Chronology

I

Introduction

Chronology, science dealing with division of time into regular periods, the arrangement of events in order of their occurrence, the assignment of correct dates to known events, and the reconciling of discrepancies in dates caused by variations in the systems employed in modern and ancient times (see Dating Methods).

II

Astronomical Time

Astronomical chronology is based on celestial phenomena and laws. The dates of celestial phenomena can be determined quite accurately by mathematical computation. By reckoning backward, the date of a historical event can often be verified or determined with precision if it was associated with an astronomical event, such as a solar eclipse (see below).

III

Geological Time

Covering the earth's entire history, beginning some 4.6 billion years ago, the scale of geologic chronology is second only to astronomical time, which covers the age of the universe (see Cosmology). Geologists of the last century had scant evidence for calculating the age of the earth and its materials, arriving at widely disparate estimates of from 3 million to 500 million years, based on the rate of accumulation of sedimentary deposits. Relying mainly on stratigraphic correlation, using fossils and other kinds of evidence (see Geology), they succeeded in constructing a relative time scale. But this first attempt at geologic chronology was of little use for comparing the stratigraphy of one continent with another, and without such comparisons the history of the earth remained largely an enigma. Then the discovery of radioactivity changed all this by laying the foundation for radiometric dating. Such dating techniques have since made it possible to calculate the absolute age of a mineral or rock and thereby date the age of the earth as well as events of the remote geologic past with an unprecedented degree of certainty.

IV

Archaeological Time

Although advanced civilizations are often dated by political chronology, some simple cultures are dated in ways more akin to the chronological methods used in geology. Thus, archaeologists carefully note the order of successive deposits (stratigraphy) containing human artifacts. The principle of stratigraphy assumes that in undisturbed strata the younger (more recent) layers overlie the older (earliest) layers, a relation often referred to as the law of superposition. The stratigraphic method of archaeological dating parallels that used in geology. In both methods, the thickness of deposits is one of the variables in time determination. Occasionally in the case of an ancient people, graves or artifacts occur in such a manner that actual geochronological methods help determine the age of the deposit. A widely used method for dating human cultures is the radiocarbon (carbon-14, or C¹⁴) technique. Based on the fact that living organisms take up a naturally occurring radioactive form of carbon during their lifetimes, this method permits investigators to determine how much radiocarbon still remains in an organic specimen found with any cultural remains. This determination relates directly to the time elapsed since the specimen was buried, and relatively accurate dates may be worked out from such information.

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