In astrology, sidereal and tropical are terms that refer to two different systems of ecliptic coordinates used to divide the ecliptic into twelve "zodiac signs". Each sign is divided into 30 degrees, making a total of 360 degrees. The terms sidereal and tropical may also refer to two different definitions of a year, applied in sidereal solar calendars or tropical solar calendars.
While sidereal systems of astrology calculate twelve zodiac signs based on the observable sky and thus account for the apparent backwards movement of fixed stars of about 1 degree every 72 years from the perspective of the Earth due to the Earth's axial precession, tropical systems consider 0 degrees of Aries as always coinciding with the March equinox (known as the spring equinox in the Northern Hemisphere) and define twelve zodiac signs from this starting point, basing their definitions upon the seasons and not upon the observable sky wherein the March equinox currently falls in Pisces due to the Earth's axial precession. These differences have caused sidereal and tropical zodiac systems, which were aligned around 2,000 years ago when the March equinox coincided with Aries in the observable sky, to drift apart over the centuries.
