41a:12 · This is the sign for the planet Mercury. Mercury is about the same size as the moon. Its orbit is the one closest to the sun in our planetary system. Mercury rotates around the sun four times while the earth makes one revolution. Its proximity to the sun accounts for this speed. With a slower speed would fall into the sun.
    That Mercury is so close to the sun means that the planet is never seen more than 28 degrees from , which in turn means that it can only be seen at specific times and then only after the sun has set, or immediately before sunrise.
    In ancient Greece the sign for the planet Mercury was drawn . In Greece Mercury was represented by Hermes, the quick-footed messenger of the gods, often pictured with wings at his ankles. Hermes was symbolized by the caduceus, the staff of the two snakes . The god in the Roman empire that the messenger of the Greek gods, Hermes, was identified with, Mercury, was the god of merchants, but also of thieves. The staff of the snakes in Rome became a symbol for trade and communications.
    This caduceus, or snakes' staff, has been mixed up with the Aesculapii staff, with only one snake, a symbol for the art of healing or medicine. Aesculapius for the Romans and Askleipos for the Greek, this god was the son of Koronis and Apollo and the god of medicine and healing, worshipped at Epidaurus. Zeus killed him with a thunderbolt for restoring Hippolyte to life at the request of Artemis.
    In astrological psychology Mercury stands for the intellect and, on a more fundamental level, for the processes of symbolizing, the basis of our communication systems. All our knowledge of the parts of the world that we cannot directly perceive reaches us via symbols and signs. When these symbols and signs (e.g. words, pictures and ideograms) reach us, we associate them with our memories of our perceptions of the world, and then true knowledge or illusions (false ideas) about the world we live in are released in us. For every sensory perception that reaches us, there is some outer physical reality. The position of in the natal chart, or horoscope, indicates the character of the individual's intellect, his or her inventiveness and communication ability, and the functioning of his or her nervous system.
    Those who are lucky enough to be born with a well placed , according to astrologers have a good linguistic ability, are fast thinkers, and like to communicate. They are said to have a well developed ability to convince others as well as a good memory for even tiny details. If is less well placed it can give restlessness, inconsistent behavior and superficiality.
    In mundane astrology Mercury stands for business, letter writing, short journeys, gossip between neighbours, schools and universities, technical and natural science institutions, printing, publishing companies, and all those people who are in one way or another working in these fields.
    Anatomically stands for the nervous system, the organs of vision and speech, and the hands, all considered in this case as instruments of the intellect. Mercury is the ruling planet of , Gemini, and , Virgo. The planet is considered not very well placed in , Sagittarius, and , Pisces.
    In alchemy all that is volatile, i.e. all that could be sublimated () or distilled (), was sometimes referred to as mercury, since one of the most important substances for the alchemists to sublimate was the white, poisonous compound mercuric chloride. Alchemists were always very interested in the fluid metal mercury (a close relative to gold, mercury having the atom number 80, gold having 79). Mercury could dissolve other metals and combine itself with them into amalgams, . The alchemists produced mercury by roasting cinnabar (see and in this group).
    Mercury is poisonous, and thus the sign for it has often been used in pharmacology to mean dangerous poison.
    Hermes was, contrary to the other Greek gods, both male and female, a hermaphrodite. For this reason the sign has been used both in biology and botany to mean double-sexed.
    As a weekday sign it stands for Wednesday. See in Group 29 for a derivation of the planets' days. In the Germanic countries this day is known as Odin's or Woden's day, in Anglo-Saxon countries as Wednesday. In Latin countries it is known as the day of Mercury.
    The hermaphroditic character of Mercury (the alchemists were interested in opposites, also the opposite sexes) gave it a special position in the field of alchemy. Apart from meaning mercury, and compounds with mercury, it was also used to signify the coniunctio, or uniting of opposites, in esoteric alchemy. (Refer to the section "Esoteric alchemy" in the Appendices.)
    Also note its appearance in , the philosopher's or wise man's stone (for more data, see , in Group 39).
    As to the age of this sign's structure, it is believed to have been used for about 3,000 years. See the sign found on a Babylonian clay cylinder.