Every major planet in astrology has a sign or two where it is most at home. We say that planet “rules” that sign.
The energy of Aries, for example, is very similar to the planet Mars. While Mars is in Aries, it is “at home” and able to more comfortably express its basic nature. You can think of Mars as the “landlord” of Aries. Mars has a special influence over the Aries house and any planets in Aries. Some planets rule two signs. Mars has the same relationship with Scorpio, the other sign it rules. The landlord has a say in what goes on in the house, whether or not the landlord is present in that house.
What’s the sign on the cusp of your 1st house? That planet is the first house ruler.
The ruling planet has a say-so in the affairs of that sign, and of the house that has that sign on the cusp. The house and the planets in it answer to the priorities of the ruling planet. And when something rattles the landlord, it rattles the houses and their tenants, too.
|Mercury||Gemini and Virgo|
|Venus||Libra and Taurus|
* Traditional astrologers used only the visible planets. Saturn was, to them, the outermost boundary of the cosmos. But then Uranus, Neptune, and Pluto came into our awareness, and we saw that they also share this connection with certain signs. Consider Scorpio, Aquarius, and Pisces as having two landlords: a traditional ruler and a modern ruler.
Some astrologers still use traditional techniques and systems of rulership (which are quite complex).