A Classical Planet

Mercury is the planet of the classical seven closest to the Sun, and in traditional astrology it rules both Gemini and Virgo. It is associated with communication and exchange, echoing the messenger god whose name it bears. That association is built into the glyph itself: the sign derives from the caduceus, the herald's staff twined with two serpents that Hermes and Mercury carried in Greek and Roman art, abbreviated over centuries of copying into the crescent-topped circle used today.

The abbreviation was not fixed all at once. Early forms survive in late Greek papyri and in Byzantine astrological codices without the small cross now attached at the foot of the sign, a Christian-era addition that manuscript evidence places some time after the eleventh century, when scribes working in a thoroughly Christianised Byzantine world added the cross to a pagan emblem much as they Christianised other inherited symbols of the period, without erasing the pagan derivation underneath. The caduceus itself had already been a borrowed image for centuries before any astrologer adopted it: Greek art gave the winged staff to Hermes as messenger of the gods, and its later Roman equivalent inherited the same attributes wholesale, so that the planetary sign compresses into a few strokes an emblem whose own history as a symbol of speed, trade and negotiation runs well outside astrology.