The debunking is based on the assumption that our solar system orbits the plane of the milky way galaxy in a linear fasion. Scientist belive this is not the case. Infact our solar system is part of the micro galaxy of sagitarius and this micro galaxy is being sucked up by the milky way galaxy.
A new infra red digital survey of the entire sky was made in 2003. Teams from the universities of Virginia and Massachusetts used a supercomputer to sort through half a billion stars to create a -- NEW STAR MAP showing our Solar System (yellow circle) to be at the exact nexus crossroads where two galaxies are actually joining.
It is my best guess that the Mayans new of this cycle.
here is the full article. see for your self.
http://viewzone.com/milkyway.html