It is based on lunar cycles. As in when the moon is full or not and etc. It is as Ping said, not some magic voodoo stuff.
Okay, every one understands its not magic voodoo, its purely science...
However, could you be so kind to give me some links to your sources about your lunar cycles idea, because I really don't think you know crap from crab apples about Mayans or their practices and works, and that your just pulling random thoughts from your head.
I didn't exactly attend a "Mayan Civilization 101" but I can tell you its not just based on lunar cycles....It's pretty much based off Earth cycles, but the only way for an ancient civilization to learn about Earth cycles is through cycles of the sun and the moon, and seasons.
And going back to your previous argument that the earth, sun, and the center of the galaxy line up every year, you misunderstood me...When i said coming into line of the center of the galaxy, i meant in line on the galactic equator. If your interested, read more about that here
http://www.artideas.com/Why2012/Why2012.html