People have been wary of Mercury for three thousand years.
The superstition is ancient. The astronomy is real. The habit is surprisingly useful.
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The messenger stalls
Babylonian skywatchers tracked Mercury on clay tablets and timed markets and omens to its moods. Rome later named the planet for the messenger god. So when the messenger appeared to stall and walk backwards, letters, deals, and journeys were assumed to wobble with it.
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The illusion is real
The backwards walk is a trick of vantage. Mercury laps the Sun every 88 days and passes Earth on the inside three or four times a year; for a few weeks it seems to reverse through the zodiac. Ancient observers saw it plainly. The telescope explained it. The superstition survived anyway.
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The caution is free
Whether or not you believe the planet meddles, retrograde season is a shared reminder to communicate deliberately: reread before sending, confirm the plans, back up the work. A calendar that nudges you toward care is useful under any cosmology.