The Planets
What each planet governs, and how it shows up in a chart.
Planets Describe Function
If signs describe style, planets describe function. The sun is identity and direction. The moon is emotional habit and what somebody needs in order to feel settled. Mercury is thinking and communication. Venus is affection, taste and what a person finds valuable. Mars is drive and how conflict gets handled.
These five move quickly enough to differ noticeably between people born in the same season, which is why a chart says considerably more than a sun sign does. Two people born a week apart share a sun sign and can have Venus and Mars in different signs entirely.
The Slow Planets Are Generational
Jupiter and Saturn take twelve and twenty-nine years respectively; Uranus, Neptune and Pluto take between eighty-four and two hundred and forty-eight. The outer three stay in a sign for years at a time, so their sign describes a cohort rather than a person.
In an individual chart their house matters far more than their sign. Pluto in Scorpio describes a generation of hundreds of millions; Pluto in the seventh house describes one person's relationship to partnership. Readings built on outer planet signs alone have the specificity of a horoscope column stretched over a decade.
Rulership
Each sign has a ruling planet, and the pairing is why the planets carry the associations they do. Mars rules Aries, Venus rules Taurus and Libra, Mercury rules Gemini and Virgo, and so on. The traditional scheme used seven planets; modern practice assigns Uranus to Aquarius, Neptune to Pisces and Pluto to Scorpio.
Rulership is also where the days of the week come from - Sunday for the sun, Monday for the moon, Saturday for Saturn, with the pattern more visible in the French mardi for Mars and mercredi for Mercury.