The Mechanism of Plana's Calendar

To menage complex social activities such as job, religion, commerce, political institutions, the mankind needs to develop tools for measuring and organizing time. Roughly speaking, such tools are clocks (for period of time less than one day) and calendars (for periods longer than a day). Since the most ancient civilized countries, calendar modeling has been related to the periodic motion of celestial bodies. Of course, from the Earth's point of view, the most important celestial bodies are the Sun and the Moon. Unfortunately, the periods of their motions are difficult to compare, and cannot be reduced each other easily. Other merely cultural traditions (for instance, the 7 days week) further complicate thinks. In summary, to design a calendar is not an easy task. This may not be a problem as far as we operate over short periods, small areas, and the amount of the required information is minimal. But the amount of information and the need of precision becomes more and more crucial, because of the evolution of the human society and the growth of complexity.