The classical zodiac of Greek astronomy as defined by Ptolemy is tropical in nature, defining the signs relative to vernal equinox regardless of the position of constellations. Western astrology traditionally uses this system.
Sidereal astrology ties its signs of the zodiac to the actual constellations.
Changing position of the vernal equinox. The red line is a section of the apparent path traced by the Sun through the Earth's year. The red/green line is a projection of the Earth's equator onto the celestial sphere. The crossing point of these two lines is the spring equinox. In 1500 BCE it was near the end of the Aries constellation, in 500 BCE it was near the beginning of the Aries constellation, and in CE 150 (the time of Ptolemy) it was in the center of the Pisces constellation.The precession of the equinoxes, a phenomenon discovered c. 130 BC by Hipparchus and known to Ptolemy, results in a shift between the two systems of about one degree every 70 years.
The vernal equinox lay near the beginning of the Aries constellation around 500 BC, consistent with a Babylonian origin of the system.
While classical tropical astrology is based on the orientation of the Earth relative to the Sun and planets of the solar system, sidereal astrology deals with the position of the Earth relative to both of these as well as the stars of the celestial sphere. The actual positions of certain fixed stars as well as their constellations is an additional consideration in the horoscope. (Over very long astronomical time scales, these fixed stars are of course themselves far from stationary.)
Some sidereal astrologers denounce tropical astrologers for failing to relate to the "actual heavens," seeing in this a fundamental degeneration of the subject (Kenneth Bowser, The Traditional Astrologer magazine, (Ascella), Issue 14, May 1997, pp.23-27 [
