Mercury the Messenger
Astronomers have traditionally used this symbol for the planet Mercury, and it is the medieval alchemical sign for the element mercury (Hg) as well. It consists of the biological sign for female (the sign used for the planet Venus), but topped with “horns” or a crescent. Mercury was the swift messenger god of classical Roman civilization, and the horns represent the winged hat he is usually shown wearing. In ancient Greece, he was called Hermes.
The god Mercury was also symbolized by a caduceus - a staff with two snakes wound about it. The caduceus is today occasionally used to represent medicine, but the true symbol for medicine is the staff of Aesculapius (the Greek god of medicine and healing) which has only one snake.