There is a correspondence between planets and signs, based on some core similarities in their natures. This correspondence is formulated in terms of the planet ruling the sign.
It is important to know that the significations of a planet are not one-to-one identical with those of the signs it rules, only that there are similarities in some essential dimensions.
Let’s start with the luminaries. The Sun rules the sign of Leo, and the Moon rules Cancer. The Sun’s signification of identity and leadership resonate with Leo, while the Moon’s signification of emotion and need for security echo Cancer’s proclivities for the same.
Starting with the inner planets and going outward to the social planets, there is a symmetry to the rulerships, as shown in the following table.
Mercury rules Virgo, which is next to Leo, and Gemini, which is next to Cancer.
Mercury, as ruler of Gemini, speaks to the airier or the ideas side of conscious thought, while as the ruler of Virgo, it weighs in on the practical side of organizing or implementing ideas toward usable solutions.
Venus rules the signs that flank Virgo and Gemini, namely Libra and Taurus, respectively. As the ruler of Libra, the inter-personal Venusian traits of love, affection, and friendship are emphasized. Whereas, as the ruler of Taurus, the sensual side of Venus, personal desirability, art, and pleasure come into play.
Then comes Mars, with its rulership of Aries on one side, and Scorpio on the other. The Aries rulership points to the assertiveness, boldness and directness of Mars. But the Scorpio rulership of Mars is less persuasive.
Scorpio is a water sign, inward looking and emotional, which doesn’t quite jell with Mars’s direct, fiery energy. The Scorpio rulership seems forced, shoe-horned just to maintain the rulership symmetry.
Continuing with the mirror-image symmetry around the line between Leo and Cancer, Jupiter rules Sagittarius and Pisces, and Saturn rules Capricorn and Aquarius. But, as with Mars ruling Scorpio, Jupiter as ruler of Pisces, and Saturn as ruler of Aquarius are questionable.
Pisces is a water sign that is all about empathy. While Jupiter can be quite the benevolent and its general outlook of optimism and faith are very Sagittarian, empathy is not its thing. Similarly, Aquarius signifies progressiveness and being different, not at all like Saturn which celebrates form and convention, qualities that evoke Capricorn.
As it turns out, the quandary of questionable rulerships can be resolved by assigning Uranus as the ruler of Aquarius, Neptune as the ruler of Pisces, and Pluto as the ruler of Scorpio.
Aquarian progressiveness and distinctiveness quite clearly strike a chord with the Uranian way of being iconoclastic and different. Piscean fluidity bring to mind Neptune’s shape-shifting nature, and formlessness. And the Scorpionic traits of seeking power, and experiencing deep transformation are very much Plutonian traits as well.