David Whitten, age 12, of Dallas, Texas, for his question:

Which planet was discovered first?

With our eyes alone, we can see only five of the other eight planets of the Solar System. Three planets had to wait for the invention of the telescope before they were discovered. The five which are visible to the naked eye have been known since before the dawn of history and they were named for four gods and one goddess of the ancient world.

Mankind has been a stargazer from the very beginning. The ancient Babylonians watched the heavens from the tops of tall, tapering buildings. They charted the stars and figured out when to expect an eclipse of the sun. The ancient Egyptians planted their crops when certain stars were in the sky. Many of these early people knew that the starry constellations change with the seasons. They knew that brilliant Orion is a constellation of the winter night sky and that Scorpius is a constellation of summer skies.

The early stargazers also knew the sun appears in the twelve constellations of the zodiac, one after another, throughout the year. All these bright heavenly bodies form an orderly parade which marches across the sky throughout the year. But the ancients also observed five heavenly bodies which did not fit into this neat scheme of things. Their paths wandered this way and that against the background of orderly fixed stars. They were called the planets, a word which means wanderers. They were named Mercury, Venus, Mars, Jupiter and Saturn. And almost everyone thought that the sun, moon, planets and all the stars revolved around our earth.

This idea began to change some 400 years ago. The Polish astronomer Copernicus suggested that the planets were wandering round the sun and so also was the earth.

The idea was proved by Galileo, early in the seventeenth century, Galileo was the first to see the heavenly bodies through a telescope, but he did not discover more than the five planets man had known since ancient times.

Uranus, the planet beyond Saturn, was discovered in 1781 and named for the Greek god of the sky. It was decided to name the newly found planet for an ancient god in keeping with the planets which were so named long ago. Neptune, whose orbit is beyond Uranus, was discovered in 1846 and named for the Roman god of the sea. Pluto, the little planet which pedals around the outside rim of the Solar System, was discovered on January 21, 1930 and named for the Greek god of the underworld. It is just possible to see Uranus with the naked eye, if you know just where to look. But the other two outer planets are always invisible without a telescope,