Recently someone asked me what a chart ruler was, and the significance behind it.
In the most fundamental terms, the chart ruler is the planet or planets associated with the sign of the Ascendant. For example, in my chart, my Ascendant is the sign Scorpio. With that being said, my chart rulers would be Mars and Pluto (the sign of Scorpio has co-rulers). If you had Gemini on the cusp of the 1st House, then Mercury would be your chart ruler. With Libra, it would be Venus, and so on. Just determine which planets rule the sign on the 1st House cusp, and there you go, simple as that.
Alright, so now you know what a chart ruler is. Locate the house in your chart where your chart ruler is located. Going back to my personal natal chart, one of my chart rulers, Pluto, is in my 12th house. Now think about how the house where your chart rulers; this placement ‘colors’ everything inside it. In my case, the mission of my 1st House persona comes into fruition via the themes of the 12th house: my Ascendant or Rising Sign functions in the manner that illustrates what is represented by the 12th; I project what people wish to see, I am private, secretive, with a rich fantasy world, and my ‘persona’ is one created from illusions (maybe even delusions), perhaps?
Maybe your chart ruler was located in the 5th house. This would play out as your personality looks to creative outlets, children, romance, fun, lighthearted pleasure as the way you manifest the ‘agenda’ of your 1st House persona. Just remember to take into consideration the themes of the house in which your chart ruler is placed in to interpret it.
Some astrologers argue that the chart ruler is the most important planet (or planets in the case of co-rulers) in the chart, even going so far as to place its importance above the luminaries (Sun and Moon). Personally, I disagree; I don’t think anything tops the Sun or Moon (I mean, come on; if we didn’t have either, our existence on Earth would be…non-existent!).
But try it out for yourself; if you hadn’t considered it before, determine your own chart ruler, and see if that makes a difference in what you know of yourself and your chart. Tinkering around with various concepts is usually a good way to get another level of understanding of astrology, especially your own personally astrology.