According to Western astrology Aries starts at spring equinox every year. The Aries starting point is fixed with respect to the movement of Sun. However according to Vedic astrology the equinoxes are moving with respect to the fixed stars every year at the rate of 50.26 seconds of arc. This is due to the rotation of earth's axis itself and it takes 26000 years to complete one rotation (http://astrosun2.astro.cornell.edu/academics/courses/astro201/earth_precess.htm). Accordingly the starting point of Aries moves slightly every year and in 26000 years will align with its original position. The current difference between these two systems are approximately 22 degrees and the Aries starts on April 14 in Vedic astrology vs March 21st in Western astrology. This shows how much thought has went into identifying the position of signs accurately. Ancient astrologers from India not only understood that the earth goes around the sun, but also understood that the earth's axis itself rotates once in 26000 years and accordingly devised a system that shows a fixed background (stars) against which all the planets and the sun are moving.