[Science Report]:The Motion of Mars


This is the VOA SpecialEnglish Science Report.

Last week, we explained that the planet Mars has passed"opposition." It passed a point opposite the Sun. This week, we tellabout 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 thatplanets travel among the stars. The planets generally follow thepath taken by the Sun through the sky. The Sun's path is called theecliptic. The groups of stars, or constellations, along the eclipticare called the Zodiac.

The motion of the planets can be confusing at times. Everyoneknows the Sun rises in the east and sets in the west. But this iscaused by the turning motion of the Earth. Planets generally movefrom west to east.

However, Mars will appear to move backwards for about two monthsthis year. This happens because the Earth is overtaking Mars on itsway around the Sun. Mars began its backward, or westward, motion onMay eleventh. It will start moving eastward again on Julynineteenth.

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

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

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

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

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

This VOA Special English Science Report was written by MarioRitter.

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