Epoch (reference date)  

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Train wreck at Montparnasse (October 22, 1895) by Studio Lévy and Sons.
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Train wreck at Montparnasse (October 22, 1895) by Studio Lévy and Sons.
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In the fields of chronology and periodization, an epoch means an instant in time chosen as the origin of a particular era. The "epoch" then serves as a reference point from which time is measured. Time measurement units are counted from the epoch so that the date and time of events can be specified unambiguously.

Events taking place before the epoch can be dated by counting negatively from the epoch, though in pragmatic periodization practice, epochs are defined for the past, and another epoch is used to start the next era, therefore serving as the ending of the older preceding era. The whole purpose and criteria of such definitions is to clarify and co-ordinate scholarship about a period, at times, across disciplines.

Epochs are generally chosen to be convenient or significant by a consensus of the time scale's initial users, or by authoritarian fiat. The epoch moment or date is usually defined by a specific clear event, condition, or criteria— the epoch event or epoch criteria —from which the period or era or age is usually characterized or described.

Examples:
by events:
The assassination of the Roman Emperor Alexander Severus triggering the Crisis of the Third Century
The defenestration of Prague triggering the Thirty Years' War
Queen Victoria ascending to the throne giving the start of the Victorian era
by criteria:
The spurt in exploration, mercantilism, and colonization in the Age of Discovery
Particular ratios of animal fossils in a rock strata —various Geology epochs




Unless indicated otherwise, the text in this article is either based on Wikipedia article "Epoch (reference date)" or another language Wikipedia page thereof used under the terms of the GNU Free Documentation License; or on original research by Jahsonic and friends. See Art and Popular Culture's copyright notice.

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