For thousands of years, man has conceived "time" in a cyclical way, because nature has an apparently regular rhythm. Days follow nights, seasons follow seasons, years follow years. Cyclical time is therefore a kind of "timeless, immortal heritage of mankind" which has both a natural and a social origin. Nature sometimes records these marks of cyclical time: seasonal production of tree-rings, stalactites in caves, glacier deposits, etc.
Dendrochronology is the scientific study of tree-rings to trace the course of time and sometimes to find out about past climates. A cut trunk has concentric rings called tree-rings. They vary in thickness and colour. Light rings indicate a period of growth (spring-summer), dark rings a period of rest (autumn-winter). Wide rings correspond to good years for growth and narrow rings to "bad" years. The ring growth is translated into data and graphs for each tree analysed. The data and the graphs are compared and then, from thousands of trees of the same species, a reference graph is established that allows us to go back a long way in time. It is thus possible to date living trees, but also dead trees, used in ancient constructions. By comparing vast amounts of tree-growth data, we are able to go back in time over centuries or even millennia.