The WYSIWYG formula. What you see in the skies is what you get in your natal chart. The sidereal zodiac is based on positions of the planets and stars in the actual constellations, rather than their positions in signs that are named for - but no longer line up with - the constellations. The sidereal approach does not adjust for the precession of the equinoxes, an astronomical phenomenon that slowly, over thousands of years, shifts which constellations are most prominent in the sky in any given month. Thus, sidereal zodiac + adjustment for precession = tropical zodiac. Western astrology is based on the tropical zodiac.