Science / Tides and Currents / Epoch: (1) Also known as phase lag. Angular retardation of the maximum of a constituent of the observed tide (or tidal current) behind the corresponding maximum of the same constituent of the theoretical equilibrium tide. It may also be defined as the phase difference between a tidal constituent and its equilibrium argument. As referred to the local equilibrium argument, its symbol is 6. When referred to the corresponding Greenwich equilibrium argument, it is called the Greenwich epoch and is represented by G. A Greenwich epoch that has been modified to adjust to a particular time meridian for convenience in the prediction of tides is represented by g or by 6N. The relations between these epochs may be expressed by the following formula: G = 6 + pl, g = 6N = G – as / 15 in which L is the longitude of the place and S is the longitude of the time meridian, these being taken as positive for west longitude and negative for east longitude; p is the number of constituent periods in the constituent day and is equal to 0 for all long-period constituents, 1 for diurnal constituents, 2 for semidiurnal constituents, and so forth; and a is the hourly speed of the constituent, all angular measurements being expressed in degrees. (2) As used in tidal datum determination, it is a 19-year cycle over which tidal height observations are meaned in order to establish the various datums. As there are periodic and apparent secular trends in sea level, a specific 19-year cycle (the National Tidal Datum Epoch) is selected so that all tidal datum determinations throughout the United States, its territories, Commonwealth of Puerto Rico, and Trust Territory of the Pacific Islands, will have a common reference. See National Tidal Datum Epoch.

National Tidal Datum Epoch

Science / Tides and Currents / National Tidal Datum Epoch: The specific l9-year period adopted by the National Ocean Service as the official time segment over which tide observations are taken and reduced to obtain mean values (e.g., mean lower low water, etc MORE

Prenatal Epoch

Science / Astrology / Prenatal Epoch: The astrological moment of conception. According to Ptolemy, the natal Ascendant or Descendant coincides with the Moon's zodiacal position at the prenatal epoch, ten lunar months prior to birth, more MORE

Polarity Epoch

Science / Geology / Polarity Epoch: An interval of time between reversals of Earth's magnetic field. MORE