The planet Mercury is only about one third the size of the earth. It is the closest planet to the sun in our solar system, and orbits the sun about once every 88 days. It is one of the most difficult planets to study because it is so close to the sun, the vast majority of the time.
It has almost no atmosphere and no natural satellites. Much is still unknown about the planet. The only spacecraft to get close to this planet was Mariner 10 (from 1974-75) and even then only about 45% of the planet was mapped. The Babylonians most likely made the earliest mention of Mercury in the third millennium B.C.
The first telescopic observation of Mercury was made by Galileo in the early 17th century. Japan is currently planning a joint mission with the European Space Agency to launch probes to study it further. The mission, called “Bepi Columbo” is planned to take place in 2013. The mission is named for Giuseppe (Bepi) Colombo, the scientist who first determined the nature of Mercury’s orbital resonance with the Sun and who was also involved in the planning of Mariner 10’s gravity-assisted trajectory to the planet in 1974.
In Western Astrology, Mercury represents, broadly speaking, the concept of communication. Also, again broadly, he’s the representation of our minds - how we think and communicate. Mercury is the planetary ruler of Gemini and Virgo, and is loosely associated with all things that move quickly: ideas, information, communication, thoughts, the media, and transportation.
Mercury is also the “God of the Crossroads” and in astrology, very much points to choice we must make, where we are going, and how we are getting there.
If you remember nothing else about Mercury from an astrological standpoint, remember that in a birth chart, he’s all about communication, speech, and the way in which the subject thinks and expresses him/herself. For example, my Piscean Mercury will be convoluted, slow, and sometimes confusing in my thought processes and expression; whereas someone with a Sagittarian Mercury may well communicate quickly, sharply, energetically, and quite clearly.
Several times a year, Mercury is said to “go retrograde.” Retrograde literally means “moving backwards.” This is actually a misnomer in that nothing in the sky suddenly literally goes backward. It merely appears to do so from our limited vantage point on Earth.