Some people think that astrology is just about the signs of the Zodiac, but with the smallest bit of knowledge of the subject, they find out just how important the planets are.
One can compare the ancient view of the sky to a stage play, in which the Zodiac signs are like the set and the stage, while the planets are like the actors. It's an old analogy about astrology that stuck so well, that we still call the best-known players in modern cinema the "Stars."
It should be made clear that it was some time before astrologers began calling planets by the name "planet." The ancients regarded the celestial sphere as consisting of two different types of stars; the "fixed stars" who change their relative positions to one another very slowly, and the "wandering stars" which make all sorts of movements against the backdrop of the constellations. Of course, these "wandering stars" are known as planets today. It is from the Greek word for "wanderer" that we get the term "planet."
In ancient times, there were the "Luminaries", the Sun and Moon, and the five wandering stars, Mercury, Venus, Mars, Jupiter and Saturn. They were the basis of all astrological analysis and prediction. Astrologers study their movements with great care in order to forecast what might happen when they change directions, move into each other's houses, form geometrical figures with one another, or appear in the same place in the sky.
One of the most important listings of the planets has to do with their apparent average speed. This is commonly called the Chaldean Order.
If the planets are arranged around a circle in this order, and straight lines are drawn to make a seven pointed star, the planets connected by these lines will go in order of the days of the week (each day of the week is named after a planet if you did not know).
This measure of speed is from the slowest to the fastest, and taken from the measurements made by an observer on the surface of Earth.
But we all know that the Moon is Earth's satellite, and that the Sun is at the center of the Solar system. So how does this ordering make sense? Well, if you take the Sun and Moon out of the listing, you have the five visible planets named in order of their distance from the Sun, Saturn being the furthest and Mercury being the nearest.
The order of distance and speed also defines the rulership of the signs, as we will later.
It should also be mentioned that the Zodiac is divided in half, such that the signs from Leo to Capricorn (moving counterclockwise) are called the "Day Houses" of the planets. The signs going from Cancer to Aquarius (going clockwise) are called the "Night Houses of the planets.
There are many other curiosities about the relationships of the planets, and most of them are purely mechanical. Jupiter travels at a speed that enables it to cover approximately one sign per year; it circumnavigates the Zodiac in just under 12 years. The Sun moves through a sign in about a month, and goes around the Zodiac in 12 months, or a year. Saturn's main period is around 28 years, while the Moon's is around 28 days. The relative speeds of the planets seem to work in numbers that speak of the "Golden Mean", Fibonacci Numbers and Sacred Geometry.
The traditional astrologer sees the 7 planets as having special places in the Zodiac. These special places were called their "dignities" and many astrological judgments are made by using them. The Sun and Moon had one sign each, while the other planets had two signs. We call them the signs that the planets "rule" but it's more accurate to say that the planets manage them, as if they were landlords.
The Moon rules Cancer, and the Sun rules Leo. Mercury, the fastest of the planets (next to the Moon) and the first planet from the Sun moving outward from the center of the Solar System, rules the signs surrounding them, Gemini and Virgo. Venus, which is slower in speed than Mercury, and the 2nd planet from the Sun, rules Taurus and Libra. Mars, the next planet in order of speed, and the next in order of distance from the Sun, rules Aries and Scorpio. Jupiter, the second slowest planet and the second furthest planet from the Sun rules Pisces and Sagittarius. Saturn, the slowest of the visible planets and the most distant from the Sun, rules Aquarius and Capricorn, the two signs furthest from those of the Sun and Moon, respectively.
It's very interesting how the Zodiac wheel, when considered in the light of the planets, shows both distance from the Sun and the speed of motion.
Exaltation was another important dignity, and to some think it's more important than rulership when judging planetary effects. Exaltation is a little different than rulership; the other planets look up to an exalted planet; they tend to have greater influence than rulers. It's sort of like visiting the home of a person who has the picture of an idol, or a religious or political leader on the wall; it might be their home, but if such a famous person came to visit, they'd probably be treated more respectfully than a family member.
The Sun is exalted in Aries, and the Moon is exalted in Taurus. Jupiter is exalted in Cancer, while Mercury is exalted in Virgo. Saturn is exalted in Libra, Mars in Capricorn and Venus in Pisces. These dignities often have much to say about the nature of individual's lives and their relationships.
This is especially true when a person is born in a sign (or has the Moon there) that is the dignity of another; for instance a person whose sign is Capricorn will probably find themselves looking up to (exalting) people who exhibit characteristics like those of the planet Mars. A person who's Moon Sign is Libra will probably be captivated by those born into Capricorn or Aquarius, or who otherwise exhibit strong traits of the planet Saturn. People who complain about Virgo's criticism may have a strong dominance of the planets Venus or Jupiter in their birth-horoscopes, because Venus is fallen and Jupiter is in its detriment in Virgo.
The concept of exaltation is not an absolute rule of compatibility, however, because it has more to do with behavior; some Capricorn people don't like the impulsiveness of Aries and a typical Aries person may be disinclined to accept the Capricorn's steady and sure approach to life. Many Virgos and Pisces make perfect couples, because the Virgo may be less inclined to the critical side of his or her nature, and the Pisces may ignore complaints, or turn the charm on in a way that emphasizes the more compatible aspects of their signs. There are many factors defining the relationships between the planets and signs of two people that can make for more exceptions than rules.
Outside of the myriad asteroids, comets, Kuiper-Belt objects, Centaurs and Trans-Neptunian objects, we have discovered three other planets that are commonly used in astrology: Uranus, Neptune, and Pluto. In the scheme of traditional astrology, they don't rule signs, but modern writers give the rulership over Aquarius, Pisces and Scorpio. They are important to a good astrological reading, but traditionalists prefer to call them co-rulers of the signs, because they are not part of the original scheme. You can learn much about them and what they mean to you by clicking on their names above.
The planets all move through the Zodiac from sign to sign at relative amounts of speed, depending on where they are in their orbits around the Sun. Other than the Sun and Moon, they all experience retrograde motion, which is something that we see because of our observation point on Earth. More of this phenomenon is discussed in the articles about the individual planets, please read them to understand more about the individual planetary natures, and their role in relationships.