Note: This article is my personal opinion which may be misleading (so read with caution!). This does not represent the views and opinions or my Guru or my Jyotish Tradition.
The astronomers from the ancient civilisations noted that the occurrence of the seasons shift gradually with respect to the fixed start of the zodiac. Later this was attributed to a phenomenon called the precession of the equinox. Although the first knowledge of precession of the equinoxes is attributed to Greek philosopher and mathematician Hipparcus, studies of the ancient civilisation such as that of the Mayan, Babylonian and Harappan reveal that the knowledge was quite common during those times, however lost with time.
The implication of precession of the equinox has not only caused the sidereal zodiac to precede with respect to the tropical zodiac with a constant rate, but it also implies a greater cycle is at work; a cycle which is much beyond our own solar system. This cycle is the movement of the vernal equinoxial point in the zodiac in an anti-zodiacal direction
Many speculated on the cause of the equinox and two widely known theories are postulated. First one is due to wobbling of Earth due to gravitational pull of the Moon and the Sun and second one is due to movement of Sun in a bigger cycle. The most accepted reason behind the second theory is presence of a Binary Star. In the universe presence of a binary start system is quite common and hence Sun's movement in a bigger cycle due to a binary pair is not a rare occurrence. While Indian classics on Astronomy and Astrology supports the second theory, more tangible proof is expected to appear in future supporting it. Nevertheless, lets see why it is important to study the great cycle of precession of equinox.
Precession of the equinox is a well known concept in Vedic astrology, since the whole discipline is based on this concept. Unlike western astrology, Vedic astrology gives high importance to the Nakshatras or the lunar asterisms in addition to the Rasis (or the signs). When a planet is seen in the sky, in Vedic astrology it corresponds to its placement with respect to a fixed star in the zodiac, the Nakshatra. Since the normal practice of recording the planetary positions is from the vernal equinox (in the ecliptic coordinate system), it has to be converted to sidereal positions (as used in Vedic astrology) by subtracting the amount of precession that the vernal equinoxial point preceded so far from the time when the vernal equinoxial point actually aligned with the beginning of sign Aries or Nakshatra Ashvini. This accumulated amount of precession is called Ayanamsa in vedic astrology or Jyotish Shastra. The term Ayanamsa is derived from Ayana and Amsa where Ayana means the portion of the zodiac in the ecliptic plane and amsa means the angular distance.
From the observations it is found that the rate of precession (ayana chalana in Sanskrit) is not constant and varies with time in a sinusoidal wave form, making it inappropriate to apply constant Ayanamsa correction to the the tropical zodiac to derive the sidereal zodiac and a better suggestion would be to find the exact start of the zodiac from the astronomical observation and apply the angular separation of this point with respect to the vernal equinoxial point to the tropical zodiac. The duration of the grand cycle is observed to be 24,000 sidereal year (year measured from movement of Sun in Ashvini Nakshatra) while due to variation of the rate of precession does gives a impression that the duration to be 22,000 or 26,000 years if the variation is not seen in a bigger timescale.
Now does the grand cycle of the equinox correspond to the cycle of the Yugas mentioned in the Vedic scriptures? Although there are definite linkages with the precessional cycle with the cycle of yugas, there are some controversies too. Before we proceed with drawing the linkages, lets understand what the yuga cycles are:
It is said that in the life of Manu, who is governor of the "Manu"sya race (Human Race) there are 71 yuga cycles and each yuga cycle consists of 4 phases from brighter to darker. The brightest and most enlightened phase is the Satya Yuga and in the order of decreasing brightness are the Treta, Dvapar and the Kali. Among the 4 yugas, the Kali is the shortest and the darkest, while Satya is the brightest and the longest. If the duration of Kali is 1 part, the duration of the other yugas are Dvapara- 2 part, Treta- 3 part and Satya- 4 part. Although there are unanimity by the scholars in the details I have provided so far, the differences arise on the duration assigned to each part mentioned above. One group of scholars believe that each part is 1200 divine years (each divine years is 360 human years, thus making each part as 4,32,000 years), while the other group believe that each part is 1,200 human years.
If we accept the duration of each part to be 1,200 years (human/ earth), then the total duration of the cycle would be 12,000 years and this correspond to half of the duration of the precession cycle. Like all cycles, the yuga cycles also comes in two major phase ascending and descending or waxing and wanning like the phase of the Moon. In the waxing phase, the level of consciousness increases as time progresses moving from the Kali Yuga towards the Satya Yuga, while in the wanning phase, the level of consciousness decreases from Satya yuga to Kali yuga again. If we accept this principle, then two yuga cycles will form a pair in a complete precession cycle with a total duration of 24,000 years.
Now let's have a re-look at whether the duration of the yuga cycle be 1200 divine years or 1200 human years. It seems that 1200 divine years could be a misinterpretation of what it was meant to be. The divine years are from Daiva varsha in Sanskrit. Daiva is from Deva and Deva is someone who is full of light. It could mean Surya (Sun), who is full of light and brighness and cause day. 1 daiva varsha could mean 1 solar year. 1 sidereal solar year would have 360 solar days and each day would measure 1 degree movement of the Sun in the sidereal zodiac. One must note that since the precession of the equinox as a phenomenon is related to the sidereal zodiac, the definition of a day should correspond to 1 degree motion of Sun in the sidereal zodiac and not the day which is caused by Sunrise & Sunset. Thus 1200 daiva varsha could mean 1200 solar years in which each year correspond to 360 sidereal-solar days.
If we accept that the yuga cycle correspond to the precession cycle, let's find out which yuga we should be in now. To understand this, we need to know when the Kali Yuga started and find references of association of Yugas with known time scale. If we try to tag the beginning of the yugas to the beginning of the sidereal zodiac, Kali yuga should start on February 11th or 12th, 285 A.D, when both the sidereal and tropical zodiac were perfectly aligned. If we accept this then the forthcoming years would have following duration:
(Note: All dates are as per Gregorian Calendar)
1. Kali - 1200 years: 285 A.D. - 1485 A.D.
2. Dvapara - 2400 years: 1485 A.D. - 3885 A.D.
3. Treta - 3600 years: 3885 A.D. - 7485 A.D.
4. Satya - 4800 years: 7485 A.D. - 11085 A.D.
Total: 12,000 years: Aarohini cycle (Ascending)
If we go backward in time, the Avarohini cycle (Descending) would be
1. Kali - 1200 years: 285 A.D. - 915 B.C.
2. Dvapara- 2400 years: 915 B.C. - 3315 B.C.
3. Treta - 3600 years: 3315 B.C. - 6915 B.C.
4. Satya - 4800 years: 6915 B.C. - 11715 B.C.
Total: 12,000 years: Avarohini cycle (Descending)
This indicates that we are currently in Dvapara yuga and 523 years have already elapsed.
If we find the angular distance the point of vernal equinox would travel in various yugas as per the association mentioned above, it would look like this (moving in an antizodical direction):
1. Kali : 18deg: Pisces 18deg - Pisces 30deg
2. Dvapara: 36deg: Aquarius 6deg - Pisces 18deg
3. Teta : 54deg: Sagittarius 12deg - Aquarius 18deg
4. Dvapar : 72deg: Libra 0deg - Sagittarius 12deg
However, this does not match with the currently accepted timescale for the beginning of the the kali yuga, which is 23rd January 3102 BC. If we align the yuga start and end with the this timescale, the dates will looks like this:
1. Dvapara (Descending) - 2400 years: 5502 BC - 3102 BC
2. Kali (Descending) - 1200 years: 3102 BC - 1902 BC
3. Kali (Ascending) - 1200 years: 1902 BC - 702 BC
4. Dvapara (Ascending) - 2400 years: 702 BC - 1698 AD
5. Treta (Ascending) - 3600 years: 1698 AD - 5298 AD
6. Satya (Ascending) - 4800 years: 5298 AD - 10098 AD
There are various other scholars who have even different opinions such as Yukteswar Giri. According to him
We are currently in the beginning stages of Dwapara Yuga, which began around 1699 A.D. This now puts us in the year 307 Dwapara according to Sri Yukteswar. Thus, we are moving closer to the grand center of our galaxy, and will pass into Treta Yuga around the year 4099 A.D. More and time and research into this area would help in clarifying the issue further and enable us to settle down on the phase we are in.
Now how can we use this knowledge? While pondering over this, I came up with an idea. We can equate the ascending and descending phenomenon of the yugas with the phenomenon of Lunar Phases. Here the middle of Satya Yuga will represent the brightest phase of Moon, full moon or Purnima and the middle of the Kali yuga would represent the darkest phase of Moon, the new moon or the Amavasya and all other time can represent one of the phases of the Moon. The aarohini yugas or the ascending yugas can correspond to the waxing phase of Moon while the avarohini yugas or the descending yugas can correspond to the waning phase of Moon. Here we can divide the zodiac into 30 parts of 12degrees, where each part correspond to 1 lunar phase. The first 12degrees from Aries (in ant zodiacal direction, which is 18deg Pisces to 30deg Pisces) can be associated with 1st day of wanning day, the shukla partipada which is ruled by the Sun. For ease of reckoning we can call this Yuga Tithi and assign them the lordship based on the Lunar Tithis. Since there will be 30 yuga tithis in the twin yuga cycle of 24,000 years, the duration of each yuga tithi would be 800 sidereal years.
Since currently the equinox in around 6 deg pisces, which is 24 degree away from the beginning of the zodiac (Ashvini nakshatra) we are getting near to the end of Yuga Tithi Dvitiya (the 2nd tithi) ruled by Moon (care, compassion, relationship, mutual co-existence, preponderance of emotions). For next 800 years when Equinoxial point would be moving from 6deg Pisces to 24deg Aquarius, we will be in the Yuga tithi of Shukla Tritiya ruled by Mars. So what are we going to expect in the coming 800 years! Wars, Strife and Struggles all over the world. Mars also rules over rationality and logical thinking process as well as Engineering. Thus the world would see predominance of move individualism, rational and logical thinking over relationship, care and compassion and creation of some of the most wonderful engineering marvels.