Dennis Marquardt, age 17, of Kalispell, Mont., for his question:
WHAT IS THE COPERNICAN PLANET THEORY?
For thousands of years, as man studied the planets moving in the night sky, it was believed that the earth was the center of the universe. Then, in the mid 1500s, the Copernican theory presented a new suggestion: the earth and the other planets travel around the sun.
The earlier theory was suggested about A.D. 150 by Ptolemy, a Greek astronomer. He and other scientists of the time thought it was obvious that the earth was the center of the universe. He thought the sun and the planets traveled around the earth once a day.
Ptolemy's theory explained what men saw in the sky. It guided people's thinking for more than a thousand years.
In 1543, Nicolaus (CQ) Copernicus, a Polish astronomer, suggested that the earth and the other planets moved around the sun. This theory made it easier to describe the motions of the. planets, and other astronomers soon began to use it.
Religious leaders called Copernicus a fool for saying that the earth was just another planet. They forbade the use of his writings until 1757.
Discoveries by other astronomers gradually convinced people that the Copernican theory was indeed correct. The Copernican theory gained support after Sir Isaac Newton of England devised his law of universal gravitation about 1665. This law described the sun's pull on the planets.
As time passed, it was discovered that the temperature, atmosphere, surface features, length of days and nights and other conditions on each planet varied widely. The conditions depended on the planet's distance from the sun, its atmosphere and rotation.
The planets nearest the sun received more heat than those far away from it. The temperature on the closest planet, Mercury, rises to about 67,235 degrees Fahrenheit during the day. Pluto, more than 100 times as far from the sun as Mercury, is more than 300 degrees Fahrenheit below zero.
All of the planets move around the sun in the same direction. Also, each planet rotates as it revolves around the sun.
The planets' rotation periods, or the time required to spin around once, range from less than 10 hours for Jupiter to 243 days for Venus. The earth, of course, rotates once every 24 hours.
Each planet spins around the rotational axis, an imaginary line through its center. The rotational axis is not perpendicular, or at an angle of 90 degrees, to the path of the planet's orbit. It tilts at an angle from the perpendicular position.
The earth's axis tilts about 23.5 degrees. Because of the tilt, the equators of earth and the other planets do not always face the sun directly. As a result, the northern and southern halves of the planets are not heated evenly throughout the year. This uneven heating by the sun produces the changes on the earth that cause the spring, summer, fall and winter seasons.