You don't need to measure a radius to set out a circle. Any radius will do and any radius will circumscribe a perfect circle, whether or not its measured.

In addition, the stones are not regularly set out - they are roughly set out around a radius.

A calendar means you know when to plant and harvest, and how quickly you are using your surplus - its a vital part of any organised agricultural society, and not only was it practically useful, the yearly cycles were part of the rituals of the society, hence festivals and holidays related to the solar year.

Dave Light