Any regularly recurring event can be used as a measure of absolute or numerical time. The Earth regularly orbits the Sun. The length of time it takes for one orbit is one year. Many natural recurring events leave a permanent record in rocks or fossils that allow us to measure absolute time. For example, you may have noticed that the cross section of a tree trunk consists of a series of concentric "rings". Each year, the tree adds a ring. We can thus tell how old a tree is by counting the rings. The thinner rings represent dry years, whereas the wider rings grew when the climate was wetter.