List of time periods

Contents

The categorization of time into discrete named blocks is called periodization. This is a list of such named time periods as defined in various fields of study. Major categorization systems include cosmological (concerning the various time periods in the origin and evolution of our universe), geological (concerning time periods in the origin and evolution of earth ) and historical (concerning time periods in the origin, evolution of mankind).

Human time periods

Based on current and debatable evidence, the human species has found its origins starting from about 250,000 years ago - when homo began to develop. It is broadly divided into prehistorical (before history began to be recorded) and historical periods (when written records began to be kept).

Further information: Human evolution

Prehistorical periods

Main article: List of archaeological periods

In archaeology and anthropology, human prehistory is subdivided around the three-age system.

The dates for each age can vary by region. On the geologic time scale, the Holocene epoch starts at the end of the most recent Ice age (about 9400 BC) and continues to the present. The beginning of Mesolithic is usually considered to correspond to the beginning of the Holocene epoch.

Historical periods

Further information: History of the world

Calendar systems

Various societies in the past have created calendars to record events, such as religious observances and agricultural tasks. A common characteristic of most known calendars is that they measure time in relation to a particular point in history, known as the epoch date. A period between epoch dates is known as a calendar era. 27!É

Mythological and astrological time periods

Cosmological time periods

13.7 billion years ago: The Big Bang

Main article: Timeline of the Big Bang

Because of the scales involved (both very large and very small), cosmological time periods are usually described in seconds. In this table, each row is defined in seconds after the Big Bang, with earliest at the top of the chart.

Start End Period
0 s 1×10−43 s Planck epoch
1×10−43 s 1×10−36 s Grand unification epoch
1×10−36 s 1×10−32 s Inflationary epoch
1×10−32 s 1×10−12 s Electroweak epoch
1×10−12 s 1×10−6 s Quark epoch
1×10−6 s 1 s Hadron epoch
1 s 3 min Lepton epoch
3 min 380,000 yr Photon epoch

Formation of Population III stars

The first stars were formed from the Hydrogen and Helium formed in the Big Bang were short lived massive Population III stars. Nuclear processes in these stars converted the Hydrogen and Helium into metals and other heavier elements. As the Population III stars died these heavier elements were released.

Formation of Population II stars

Population II stars contain metals formed in the Population III stars. These were longer lived than the Population III stars and some of them are still around. In addition to the metals these inherited from the Population III stars the Population II stars also formed metals by nuclear reactions and when the stars died much of that material was returned to be used as the building blocks for the next generation of stars.

5 Billion Years ago - Formation of Population I stars

Population I stars are also known as metal rich stars. Our own sun is a Population I star and was formed about 5 billion years ago.

Geologic time periods

Main articles: Geologic time scale and List of Global Boundary Stratotype Sections and Points

The geologic time scale covers the extent of the existence of Earth, from about 4600 million years ago to the present day. It is marked by Global Boundary Stratotype Sections and Points. Geologic time units are (in order of descending specificity) eons, eras, periods, epochs, and ages; and the corresponding chronostratigraphic units, which measure "rock-time", are eonothems, erathems, systems, series, and stages.

The second and third timelines are each subsections of their preceding timeline as indicated by asterisks. The Cenozoic is sometimes divided into the Quaternary and Tertiary periods, although their use is no longer official.




Millions of Years

See also

References