Eastern based system of astrology uses fixed zodiac (sidereal zodiac), and therefore, in relation to position of a star the zodiac positions are fixed.
In the Western based system, the zodiac moves according to precession of equinoxes caused due to wobbling of the earthís polar axis. As a result, the equinoxes and zodiac move. Thus, once about every 72 years, the zero degree of Aries shifts by one degree with respect to a fixed star. The Western based zodiac is also referred as tropical zodiac.
The difference between both systems is the starting point of zodiac. Currently western zodiac is ahead of the eastern zodiac by about 23 degrees and 45 minutes. Thatís why the sun sign Aries as per western system corresponds to March 21 through April 21 time period while as per eastern system the same corresponds to the time period of April 15 through May 15.