The major difference between Hindu Astrology and Western Astrology is that Hindu Astrology follows Nirayana system and the Western Astrology follows Sayana system.

Nirayana System:
In this planets are taken as fixed while Sun is taken as moving, because this is the way it appears.

Sayana System:
In this, planets are taken as moving while Sun is taken as stationary, because this is the way it actually is.

Aynamsha:
The exact period when both the zodiacs were in the first point is called Aynamsha or the precessional distance.  It is the increment between the beginning of the fixed and moveable zodiacs and varies from 19° to 23°.  It is taken roughly as 50 1/13 seconds of arc per year.