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epoch

(1) A particular instant of time from which an event or a series of events is calculated; a starting point in time, to which events are referred. In particular, a date and instant corresponding to the position of a coordinate system, all subsequent positions being referred to the position at that epoch. In this sense, it is equivalent to date (1). Many writers prefer to use date for this meaning, since it denotes an instant unambiguously. However, the two terms have different connotations. Epoch is associated with a point in time to which events are referred; date is associated with a value of time and is associated with only the event that occurred then. Also, date is commonly used for civil date, which is not an instant. (2) The angular lag of the maximum of the observed tide or tidal current behind the corresponding maximum of the same constituent of the theoretical equilibrium tide. When the theoretical, local equilibrium tide is meant, the epoch is represented by κ. When the corresponding equilibrium tide at Greenwich is meant, it is called the Greenwich epoch and is represented by the symbol G. (3) A specific period of time.

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