Chart rulers
Classic astrology
Chart rulership has changed over time due to the discovery of previously unknown planets. In classic astrology, Mars, Venus, Mercury, Jupiter, and Saturn were the only planets visible to the naked eye and ruled the astrological signs along with the Sun and Moon. While astrologers are aware that the Sun and Moon themselves are not planets, they are often grouped as such due to their movement around the Earth (since birth charts are mapped from the perspective of a person's position on Earth). The Sun and Moon are also referred to as luminaries and each have their own significance in horoscopic astrology alongside their roles as planets. The Sun, Moon, and aforementioned observable planets are known as the classic or ancient rulers are still used by some astrologers today.
Modern astrology
After the discovery of Uranus, Neptune, and Pluto starting in the 18th century, chart rulership was adjusted to include these previously unobserved planets accordingly. Today all eight planets, as well as the Sun and Moon, are commonly attributed in chart rulership. Some astrologers also include Ceres as a newly discovered dwarf planet, however, consensus has not yet been reached on its position as a chart ruler. The modern interpretation of chart rulers is most often used though some astrologers will use both classical and modern rulers for reference when interpreting an individual's natal chart. Often in this case the inner planets (Mars, Saturn, and Jupiter) will be referenced as a chart's primary ruler while the outer planets (Uranus, Neptune, and Pluto) become a secondary ruler.
The inner planets refer to planets visible to the naked eye. These are the Sun, Moon, Mercury, Venus and Mars. Jupiter and Saturn may or may not be included in this category. They are often instead known as the “transpersonal” planets because they are a transition from the inner personal aspects of one's life to the outer impersonal. Outer planets include those that need a telescope to be seen, such as Uranus, Neptune, and Pluto. The inner and outer planets are significant to chart rulership because they are said to represent an individual's basic drive and the unconscious. Knowing how many cycles a planet makes around the Earth and how fast a planet makes these cycles allows individuals to understand and master the energy and lessons of that particular planet.
Both inner and outer planets have an influence on an individual's character and behavior. The inner planets influence our everyday reality because of the fast speed they move at. The inner planets cycle around the Earth quickly which is why they have a stronger and more immediate personal effect. The outer planets take longer to orbit the Sun and represent an overarching theme within someones life. Since these planets take longer to cycle, their effect can last over generations. For example, Saturn is the slowest planet within the inner planet group as it completes a cycle around the Sun every 29.5 years. It spends an approximate 2.46 years in each sign of the zodiac. Uranus, as the fastest outer planet, takes 84 years to complete a cycle around the Sun and spends about 7 years in each sign.
Despite the modern use of outer planets as ruling planets, a case can often be made for still using classic rulers in place of or alongside these modern rulers. Many astrologers will recommend investigating both planets as chart rulers if you fall into one of these signs with some going so far as to recommend co-rulership by both if you cannot determine one's strength over the other. The planet ruling over an individual's chart represents a piece of that person's character that is more important or vital to them and their approach in life than others and thus is crucial in understanding and interpreting a birth chart.
Determining chart rulers
In ancient astrology, planets were originally assigned to rule various signs based on their geometrical order of the distances between the planets and the Sun, as well as their speeds. As astrology became more simplified over time, rulerships were established by the planets’ and the signs’ shared commonalities. Today's more modern use of the term “rulership” is a system of affinities; meaning that the ruling planets are often similar to the astrological sign they rule rather than an essential dignity.