1911 Encyclopædia Britannica/Epoch

EPOCH (Gr. ἐποχή, holding in suspense, a pause, from ἐπέχειν, to hold up, to stop), a term for a stated period of time, and so used of a date accepted as the starting-point of an era or of a new period in chronology, such as the birth of Christ. It is hence transferred to a period which marks a great change, whether in the history of a country or a science, such as a great discovery or invention. Thus an event may be spoken of as “epoch-making.” The word is also used, synonymously with “period,” for any space of time marked by a distinctive condition or by a particular series of events.

In astronomy the word is used for a moment from which time is measured, or at which a definite position of a body or a definite relation of two bodies occurs. For example, the position of a body moving in an orbit cannot be determined unless its position at some given time is known. The given time is then the epoch; but the term is often applied to the mean longitude of the body at the given time.