The sidereal zodiac is based upon the constellations themselves, as seen in the night sky. These dates may vary by one day, depending upon time zone and period within each Olympiad. However, due to the Precssion of the Equinoxes, these dates will appear to move forward at the rate of one day for each seventy years, i.e., Pisces will move from starting on 15th March in 2003 to 16th March in 2073.

Currently, approximately 75% of people in a tropical sign (the first three weeks of that sign), are in the preceding sign in the sidereal zodiac: viz, for the tropical star sign of Leo, the first 75% of this group were born when the sun was transiting the constellation of Cancer.