Chinese astrology uses a 60 year Stem-Branch cycle.
Where does it come from?
Well, a sixty year cycle seems to pop up quite frequently in nature. For example in climate, in sea levels, in length of the day variations, in changes of water levels in lakes, in solar activity.
This article lists some studies that found a 60 year cycle: http://www.appinsys.com/globalwarming/SixtyYearCycle.htm
This sixty year rhythm corresponds to the so-called “tri-synodic” period of Jupiter and Saturn, which is 59.6 years.
In fact, we could say that the 12 Branches track the movement of Jupiter because Jupiter takes about 12 years to orbit the Sun. And the 10 Stems keep up with Saturn, which takes about 30 years to orbit the Sun. Three cycles of the 10 Stems in the Year thus correspond to one orbit of Saturn around the Sun.
What is this tri-synodic period?
The ancients observed long time ago that Jupiter and Saturn meet in the sky every 20 years. That’s the likely origin of the twenty year periods used in flying stars feng shui. But they probably also noticed that Jupiter and Saturn meet in exactly the same part of the sky every 60 years. That’s the tri-synodic period.
For example in May 1782 Jupiter and Saturn were conjunct at 28 degree Sagittarius, very close to the winter solstice point. 60 years later, in January 1842, Jupiter and Saturn were again conjunct, now at 8 degree Capricorn, just 10 degree of arc away from their conjunction 60 years earlier. And another 60 years later, Jupiter and Saturn were again conjunct, at 14 degree Capricorn, again only 6 degree of arc away from where they met 60 years earlier. And that’s how it goes on forever.
You can take any random date and calculate the position of Jupiter and Saturn in the sky. Then calculate the position of these planets 60 years ago and you will find both of them in nearly the same place. E.g:
* Today 28 April 2013. Jupiter is at 16 degree Gemini, Saturn is at 8 degree Scorpio.
* Go back 60 years and find the positions for 28 April 1953: Jupiter was at 27 Taurus and Saturn was at 23 Libra.
* Go forward 60 years and find the positions for 28 April 2073: Jupiter will be at 7 Cancer and Saturn at 23 Scorpio
There is only about 10 degree shift every tri-synodic cycle. But the bottom line is that every 60 years Jupiter and Saturn form a very similar constellation in the sky, and that’s the likely origin of the 60 year cycle used in Chinese astrology.