The astronomical model of precession asumes that the current movement of the vernal point is a constant motion.
This model guides to a period of 25771 years until the vernal point returns to its initial point. But this number can be wrong if the motion of the vernal point has long term fluctuations. Such an effect may be caused by unknown galatical phenomenons or fluctuations in gravitational fields.
Can be configured in the configuration dialog, see Ephemeris.
Standard configuration of custom Ayanamsa is based upon a reference value at a given date (normally Jan 01 1990 = 2415020.5).
Caution: changes of these parameters can cause major calculation errors!
Sri Yukteswar exposes his view about the yugas and the movement of the vernal point in the first chapter of his illustrious book The Holy Science (Kalyana Darshanam).
According to the Holy Science the period for the movement of the vernal point is 24,000 years.
Sri Yukteswarji combines the movement of the vernal point with the theory of the Yugas. According to his view Kali Yuga's duration is 2400 years (ascending and descending each 1200), and the start of ascending Kali Yuga begins when the vernal point is 0 Aries. Dwapara Yuga follows and has double duration, each Treta Yuga has 3600 years and each Satya Yuga 4800 years.
Therefore we have the sequence of Yugas shown in the table below.
Yuga Duration (Years) Vernal Point (Start) Vernal Point (End)
Kali (ascending) 1200 00 Aries 12 Pisces Dwapara (ascending) 2400 12 Pisces 06 Aquarius Treta (ascending) 3600 06 Aquarius 18 Scorpio Satya (ascending) 4800 18 Scorpio 00 Libra Satya (descending) 4800 00 Libra 18 Cancer Treta (descending) 3600 18 Cancer 24 Taurus Dwapara (descending) 2400 24 Taurus 18 Aries Kali (descending) 1200 18 Aries 00 Aries Total 24,000
This ayanamsa can be emulated with several methods.
First method of emulation is to take a fixed start date with known Ayanamsa and use the astronomical period. This is what the Sri Yukteswar Ayanamsa of swiss ephemeris does.
Disadvantage of this method is that long term predictions are not correct because the period is wrong.
Second method is to take (again) a fixed start date with known Ayanamsa, but to use the period of 24,000 years given by Sri Yukteswar. This kind of calculation can be configured in Maitreya.
Disadvantage of the method is that the actual true movement of the vernal point is neglected.
This method is a mix of the above methods. It assumes that the current precession is correct but has sigificant fluctuations that lead to a total period of 24,000 years.
The current precession is too slow to get a period of 24,000 years. So it must be near a minimum. The speed must grow in later periods to get the desired total period.
Possible positions for the minimum could be the point 0 Aries or the point in opposite to the supergalactic center (i.e. around 26 Pisces).
It is possible to give mathematical equations for the 3rd method. A simple eliptic equation could be satisfiying for the first time.
Investigations regarding long term Ayanamsa can be made taking old sources that rely on chitra paksha (i.e. Lahiri) ayanamsa. Most contemporary authors do not rely on old sources and pretend that they are not correctly calculated.