Why do you think some early cultures based their calendars on the cycles of the moon?

There are several major reasons as to why the Moon was used by early cultures for creating their calendars. One of the reasons was that the Moon was an object on the sky that was easily observable and was on the sky constantly. Another major reason is that the Moon has phases, so they can be used like a natural calendar and establish a man made one because the phases repeated themselves. Once one phase of the Moon was taken as a staring point, in roughly 28 days the Moon was going to be in the same phase again, so it was a nice and practical way to construct the months.

During ancient times, moon was considered sacred. It was the biggest thing visible to the human eye in the sky. The need of a calendar was important to keep a track of the crops growing in different seasons.

There the changing shaped of moon was observed and the placement of moon in the sky also changes with the changing seasons. Therefore, the changing shapes were considered as dates and the placement is considered as the season. It was first established by the Sumerians. Moon is still considered sacred.  

a social contract.

"a social contract is an agreement between citizens and their leader where the citizens give up certain freedoms in exchange for protection. the constitution of the united states is an example of a social contract."