One of the main differences between the western and the eastern tradition of astrology is the definition of the zodiac.

Western astrology uses the so-called tropical zodiac which defines 0 Aries as the vernal point (the celestial point where the Sun stands at the beginning of spring).

The so-called sidereal zodiac is used in eastern astrology. Because the vernal point slowly moves through the constellations and completes its cycle once in 26,000 years, tropical Aries moves through all sidereal signs, staying in each one for roughly 2,160 years. Currently, the vernal point, and the beginning of tropical Aries, is located in sidereal Pisces.

There are many different definitions of the sidereal zodiac, and they differ by several degrees.
Sidereal planetary positions are usually computed from an equation:-
sidereal position = tropical position - ayanamsa, where ayanamsa is the difference between the two zodiacs and changes with time.