Below is an article written by Dr Ong Hean Tatt and if what he wrote is true, it will shake the Feng Shui world! I have reproduced it here due to its importance and research value. You can find the article on his Gui Management website also.
ERRORS IN COMPUTATION OF FENG SHUI AND ASTROLOGICAL YEARS
2. ISSUE OF 20 YEARS YUN AND SATURN JUPITER CONJUNCTION
Prof. Dr. Ong Hean-Tatt 10th January 2008
Most are using the wrong Yun Sign in areas like Xuan Kong, etc.!
As each Chinese New Year approaches, there would be a spate of forecasts about the coming year by both astrologers and Feng Shui masters. The forecasts can be conflicting or do not fit what eventually happened in the said year. There can be good reasons why this is so, which are generally that the current masters failed to take into account the "precession of the Equinoxes" in their dating methods!
Another important issue which creates date inaccuracies is the 20 years Yun used in San Yuan Feng Shui , which is due to the Jupiter Saturn conjunction, whose periodicity of 20 years is also the basis of the astrological Age of the Hebrews.
Saturn, the "Ancestral Temple" Planet
The Chinese astrology terms the beautiful ringed planet Saturn as the "Ancestral Temple" star. It makes one orbit around the Sun in 29.5 years. The conjunctions of Saturn with the other planets form important markers in astrological Time. The Saturn Jupiter conjunction is fundamental in the Chinese astrological period of the 20 years Yun: Jupiter has an orbit of 11.882 years. The conjunction of Saturn with Jupiter occurs every 19.84 years.
The Saturn Jupiter conjunction is the basis of the Chinese astrological 'Yun'. Feng Shui and astrology practitioners should understand the astronomy of their arts.
Jupiter Saturn Conjunction as Basis of Yun of 20 Years Duration
Each San Yuan period of 180 years will have 9 Yuns, with 3 successive Yuns forming the 60 years Yuan. If a Jupiter Saturn conjunction occurs at one point on the orbit around the Sun, it will return to this same point 60 years later, forming the Yuan era of 60 years. The 1st, 2nd and 3rd conjunctions will occur at 3 different points along the orbit. The point of the 1st conjunction will also be that of the 4th and 7th conjunctions, while the point of the 2nd conjunction will also be that of the 5th and 8th conjunctions, while the 3rd, 6th and 9th conjunctions will be at the same point. This yields the auspicious 1-4-7, 2-5-8 and 3-6-9 Parent Strings combinations in San Yuan Feng Shui forecasts.
Hebrews add One Jupiter-Saturn Cycle every 2,000 Years
By convention, since Emperor Yao's time circa 2405 BC, the Yun starting from 2004 is supposed to be Yun 8. However, the Jupiter Saturn conjunction is not exactly 20 years but 19.84 years. The difference is insignificant in the short term but will build up into a serious difference every 2,000 years. The Hebrews will adjust for this discrepancy by adding an extra Jupiter Saturn Age or cycle every 2,000 years. But the Chinese had not done this since Yao's time.
Need for Adjustments of Yuns in Feng Shui and Astrology
In the over 4,000 years since Yao, the Jupiter Saturn conjunction will jump at least 2 cycles - meaning what is now taken as Period 8 is actually Period 1! We note that about every 2,000 years, the Jupiter Saturn conjunction will jump one Yun cycle. Coincidentally, every 2,000 years the precession of the equinoxes will also cause the Zodiac sign to shift one sign. That is, every 2,000 years there will be a need to shift one Yun sign and also one Zodiac sign.