That of Rulership is an ancient tradition dating back to the early stages of Astrology. Ancient Greek sources tell us that the Babylonians already adopted a system of planetary exaltations, calling the signs in which they believed planets to be exalted, their “Houses” (not to be confused with the Natal Chart Houses). The Greeks developed this tradition, starting to associate the Planets in sight to the Zodiacal Signs in which they thought said planets could have their most powerful influence.
Planets rule the signs they have the strongest affinity with; as basic forces/functions of the Soul/Psyche, they “embody” the particular energy of one or more signs. Since there are several branches of Astrology, we also have multiple systems that assigned Rulerships according to different criteria. The Introduction to Astrology serie, however, is mostly aimed to those who are not yet familiar with Astrology as a whole, so it’s better to keep things simple for the time being, and get a grasp on the standard Rulership system used in Western Tropical Astrology for starters.
The Sun, planet of the Self, self-expression, Animus, and Ego, has rulership over the sign of Leo; Moon (emotions, the Jungian Anima, past, mother, needs) rules Cancer, which starts with the Summer Solstice, marking the Sun’s change of direction. Mercury (intellect, communication, thought process, cognitive abilities) comes next, and rules both the signs of Gemini and Virgo. Venus (expression of Love and affection, artistic talents) also rules two signs, Taurus and Libra, then Mars (objective manifestation, willpower, assertion, sex drive) which has rulership over Aries and Scorpio; Jupiter (principle of expansion, luck, wealth, Life philosophy, the Higher Mind) rules Sagittarius (and it’s also the secondary ruler of Pisces). Finally, we have the slowest planets, symbolizing the forces that transcend our individualized Self: Saturn (principle of restriction, constraint, limits, fear) has rulership over the signs of Aquarius and Capricorn. The outer planets, who have been discovered in relatively recent times, have been added to this model as well: Uranus (change, innovation) rules Aquarius, Neptune (transcendence, Spirituality, Divine Inspiration) rules Pisces, and Pluto (Death and Rebirth, Transmutation, transformation, Power, occult) is now the main ruler of Scorpio, alongside Mars.
Rulerships also apply to individual Birth Charts, according to a slightly different pattern. First off, a person’s ruler is said to be the planet ruling the sign found on the Cusp of the Ascendant (if you don’t know what this means, read here). For instance, someone with Aries rising is ruled by Mars, while a Cancer rising like myself is ruled by the Moon. This means that the transits (again, if you don’t know what I’m talking about, read here) of the planet that rules the individual are felt much more deeper in everyday Life. Planets also rule the Houses of your Birth Chart that have their ruled Sign on the Cusp. I have Cancer rising, and my II House starts in the sign of Leo; in this case, the Sun is the ruler of my II House. Generally speaking, you might want to take a look at where your Ruling Planet is placed in your Chart, by House and Sign. This is sure to provide a valuable insight in your Nature.
Just as every planet has a naturally associated Sign, there are also Signs where its expression and emanation is altered and/or diminished. So, depending on its placement in the Birth Chart, a planet is referred to as dignified or debilitated. It’s Dignified when it’s found in the sign it rules, or in the sign of its Exaltation. It’s debilitated when in Detriment or Fall. A fifth state is referred to as Peregrine, when the planet is found in none of the four states enumerated above. It might seem overcomplicated, but it’s not: just look at the table below.