Mercury is a small planet, by size similar to Moon, and 58 million km from Sun. His surface is filled with craters (mush alike Moon's). Its element is earth. It passes 1 - 2° daily, and it takes him about a year for the Zodiac to pass. It's nigh domicile is in Geminis, a daily one in Virgos. In mythology, Mercury (Hermes) was a messenger tot he gods, so due to his common sense and golden tongue, he became the God of communications. He enjoyed deceiving others, that thieves honoured him.
In astrology it represents our capability to communicate, analytically think, sense of business, public performance. It is the symbol of puberty, brothers, colleagues, family, friends, co-workers, cunning people, spies, astronoms. mathematicians, thieves, literature, all ways of communication and transportation, commercial exchanges, travelling, studying, makes good orators, but moody people as well. It is unpredictable, causes constant changes.
With people, he controls the lungs, nerve and cerebral system, hands, guts, fingers and toes, memory, imagination, body moves. If harmonical, gives common sense, good hands, speech, capability to analyse, agility, sense of humour. Unharmonical causes irony, sarcasm, inquisitory mood, sterile polemics, cinism, hypocrisy and instability.
Mercury rules Gemini and Virgo, as well as the 3rd and 4th house of Zodiac's.
Psychological manifestations: good power to judge, criticality, ability to express and write, diplomacy, intellectual ambitions.
Biological manifestations: nerve system, nervousness
Sociological manifestations: intellectual workers, young people, office workers.