This is the VOA Special English Science Report.

Last week, we explained that the planet Mars has passed "opposition." It passed a point opposite the Sun. This week, we tell about the planet's surprising motion among the stars.

Our English word "planet" comes from the Greek word meaning "traveler." For thousands of years, people have recognized that planets travel among the stars. The planets generally follow the path taken by the Sun through the sky. The Sun's path is called the ecliptic. The groups of stars, or constellations, along the ecliptic are called the Zodiac.

The motion of the planets can be confusing at times. Everyone knows the Sun rises in the east and sets in the west. But this is caused by the turning motion of the Earth. Planets generally move from west to east.

However, Mars will appear to move backwards for about two months this year. This happens because the Earth is overtaking Mars on its way around the Sun. Mars began its backward, or westward, motion on May eleventh. It will start moving eastward again on July nineteenth.

Mars' apparent motion has been a mystery to astronomers for hundreds of years. Most early theories of the solar system argued that the Sun and planets turned, or revolved, around the Earth. But the sudden westward motion of Mars presented a problem. Why would Mars move west for two months when it nears opposition?

In Fifteen-Forty-Three, a Polish church worker named Nicolas Copernicus published a different theory. His theory said the Earth and planets moved around the Sun in perfectly circular orbits. Copernicus' theory was simpler. But his Sun-centered system still did not explain the observed motion of Mars very well.

Finally, a German mathematician named Johannes Kepler published a complete theory of the motion of the planets in Sixteen-Nineteen. He had carefully studied the motion of Mars for many years.

Johannes Kepler discovered that the planets do not move in circular orbits around the sun. Instead, they travel in flattened orbits called ellipses. Mars' elliptical orbit is the cause of its unusual brightening this year and its apparent large size.

During the next several weeks, you can see for yourself why the mysterious motion of Mars has caused so many people to wonder.

This VOA Special English Science Report was written by Mario Ritter.