Just about everyone knows his or her sun sign, but did you know that's not the only important planet when casting a horoscope?
For starters, we need to define what a sun sign is, exactly. Ancient astrologers divided the sky into 12 sections, based on the 12 constellations along a huge circle in the sky. This is the Zodiac, and it consists of the 12 signs everyone knows: Aries, Scorpio, Leo and so forth. Against this fixed backdrop, the sun and other astrological planets appear to move -- when you say you're a Virgo, what you mean is that the sun appeared to be in the section of the sky called Virgo at the time of your birth. Other planets also move, and a full astrological chart includes all of their positions.
The sun sign changes roughly once a month, but the other planets have different periods depending on how fast they move compared to the Earth. Saturn, for example, takes about 13 years to complete a circuit of the sky, and further-out planets take even longer.
The classical planets were the Sun, Mercury, Venus, the Moon, Mars, Jupiter and Saturn; later discoveries added Uranus, Neptune and Pluto (even though it's not an astronomical planet anymore). Some astrologers also add the large asteroids Juno and/or Ceres. Each of them can be in a different sign at any given moment and in different orientations to each other.
But what do all of these planets mean? The sun sign is thought to determine general personality, but the other planets have more specific roles. For example, Mercury is concerned with communication. The sign in which Mercury appears on your birth chart can have an effect on how well you communicate and what methods you like to use to get your point across; someone with Mercury in analytical Virgo will tend to make logical, carefully-worked-out arguments, while a Mercury in Leo, sign of charisma and flamboyance, may try to overwhelm a conversation with sheer energy.
Similarly, Venus rules love and relationships, so its sign tells the astrologer something about how a person reacts to new friends or problems in a marriage. Venus in Scorpio can mean great passion (but also great potential for jealousy); Venus in Aries means a tendency to take the lead in romance.
The planets' relationships to each other are also meaningful; some arrangements cause their influences to increase, while others cause the opposite. To take a very simplistic example, someone who has Venus in opposition (a subtractive arrangement) to Mercury can have trouble communicating about love.
Each planet also rules one or more signs, and when it appears in its sign its influence is magnified. The Moon rules Cancer, so having the Moon in Cancer means that your likelihood of being a nurturing person who cares deeply about feelings is greater.
For a complete list of the planets and their meanings and influences, check an in-depth discussion of Western astrology. That way, the next time someone asks you your sign, you'll realize how much they still don't know about you!
