The so-called “sign” is the position of the Sun at birth. It's just the position of one of the many elements of a natal chart.
Sun sign astrology was created in the 1930s as a very simplified method of astrology to fulfill the needs of the newspapers.
The signs don't always start and stop on the same day of every year. According to the year, there are shifts of one, or even two days at the beginning or end of the Sun position.
The sign doesn't start or stop exactly at midnight either. In case of doubt, and the need to know the exact sun sign on a certain date and time, we need to consult the ephemeris or run astrological software.
Sun sign astrology places the most importance on external personality. Astrology however goes much deeper than that. The full analysis of a birth chart also reveals the hidden gems and demons of a person, and connects the soul to the timeless archetypes acting on our psyche.
Astrology cannot be reduced down to any single element, but if we really want to set a single guideline to condense a complex natal chart, we might use the chart's dominant planet. No, the “dominant” planet isn't connected to anything like 50 Shades of Grey. The dominant planet is the stronger planet in a chart.
Figuring out the dominant planet involves long and complex calculations which involve an accurate weighting of every planet’s position in the signs and house and every other aspect.
However, nowadays, through computers we can find our important planet with just a few clicks. Of course that's not the end of the story, but it is a much better approximation than the Sun sign.
In a significant percentage of cases, there isn't a clear dominant planet; there are two, or even three, most important planets.
Given that the planets are intimately connected to the signs, some of the sign’s characteristics are common with the planets. For instance, Mars and Aries share passion, energy, courage, competition, and decisiveness, which can also transform into aggression and destruction in the case of non-harmonic aspects.
The dominant planet shapes both the psychological and the somatic traits, so that, to keep the previous example, a person who is Mars dominant might have an athletic or muscular body and a fiery attitude. This is obviously a simplification since nobody is influenced by just one planet.
Let's say the second most important planet is Mercury. In that case, the psychological qualities combine energy, competition and the Mercurian mental peculiarities. Combined with Mars, the person might have a quick and energetic mind, as in becoming a ranting journalist. The body might be athletic but slim, with good reflexes and fast movements, suited for instance, to a table tennis player.
At the other end, Mars and Jupiter might display a muscular and big body that is suited for heavy work. Mars and Venus might produce a harmonic, fit body, something like a “yoga” or “Pilates” body.
We didn't consider the aspects or the astrological houses or signs involved in the dominant planets. Being that astrology is a symbolic body of knowledge, nothing is black or white, or 0/1 as in the digital systems.
Even with their limitations, the dominant planets certainly provide a better understanding of a person than the sign. So, next time somebody asks which is your sign, instead of "Libra", you might instead answer “I'm dominant Neptune” or “my most important planet is Jupiter”.
Which are your most important planets? You can find out in the “Astro Reports” page. In the “Astrological birth qualities” box, click on “Calculate” then click on the “Most important planet” sub report.