Cosmic Illusion: Mars to Move Backward

We're now coming into the home stretch of the last good apparition of Mars until 2016. Now blazing in the late-evening east-northeast sky like an eye-catching yellowish-orange "star," Mars is less than six weeks away from its closest approach to Earth during this apparition.

At the beginning of the year, the red planet was 221 million miles (356 million kilometers) from Earth. This week, it will be 63 million miles (102 million kilometers) away and it now shines some 10 times brighter than it did on New Year's Day.

Since Jan. 1, Mars has progressed more than halfway around our sky and now is on an easterly course through the background stars of the Zodiac. It currently resides smack in the middle of the constellation of Gemini, the Twins.

But on Thursday, Nov. 15, that steady eastward course is going to come to a stop.

Wandering 'star'

Actually, for the past few weeks, Mars has appeared to slow in its eastward trajectory as seen from Earth. It seems to waver, as if it had become uncertain. Finally, on Nov. 15, it will pause and come to a halt.

Then, for about the next 11 weeks, the "wandering star," as ancients called it, will reverse its course in the heavens and move backward against the star background – toward the West. Then, on Jan. 30, 2008, it will pause again, before resuming its normal eastward direction.

All the planets exhibit this "retrograde motion" at one time or another. But for the longest time, the ancient astronomers were unable to come up with a satisfactory explanation for it. For one thing, while behaving in this strange manner, Mars will also appear to deviate somewhat from its normal course; the retrograde motion will appear to bring it a little above its regular orbital track. In other words, for those of us watching from Earth, Mars will appear to travel in a loop.

Yet, the Greeks staunchly believed that the sun, moon and planets all moved around the Earth in perfect circles. They had a great difficulty in representing and calculating this mysterious loop and for a long time they had no adequate explanation for it.

The Greeks finally explained away this anomaly by assuming that the planets moved around the Earth in smaller "epicycles" – that is, a small circle whose center moves along its main orbital circle around Earth, resulting in complex, almost coil-like curves. Unfortunately, the actual observations of the planets never seemed to fit this strange orbital mechanism, ultimately making the Greeks explanation quite useless.

The truth emerges

It was not until the year 1543 when the great Polish astronomer Nicolaus Copernicus (1473-1543) had his lifelong work "De revolutionibus" published, that the secret of the odd retrograde loops were finally revealed. By demoting the Earth from its hallowed position at the center of the solar system and replacing it with the Sun, he was able to triumphantly explain the riddle of the apparent "backwards motion effect" of the planets.

In fact, it's the same effect obtained when passing another car on a highway: Both cars are going in the same direction, but one is moving more slowly. As they pass, the slower car will appear to be moving backward in relation to the faster one.

Copernicus simply applied the same effect to the planets. In the present situation, both Earth and Mars are moving in the same direction around the Sun, but the slower one, Mars, appears to move backwards compared to the faster one, Earth.

Just an illusion

The retrograde motion of Mars – like the apparent motion of the slower car on the highway – is nothing more than an illusion. The last two times Mars underwent retrograde motion (in 2003 and 2005), I received numerous inquiries asking if the sun, as seen from Mars, would also appear to stop and move backwards across the sky.

The answer is most definitely "no."

The apparent backward motion will manifest itself after Mars arrives at its first stationary point on Nov. 15. Mars will then begin to loop back toward the west. Earth will overtake Mars on Christmas Eve. Finally, on Jan. 30, 2008, the combined movements of Earth and Mars will cancel-out the apparent backward motion, with Mars reaching a second stationary point. From then on, Mars will loop back to the east, resuming its normal eastward path among the stars.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.