David Todd., Age 12, Of Pierson, Ia., for his question:

What is the zodiac?

Each night, an orderly parade of stars rises in the east and sets in the west. You can see part of the parade in an hour and still more if you watch all night. One section of the starry parade is called the zodiac; and if you watch for a whole year, you see the entire zodiac swing over the sky.

The word zodiac means circle of animals. You can understand it if you pretend that the orbiting earth is a giant carousel. The zodiac animals are no more real than our gaudy carousel animals. They are 12 starry constellations. With each orbit, our carousel earth gives us a nighttime view of the entire zodiac, and the 12 constellations seem to circle over the sky with the changing seasons.

From the face of our spinning globe, we look out at the starry heavens in all directions. The earth's orbit is almost level with the sun's equator. This belt in the heavens is called the ecliptic, and it is the path in which our sun and the major planets parade over our skies. The axis of the spinning earth is tipped at an angle to its orbit, which is the plane of the ecliptic  and this makes the apparent path of the sun slope over the sky.

In one orbiting carousel ride, the earth swings clear around the sun. The view of the sky changes as we circle around. The sun appears against a changing background of fixed stars in the far distance. In one year, the sun passes through the 12 constellations of the zodiac. In early summer, the sun rides over the sky against the background of taurus the bull, outdazzling its glittering stars. At night, we see the constellation of scorpius, which is on the opposite side of the zodiac circle.

In the rmote past, all the constellations of the zodiac may have been named for sacred or totem animals. Some of the names have been changed to represent legendary people, but the zodiac constellation libra represents a pair of scales.

The circle of stars along the ecliptic is divided into 12 edual sections called the signs of the zodiac. In the past 2000 years, the earth's axis has moved a little  and this has shifted the circle of animals slightly westward. The constellation aries the ram now occupies the sign occupied back in roman days by taurus the bull.

Each month, the daytime sun hides a different constellation of the zodiac and outdazzles its stars as they ride together over the sky. At night, we fact away from the sun and look at an arch of constellations on the opposite side of the zodiac. In ancient times, each sign of the zodiac was named for the constellation which rode over the sky with the sun.