Ayamansa in Astrology
The longitudal difference between the tropical (Sayana =Krantiya) and sideral (Nirayana=nakhastrika) in the zodiac. Because of precision(moving ahead) of Earth causes the pole star (north star) to change. So there will be inconsistency in star level in a chart. To compensate this quantity is added to longitude to find values coherent with the past. The one use of this application is Lahari/chtrapaksha Ayamansa. (Foot note: tropical = the movement of Sun and the Earth.)
The intersection of the celestial equator and ecliptic path of the Sun results in two intersection points and the one of the point is called as Vernal Equinox. What is equinox and then vernal equinox : The equinox means a time of equality : day and night--- occurs thrice a year, and marks the beginning of the spring and Autumn. Vernal equinox, two moments in the year when the Sun is exactly above the Equator and day and night are of equal length; also, either of the two points in the sky where the ecliptic(the Sun’s annual pathway) and the celestial equator intersect. In the Northern Hemisphere the vernal equinox falls about March 20 or 21, as the Sun crosses the celestial equator going north. In the Southern Hemisphere the equinox occurs on September 22 or 23, when the Sun moves south across the celestial equator. According to the astronomical definition of the seasons, the vernal equinox also marks the beginning of spring, which lasts until the summer (June 20 or 21 in the Northern Hemisphere, December 21 or 22 in the Southern Hemisphere).
The Western astrology uses the Vernal Equinox as the point of reference which is known as the Tropical Zodiac(Sayana).
Sidereal and tropical are astrological terms used to describe two different definitions of a year. They are also used as terms for two systems of ecliptic coordinates used in astrology. Both divide the ecliptic into a number of "signs" named after constellations, but while the sidereal system defines the signs based on the fixed stars, the tropical system defines it based on the position of vernal equinox in the northern hemisphere (i.e., the intersection of the ecliptic with the celestial equator). Because of the precession of the equinoxes, the two systems do not remain fixed relative to each other but drift apart by about 1.4 arc degrees per century. The tropical system was adopted during the Hellenistic period and remains prevalent in Western astrology. A sidereal system is used in Hindu astrology, and in some 20th century systems of Western astrology.
Whereas the Indian system adopts a fixed star as the point of reference which is known as Sidereal Zodiac (Nirayana). The angular difference in the longitudes between the sidereal zodiac (Nirayana) and the tropical zodiac (Sayana) is called Ayamansa. This difference is the result of the “wobble”(unsteady movement) in the Earth’s axis of rotation and thus there is a gradual shift from the vernal equinox. For each spring at the vernal equinox, the Sun does not return exactly to the same point of 0° Aries as observed from a fixed star and misses by a tiny amount of 50’.2388475 seconds per year according to Newcomb, Western Astrology site. This phenomenon is known as precession of the Vernal Equinox. But this tiny difference accumulates over the years, the discrepancy between these two systems is about 24°.
Nirayana Longitude = (Sayana Longitude) – (Ayanamsa)
To compute the Sidereal Zodiac, the ayanamsa has to be subtracted from the Sayana longitude to get the Nirayana longitude. There are different ayanamas values used by the researchers that because of the different Epoch year is used. Epoch year is the one where the ayanamsa value is zero. For example, Surya Siddhantha uses 490AD, Lahiri uses 285AD, Krishnamurthy use 291AD as epoch year.
In stellarastroloy web app, the KP Straight Line Ayanamsa (KPSTLA) is used as the default anyanamsa. The KPSTLA epoch year is 291AD and the annual precession rate (50.2388475 seconds) is applied, but the Adjustment Rate of Precession Rate (0.000111 seconds) is not applied.
The following formula is used to calculate the ayanamsa using KPSTLA.
NEWCOMB_PRECESSION_RATE = 50.2388475
#Y = Number of years elasped for the given date from epoch year 15th April, 291AD
#D = Number days elasped for the given date from 15th April
T1 = #Y * (NEWCOMB_PRECESSION_RATE / 3600)
T2 = ((#D/365d) * NEWCOMB_PRECESSION_RATE)/3600
KPSTLA Ayanamsa = T1 + T2
The choice of ayanamsa varies to different astrologers and that is the reason for the same birth details there might be two different types of horoscopes. It is recommended that regardless of the ayanamsa type used, always ensure that the generated horoscope is corrected by birth rectification techniques or ruling planets or confirm with the past events.