epoch

• (astronomy) an arbitrarily fixed date that is the point in time relative to which information (as coordinates of a celestial body) is recorded
• a period marked by distinctive character or reckoned from a fixed point or event
• a unit of geological time that is a subdivision of a period and is itself divided into ages
• A fixed point of time, established in history by the occurrence of some grand or remarkable event
• A period of time, longer or shorter, remarkable for events of great subsequent influence
• A division of time characterized by the prevalence of similar conditions of the earth
• The date at which a planet or comet has a longitude or position
• An arbitrary fixed date, for which the elements used in computing the place of a planet, or other heavenly body, at any other date, are given

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