ayanamsa From the Hindu ayana, the arc that describes the increasing gap between the tropical and sidereal zodiacs. The ayanamsa, which changes continually at the rate of approximately 50" a year, is currently about 24°.
The difference between the Tropical (Sayana) and the Sidereal (Nirayana) longitudes.
Ayanamsa - The difference between the starting point of the Tropical and Sidereal Zodiacs, due to the precession of the equinoxes. There are several in use in India today, but the official government-sanctioned Lahiri ayanamsha is the most widely used.
Lahiri AyanamsaMost widely used Ayanamsa (22°27'37.7"). There is disagreement among authorities on the precise arc of this ayanamsa. Lame (degrees)Deficient degrees, held to cause lameness or other deformities and disabilities.
Savanna and Nirayana and Ayanamsa
The celestial equator is a great circle of the celestial sphere marked out by indefinite extension of the plane of terrestrial equator.
The sidereal coordinate system takes into account the ayanamsa, a Sanskrit word where literally ayan means transit or movement and amsa means small part i.e. movement of equinoxes in small parts.
To convert a Tropical zodiac into a Sidereal zodiac one needs to know the current ayanamsa and subtract it from the tropical longitudes of the planets. The most widely accepted ayanamsa in India was introduced by N.C. Lahiri. He calculates the precessional rate to be around 48 seconds per year.
This really isn't much news, as this phenomena was discovered as early as 130 BC by Astrologers/Astronomers of the time, especially in India where they compensate for this and call it Ayanamsa (the most accurate and popular is the Chitrapaksha or Lahiri Ayanamsa).
The two systems are about 23 days apart (called the "ayanamsa") from where they mark the first point of the sign of Aries, which is the start of the circle of the zodiac (you have to start a circle somewhere). Again, this is due to that slow precessed, backward slipping of the earth.
Interestingly, both the Mayan and some Hindu systems are sidereal rather than tropical; indeed, the word "ayanamsa" is borrowed from the Hindu Vedic system to name the angular difference at any given time between the sidereal and tropical coordinate systems.
Although '́nitial point' would generally indicate the First Point of Aries, Yeats is probably disputing the date of when the Tropical and Sidereal Zodiacs coincided. This is usually considered to be after 200 AD/CE and 'Lahiri's ayanamsa', the most widely used in India, puts it at 285 CE.
See also: What is the meaning of Sun, Astrology, Signs, Zodiac, Planet?