers suspected that the zodiacal light was a cloud of dust particles, each reflecting a small glow of sunlight. IGY Scientists planned to test this theory and prove it right or wrong. The eerie glow was studied by three teams in different parts of the tropics. The glow was found to be polarized light, and at first it was assumed that such light must come from electrons, perhaps 600 to 800 electrons per cubic centimeter.
After further studies, the IGY experts concluded that most of the zodiacal light is sunlight reflected from dust particles. There seem to be no more than 20 or 30 electrons in a cubic centimeter, a space equal to about half a thimbleful. The eerie glow comes from a thin, flat cloud of dusty debris which circles the sun on a level with the earth's orbit.
Sometimes it glows in the opposite side of the Sky from the sun. This part of the hazy light is called the counterglow or the gegenschein. In the tropics when the zodiacal light and the counterglow appear together, they sometimes taper to meet at the top of the sky. At such times, they may outshine the arch of the Milky Way.
The zodiacal light is of great interest to space scientists. We know now that the space between the planets and the stars is not an empty vacuum. It is filled with the finer than fine particles of plasma. The zodiacal light reveals other particles to be met and coped with in the vast spaces between the planets of the Solar System.