For example, the results of dendrochronology (tree-ring) analysis may tell us that a particular roof beam was from a tree chopped down in A. For example, the stratum, or layer, in which an artifact is found in an ancient structure may make it clear that the artifact was deposited sometime after people stopped living in the structure but before the roof collapsed.
However, the stratigraphic position alone cannot tell us the exact date.
Prior to the development of radiocarbon dating, it was difficult to tell when an archaeological artifact came from.
Unless something was obviously attributable to a specific year -- say a dated coin or known piece of artwork -- then whoever discovered it had to do quite a bit of guesstimating to get a proper age for the item.
Fortunately, Willard Libby, a scientist who would later win the 1960 Nobel Prize in Chemistry, developed the process known as radiocarbon dating in the late 1940s. In a nutshell, it works like this: After an organism dies, it stops absorbing carbon-14, so the radioactive isotope starts to decay and is not replenished.
Archaeologists can then measure the amount of carbon-14 compared to the stable isotope carbon-12 and determine how old an item is.
Until this century, relative dating was the only technique for identifying the age of a truly ancient object.
By examining the object's relation to layers of deposits in the area, and by comparing the object to others found at the site, archaeologists can estimate when the object arrived at the site.
Inscribed objects sometimes bear an explicit date, or preserve the name of a dated individual. However, only a small number of objects are datable by inscriptions, and there are many specific problems with Egyptian chronology, so that even inscribed objects are rarely datable in absolute terms.
In the archaeology of part-literate societies, dating may be said to operate on two levels: the absolute exactness found in political history or 'history event-by-event', and the less precise or relative chronology, as found in social and economic history, where life can be seen to change with less precision over time.
Without the ability to date archaeological sites and specific contexts within them, archaeologists would be unable to study cultural change and continuity over time.
No wonder, then, that so much effort has been devoted to developing increasingly sophisticated and precise methods for determining when events happened in the past.
Carbon-14, or radiocarbon, is a naturally occurring radioactive isotope that forms when cosmic rays in the upper atmosphere strike nitrogen molecules, which then oxidize to become carbon dioxide.