Why Emperors Consulted Chen Xi Yi
The interesting question is why the Sung founding emperors should consult Chen Xi Yi and his Zi Wei Dou Shu when they could have easily used the Tang Ba Zi Eight Characters.
This was because they knew that the Zi Wei Dou Shu known by Chen Xi Yi was a more original astrology much more potent than Ba Zi Eight Characters. It was always a system like Zi Wei Dou Shu which the emperors knew was what the royal circle needed. The imperial circle had no use for Ba Zi Eight Characters, which could only forecast ordinary things and which could not fulfill their high needs.
In fact, the emperors would hope that the people would never access the high astrology used by the imperial circle. However, the people too wanted to be able to forecast their future. So, what could the emperors do?
The emperors during the time of the Tang dynasty already decided that they could prevent the people from accessing the secretive imperial astrology by inventing another less potent system for the people! This was the real origin of Ba Zi Eight Characters divination.