In chronology and periodization , an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era . The "epoch" serves as a reference point from which time is measured.
9-419: The moment of epoch is usually decided by congruity, or by following conventions understood from the epoch in question. The epoch moment or date is usually defined from a specific, clear event of change, an epoch event . In a more gradual change, a deciding moment is chosen when the epoch criterion was reached. The official Japanese system numbers years from the accession of the current emperor , regarding
18-463: A calculated point during a continuous action (such as in aviation ). A particular irreversible action (such as setting off an explosion or signing a contract) can be a point of no return. The phrase "point of no return" originated as a technical term in air navigation to refer to the time and/or location during a flight at which the aircraft no longer has enough fuel to return to its originating airfield. Important decisions may need to be made prior to
27-466: A return flight, so the "point of no return" may represent the point before which the pilot must return or else risk catastrophe. It can also mean the instance in which an aircraft taxis down a runway, gaining a certain speed, and must become airborne in lieu of a crash or explosion on the runway ( V1 speed )—for example, Charles Lindbergh 's takeoff in The Spirit of St. Louis in 1927 in which there
36-635: Is J2000, which is noon on January 1, 2000, Terrestrial Time . An epoch in Geochronology is a period of time, typically in the order of tens of millions of years. The current epoch is the Holocene . Point of no return The point of no return ( PNR or PONR ) is the point beyond which one must continue on one's current course of action because turning back is no longer possible, being too dangerous, physically difficult, or prohibitively expensive to be undertaken. The point of no return can be
45-650: The calendar year during which the accession occurred as the first year. A similar system existed in China before 1912, being based on the accession year of the emperor (1911 was thus the third year of the Xuantong period). With the establishment of the Republic of China in 1912, the republican era was introduced. It is still very common in Taiwan to date events via the republican era. The People's Republic of China adopted
54-482: The common era calendar in 1949 (the 38th year of the Chinese Republic). An epoch in computing is the time at which the representation is zero. For example, Unix time is represented as the number of seconds since 00:00:00 UTC on 1 January 1970, not counting leap seconds . An epoch in astronomy is a reference time used for consistency in calculation of positions and orbits. A common astronomical epoch
63-403: The course of the story, the character faces two "point of no return" realities: first, that his quest for a big promotion will mean either triumph or a dead end to his career, and second, that he can never go back to the small-town life he abandoned as a young man. There are a number of phrases with similar or related meaning: The point of safe return (F) is the last point on a route at which it
72-440: The point of no return, since it will be unsafe to turn around and fly back if the pilot changes their mind after that point. Otherwise, it may correspond to the aircraft's maximal safe range in a situation where the only possible landing site is the takeoff site, for example in the case of an aircraft to an aircraft carrier that is underway and distant from any airfield. In those conditions, an aircraft must always have enough fuel for
81-416: Was uncertainty about the plane's ability to take off from a 5,000-foot mud-soaked runway while fully loaded with aviation fuel. The first major metaphorical use of the term in popular culture was in the 1947 novel Point of No Return by John P. Marquand . It inspired a 1951 Broadway play of the same name by Paul Osborn . The novel and play concern a pivotal period in the life of a New York City banker. In
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