For thousands of years, man has conceived the notion of time in a cyclical manner, since nature apparently has a regular rhythm that repeats itself infinitely: days follow nights, years follow years... Cyclical time is therefore a kind of "immemorial heritage of humanity" which has both a natural and a social origin.
Nature records the marks of cyclical time: seasonal production of tree rings, stalactites in caves, etc.
Dendrochronology, or tree-ring dating, is the science that analyses tree-rings to determine dating of events and environmental change and learn about paleoclimates, or past climate-changes. A cut trunk has concentric rings (tree-rings) of varying thickness and colour. Light rings indicate a period of growth (spring-summer), dark rings a period of rest (autumn-winter). The wide rings correspond to years that are favourable for growth, the narrow rings to "bad" years for growth. In this way, the age of the tree can be deduced and information can be obtained about the climate of past centuries, as some trees live for several centuries. The growth rate of the rings is translated into data and a graph for each tree analysed. The plotted graphs of thousands of trees of the same species are compared to obtain a reference graph used to date living trees, but also dead trees, especially those used for beams and elements of wood construction found in old buildings.