As Mars has grown closer and brighter daily for several months, it has gradually moved easterly in relation to background stars in the pre-dawn sky. That's about to change as the red planet begins to backpedal in our sky, moving steadily westward.

Astronomers call this backward motion "retrograde." The shift comes as Mars is gradually becoming visible in the late evening, too, just in time for the historic close approach to Earth that will occur in late August.

Mars is already rising before midnight, blazing in the southeast skies like a pale red or sometimes yellowish-orange lantern.

Since Jan. 1, the red planet has progressed along an easterly course through the background stars of the Zodiac. The motion is not noticeable in a single morning, but could be detected from one to the next by astute observers.

For the past few weeks, Mars has appeared to slow in its eastward trajectory, almost seeming to waver, as if it had become uncertain.

On Wednesday, July 30, that steady eastward course will come to a stop. Then, for the next two months, the planet will move backward against the star background - toward the west. On Sept. 29 it will pause again before resuming its normal eastward direction.

All the planets exhibit retrograde motion at one time or another. Ancient astronomers were unable to come up with a satisfactory explanation for it. The motion is tricky. 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 below its regular orbital track.

In other words, as seen 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.

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, Copernicus was able to triumphantly explain the riddle of the apparent "backwards motion effect" of the planets.

It's the same effect obtained when one car passes another on the highway: both cars are going in the same direction, but one is moving more slowly. To an observer with no other reference points, as they pass, the slower car will appear to be moving backward in relation to the faster one. Copernicus simply applied the same effect (though he probably wasn't thinking of automobiles) to the planets out in space.

Right now, 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, the Earth.

This backward motion will manifest itself after Mars arrives at its first stationary point on July 30. Mars will then begin to loop back toward the west. Earth will overtake Mars on Aug. 28.

Finally, on Sept. 29, 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 path among the stars.