Because I have so much to do and because this subject is well-known, and the Wikipedia page well written, rather than re-invent the wheel I'm quoting here from Wikipedia's "Planets in astrology". When I have more time I'll fill in the corresponding archetypes and equivalences with other symbolic and psycho-c\socio-spiritual systems of thought.
Mercury is the ruling planet of Gemini and Virgo and is exalted in Virgo (one of his houses) and possibly in Aquarius. In Roman mythology Mercury is the messenger of the gods, noted for his speed and swiftness. Echoing this, the scorching, airless world Mercury circles the sun on the fastest orbit of any planet. Mercury takes only 88 days to orbit the sun, spending about 7.33 days in each sign of the zodiac. Mercury is so close to the sun that only a brief period exists after the sun has set where it can be seen with the naked eye, before following the sun beyond the horizon.
Astrologically Mercury represents the principles of communication, mentality, thinking patterns, rationality and reasoning, and adaptability and variability. Mercury governs schooling and education; the immediate environment of neighbors, siblings and cousins; transport over short distances; messages and forms of communication such as post, email and telephone; newspapers, journalism and writing, information gathering skills, and physical dexterity. The first-century poet Manilius described Mercury as an inconstant, vivacious, and curious planet. In medicine Mercury is associated with the nervous system, the brain, the respiratory system, the thyroid and the sense organs. Traditionally held to be essentially cold and dry, but variable in temperament according to its placement in the zodiac and any aspects to other planets. It is linked to the animal spirits, alongside the Moon.
Today, Mercury is regarded as the ruler of the 3rd and 6th houses; traditionally, it ruled the 6th house, and had joy in the 1st house (the house of 'I' or 'we': Mercury facilitates self-expression here). Mercury is the messenger of the gods in mythology, that's why it is the messenger in Astrology as it is in Mythology. It is the planet of day-to-day expression and relationships. Mercury's action is to take things apart and put them back together again. It is an opportunistic planet, decidedly unemotional and curious.
Mercury rules over Wednesday. In Romance languages the word for Wednesday is often similar to Mercury (mercredi in French and miercoles in Spanish). Dante Alighieri associated Mercury with the liberal art of dialectic. In Chinese astrology Mercury is ruled by the element water which is diplomatic, kind and intuitive. In Indian astrology, Mercury is called Budha, a word related to Budhi ("intelligence") and represents communication.
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