Casting the chart for a given time and place:
This is a laborious process of calculation of Ascendant and other planets for a given time & place involving lot of mathematical calculations, considering Local Mean Time (LMT) Sidereal Time ( ST ) Ayanamsha, Longitude and latitude of the place.
Now we have many soft wares and Apps which calculates all this and produce the Zodiac Chart, Bhava Chart along with 16 Vargas, Shadbala, Graha Avasthas, Ashtakavarga, Balance of dasa and many more details and thus an important part called Ganita Bhaga in Jyotishya Sastra is made very simple.
Still what is important to know is the Ayanamsha applied by the Soft ware or App used by us.
Now let us know what the Ayanmasha is.
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, this causes the starting 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. We know that earth is bulged at the equator than at the poles.
For Example, this year when the Sun enters into Aries zodiac on April 14th at a particular time, the following year after the Sun completes one revolution around the Sun, Earth would not enter into Aries exactly on April 14th at the same time, 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 larger 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 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 phenomenon such as Solar Eclipses or Lunar Eclipses & date of Purnima or Amavasya or Sankranti (Solar ingress). To bring about a standardization in the dates the Union 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 on 1 st August 2020, this angular difference in between the Tropical zodiac and the sidereal Zodiac is 24 Degrees 8 m and 38 Secs according to Lahiri Ayanamsa, ( 24-13-42 if calculated manually using the formula beginning from 285 CE.) It is 22-41-60 according to Ramana Ayanamsh, 24-2-59 according to KP old and 24-3-14 according to KP new Ayanasha. The table below shows as to why the Ayanamshas are different between Lahiri, BV Raman and KP.
Ayanamsha Precession rate Start year
Lahiri 50.2719 285 CE
Krishnamur 50.2388475 291 CE
B.V. Raman 50.33 397 CE
50.2564 is said to be the precession rate for the year 1900. This value is used by all soft wares to calculate the Ayanamsha for a particular year. In my opinion considering 50.2719 for 285 CE and calculating the Ayanamsha for the particular gives the perfect value but as such we have no Soft wares casting chart on the lahiri Ayanamsha calculating on 50.2719 for 285 CE.
The difference is about 5 minutes and 4 secs.
Now we will see that charts drawn to day at 6 AM for Kanyakumari, from South, Srinagar from North, Sabaramati from West, Gangtok from East.
Kanyakumari 11-25-37 12-53-08 15-05-08
SriNagar 17-52-37 12-53-08 15-05-36
Sabaramati 12-02-09 12-53-08 15-05-36
Gangtok 27-16-57 12-53-08 15-05-36
It is always good to use the software which provides the Lagna sputha and Graha Sphuta ( Lagna degrees and Planet degrees) upto Seconds level.