When a planet is retrograde, it looks like it's reeling backward through the zodiac. In truth, the planets move forward constantly, but that's not the way it looks. Although the Sun and the Moon clearly revolve in the same direction all year every year, the planets seem to follow a less consistent pattern. On a regular schedule, each of the planets appears to slow down, reverse direction, and retrace its path, arcing backward across the zodiac. For weeks or months at a time (depending on the planet), it wheels against the planetary tide. Then once again, it seems to slow down, turn around, and resume normal movement (which is referred to as going direct).
When ancient astronomers saw the heavenly bodies whirling backward, they invented all kinds of schemes to account for the phenomenon. In the second century BCE, for example, astronomers in Greece were convinced that the planets looped around their usual orbits on little spheres carved from the purest crystal. Needless to say, they were wrong. The planets never actually switched direction.
Retrograde motion is solely a perception — an illusion caused by the fact that the planets, including Earth, are always in motion, tracing arcs across the sky as they loop around the Sun at varying speeds. You can experience the same disconcerting effect in a train. If two trains pull out of the station together but your train is moving faster, the train on the adjacent track appears to slide backward. That backward motion, like the retrograde motion of the planets, is an optical illusion.
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