Mercury is one of the most interesting and enigmatic of planets. In ancient times, the appearance and disappearance of Mercury was something that stargazers and priests kept careful records of, because its phenomena were considered to be important omens.
Because of its proximity to the Sun and modern disadvantages like city lights and smog, most people never see it in their lifetimes.
Because Mercury's orbit is between the Earth and the Sun, it can never appear to be very far away from the Sun. It appears to us to go back and forth across the Sun in an approximately 116 day cycle.
When it gets to its maximum elongation ahead of the Sun in the Zodiac, it begins to retrograde or move opposite the direction of the signs. After a while it's hidden by the light of the Sun, but will soon re-emerge from it. Mercury eventually reaches maximum elongation behind the Sun, then moves forward in the direction of the signs, until it is close enough to the Sun be obscured by its light, until it emerges on the other side once again. These retrograde patterns are fairly common in conversations about astrology, but often misunderstood.
It's usually said that during the retrograde period of Mercury that anything connected with communication, technology or transportation can go wrong, but that's not necessarily the case. Issues concerning these matters may arise, but mostly to bring our attention to them. The retrograde period usually brings opportunities to take things "off the back burner", or to be more precise with tasks that require attention to detail.
Mercury's day house is Virgo and its night house is Gemini, and these two signs express the nature of Mercury better than any others. People of both signs exhibit Mercury's changeable nature through their modes of speech, their wide range of interests, and their skills.
Mercury is exalted in Virgo, making Virgo the most Mercurial of all signs. It's seldom that one meets a Virgo who is not knowledgeable about the topic of their witty conversations. People born under Virgo tend to cultivate Mercurial characteristics.
Pisces is the sign where Mercury is in its fall. People born under Pisces tend to value imagination over knowledge.
Conversational skills, a wide range of knowledge and conversational ability are the best known traits of Mercury. In ancient times Mercury was the god of scribes, messengers and athletic events (from the standpoint of the exhibition of skill).
Mercury was also, however, the god of thieves and con-artists. Mercury is not considered benefic or malefic; it takes on the nature of planets combining with it. Mercury's skill is just as necessary to an environmental non-profit as it is to a tax-evading gangster. Mercury can enable a musician to play a concert hall one day and a bar the next. Mercury is about the skill, not the values.
Mercury also has a great deal of youthful energy; many people with Mercury in their rising signs or otherwise prominent appear much younger than their chronological ages.
Mercury people are generally quick-witted, love variety, have good powers of retention (Mercury rules the short-term memory), and flexible in both mind and body.
The more negative traits of Mercurial people are that they can be distracted easily, find commitment difficult, and can be deceptive or manipulative.
Mercury is very important in both business and personal relationships because it is the planet of communication. There is an entire method of comparing horoscopes based on the position of Mercury on the days of birth of the two people involved. Usually we notice a Mercury connection in couples that they say the same thing at the same time or complete each other's sentences; "great minds think alike" is a typical Mercurial expression.
If there is a "sign aspect" between your Sun Sign and another's, there is a good chance that your Sun or Mercury is connected with theirs. This is because Mercury never gets very far away from the Sun from the standpoint of earthly observers. But there can also be a Mercury-to-Mercury, Mercury to Sun or Mercury to Venus connection between two signs that are out of aspect.
This often explains couples who have a good relationship, but do not have common compatibilities between their Sun Signs. Not only that, but the reverse can also be true: two people who appear to have perfectly compatible Sun Signs on the surface may have poor connections involving Mercury.
Sometimes relationship issues defy the conventional wisdom about the signs, and very often Mercury, who can be quite a trickster, is the responsible party. This is only one reason to learn more about personal astrology than just the Sun Sign.