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Most fundamentally, the change from prograde to retrograde motion is an illusion born of our orbital perspective.

When, on the far side of its orbit and in the midst of its prograde cycle, Mercury passes behind the sun, the planet soon appears as an evening star—a lovely if somewhat dim point of light in the western twilight sky after sunset.

We call this “apparent prograde motion,” meaning forward motion.

Scientists wondered what they were and noticed that sometimes the wanderers move in the same direction as the sun—in “prograde” motion—then come to a stop and reverse direction to move in “retrograde.”

After a while they reach another stationary point and resume prograde motion again.