Sidereal and tropical astrology

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.

[2][3][4] 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.

[5][6][7] Sidereal astrology accounts for the Earth's axial precession and maintains the alignment between signs and constellations via corrective systems known as ayanamsas (Sanskrit: 'ayana' "movement" + 'aṃśa' "component"), whereas tropical astrology, to reiterate, is based upon the seasonal cycle of the Northern hemisphere and does not take axial precession into consideration.

[8] Per tropical calculations, in contrast, persons born between March 21 – April 19 would have the sun sign of Aries.

The dates the Sun passes through the 12 astronomical constellations of the ecliptic are listed below, accurate to the year 2011.

Precession of equinoxes, the changing position of the vernal equinox over the course of about 25,800 years. The yellow line is a section of the ecliptic , the apparent path the Sun appears to follow over the course of an Earth year . The purple line is the celestial equator , the projection of Earth's equator onto the celestial sphere . The point (red) where these two lines cross is the vernal equinox . In 1500 BCE, it was near the end of Aries ; in 500 BCE, it was near the beginning of Aries; and in 1000 to 2500 CE Pisces .
A map of the IAU -defined constellation boundaries with the equal length signs used in tropical astrology overlaid. One can see that, due to precession and the inequality in the sizes of constellations, it appears that the constellations the signs are based on have moved eastward by nearly a month (or 30 degrees). For example, the sign of Cancer mostly corresponds to the constellation Gemini.