Earth revolves round the Sun in an elliptical orbit and also rotates around its own axis. We know that the 12 Zodiacs (Rashis) are arranged in the outer space forming a 360 degree circle with starting point of the zodiac sign being Aries and continuing till Pisces. The Earth revolves around the Sun and it traverses from Aries till Pisces completing one complete year. Now since the axis of rotation of earth is not stable and it wobbles like a spinning Top (as shown in the figure below), this causes the staring point of the Aries to be different after the Earth completes its revolution year after year. The wobbling of earth’s axis of rotation arises because of the unequal gravitational pull of the Sun at the equator than at the poles (please note that earth is bulged at the equator than at the poles). This wobble takes approximately 26,000 years to complete one circle.
For Example, this year when the Sun enters into Aries zodiac on April 14th, the following year after the Sun completes one revolution around the Sun, Earth would not enter into Aries exactly on April 14th because the starting point of Aries has shifted owing to the wobbling effect of earth. This time difference is very minute if one considers one year but it accumulates to larges values as years goes by. To account for this difference, our ancient Sages had postulated the concept of Ayanamsha. Thus Vedic Astrology defines the concept of Sidereal year and it accounts for the relative backward movement of the starting point of the Aries Zodiac viewed from the perspective of Earth which is approximately 0 Deg. 0 Minute and 53 Seconds every year.
On the other hand, Western Astrology has no concept of Ayanamsha and it does not consider the wobbling effect of Earth because of which Western Astrology assumes that the starting point of Aries is the same Year-on-Year as Earth revolves around the Sun. This is defined as the Tropical Year.
Thus Ayanamsha is the angular difference between the start of Aries in the Tropical Zodiac and the start of Aries in Sidereal Zodiac.
Now, continuing further we know that Aries Zodiac comprises of Ashwini Nakshatra, Bharini Nakshatra and Krittika Nakshatra with Ashwini Nakshatra being the starting point of Aries. As stated above, because of the wobbling effect of earth the starting point of Aries i.e. Ashwini Nakshatra will gradually shift over time and Aries zodiac would no longer start from Ashwini Nakshatra when Sun enters into Aries. This is totally inconsistent with the teachings of our Rishis since Aries has to start from Ashwini Nakshatra. It is because of this reason that Ayanamsha correction has to be factored into the Tropical Zodiac to obtain the Sidereal Zodiac and in turn the correct Sidereal Year.
In various parts of India, since ages the dates of various festivals differed slightly since there was a difference in Ayanamsha used by various Astrologers because of which there was a difference in the Astronomical phenomenons such as Solar Eclipses or Lunar Eclipses or the dates of Purnima or Amavasya or Sankranti (Solar ingress). To bring about a standardization in the dates the Nehru Government in 1952 brought in ‘ Calendar Reform Committee ‘ which was headed by the famous Astrophysicist Mr N. C. Lahiri.
To bring in a standardization of Ayanamsha and in turn to the dates of the Festivals throughout India, the ‘ Chitrapaksha Ayanamsha’ which is also commonly known as the ‘ Lahiri Ayanamsha’ was brought in. This was exactly what was mentioned in ‘ Surya Siddhanta ‘ by Rishi Aryabhatta.
This takes into account the Yog Tara of Chitra Nakshatra ( commonly called as Spica), which is the brightest star of the constellation and fixes the star which is exactly 180 Degree opposite to this Yog Tara to be the starting point of Aries. This serves as the anchor point for the start of Aries zodiac and this is most commonly used in Vedic Astrology.
The ‘ Chitrapaksha Ayanamsha ‘ baselines the starting year in which the Tropical Zodiac and the Sidereal Zodiac exactly coincided as the year 285 AD and then considers the angular difference between the start of the start of Aries in the Tropical Zodiac and the start of Aries in Sidereal Zodiac to be approximately 0 Deg. 0 Minute and 53 Seconds every year then onwards.
Today, this angular difference in between the Tropical zodiac and the sidereal Zodiac is approx. 24 Degrees i.e. the start of Aries has shifted by 24 Degrees since 285 AD. Hope, this elaboration clears the ambiguity as to why we should consider the Ayanamsha while generating the horoscopes. The main birth chart (Lagna Chart) of an individual might not grossly differ if one does not account in Ayanamsha but the Varga Chakras (Divisional Charts) would change grossly if one does not use Ayanamsha which would result in an erroneous prediction.