When your plane reservations get mixed up, or your voicemail goes on the fritz, you may have begun to hear an unusual excuse. "Well, isn't Mercury retrograde? What can you expect?"
What is Mercury retrograde and why does it have such a bad reputation?
Back when it was believed that the stars were all equidistant from the earth, embedded in a hard shell that moved above us, the planets required some explanation. They didn't remain in the same positions with respect to the fixed stars, but wandered through a band of constellations that became the Zodiac.
If that wasn't confusing enough, periodically, they would reverse direction and go back the way they came for a while, before turning once again to move in the same direction with the rest of the star field. When they do this, they are in 'retrograde motion', moving against the tide of all the stars and other planets. Hence, Mercury retrograde refers to the appearance of the planet 'back-pedaling'.
Now that the relative positions of the earth, the planets and the stars is understood, it is easy to see that while the earth is in motion as well, often the planets will appear to reverse direction in the sky. But when astrology was the main science, all planetary retrogrades--including Mercury retrograde--took on more mystical meanings.
Since Mercury is the closest planet to the sun and its orbit is quite short, there are more instances of Mercury retrograde than any other planet, reversing its direction three or four times per earth year. And since Mercury the mythological figure was the messenger of the gods, Mercury retrograde is thought by the mystical-minded to wreak havoc with human communications.
Astrological associations with each planet link Mercury retrograde to foul-ups in connections, computers and other communication-related activities. This notion has moved into the mainstream as a nouveau Murphy's Law, a shoulder-shrug, a 'who knows?' So when your boss asks why you haven't replied to his email, or your document went astray, check your ephemeris and then (if appropriate) blame Mercury Retrograde. Sometimes it makes as much sense as anything else.