The tropical zodiac is based upon tropical time, or the apparent motion of the Sun across the sky over the course of a year. The tropical zodiac fixes the position of the Sun in the heavens at the northern hemisphere vernal equinox as the first point of the sign of Aries. The signs of the zodiac in tropical astrology are therefore currently divorced from the actual constellations of the zodiac. This is due to the Precession of the equinoxes. The tropical zodiac therefore uses a view of the heavens as seen from Earth approximately 2600 years ago when the northern hemisphere vernal equinox did occur within the constelation of Aries.
The tropical zodiac is largely exclusive to the Americas and western European astrologers.
The Thelemic Calendar is based on a tropical zodiac.