The basic difference between the two systems is that the Vedic zodiac is Sidereal and the Western is Tropical.
• In Sidereal astrology, the zodiac is aligned with 27 constellations, or fixed star groups. Aries, the first sign of the zodiac, aligns with the first constellation, called Ashwini.
• In the tropical zodiac, the point in space when the Sun crosses the earth's equator is the beginning point of the zodiac, or Aries.
The earth's equator shifts backward through the signs of the zodiac at the rate of about 51" of longitude per year known as "precession of the equinoxes." Because of this phenomenon, the theoretical Aries of the western zodiac drifts further and further from the Aries used by Vedic astrologers -- at the rate of about 1 degree every 72 years.
The tropical (Western), and the Vedic Aries were on the same zodiacal plane around 285 AD because of the precession of the equinoxes. At present, there is approximately a 23 degree difference. Thus, all of the planets in a Vedic horoscope, including the rising sign, will be about 23 zodiacal degrees earlier than they would be in a Western chart. Since, planetary placements will vary considerably; Vedic astrologers cannot rely on the interpretation made by western astrologers.