overtide— A harmonic tidal (or tidal current) constitiient with a speed tliat is an exact 

 multiple of the speed of one of the fundamental constituents derived from the 

 development of the tide-producing force. The presence of overtides is usually attributed 

 to sliallow-water conditions. The overtides usually considered in tidal work and tlie 

 harmonics of the principal lunar and solar semidiurnal constituents M2 and S2 are 

 designated by tlie symbols M4 , M6 , Mg , S4 , 85 , etc. The magnitudes of tliese harmonics 

 relative to those of tlie fundamental constituents are usually greater in the tidal current 

 than in the tide. 



perigean tides or tidal currents— Tides of increased range or tidal currents of increased speed 

 occurring monthly as the result of the Moon being in perigee or nearest tlie Earth. The 

 perigean range (Pn) of tide is the average semidiurnal range occurring at tlie time of 

 perigean tides and is most conveniently computed from the harmonic constants. It is 

 larger than tlie mean range where the type of tide is either semidiurnal or mixed, and is 

 of no practical significance where the type of tide is diurnal. 



perigee— The point in the orbit of the Moon at which it is nearest the Earth. When used in 

 connection with the solar tides it is quahfied as solar perigee and refers to tlie position of 

 the Sun when nearest the Earth. 



perihelion— The point in the orbit of the Earth (or otlier planet) nearest the Sun. 



period— Interval required for the completion of a recurring event, such as the revolution of a 

 celestial body, or tlie time between two consecutive-hke phases of tlie tide or tidal 

 current. A period may be expressed in angular measure and is then taken as 360°. The 

 term is also used to express any specified duration of time. 



phase— (1) Any recurring aspect of a periodic phenomenon, as new Moon, high water, 

 strength of flood, etc. (2) A particular instant of a periodic function expressed in angular 

 measure and reckoned from tlie time of its maximum value, the entire period of tlie 

 function being taken as 360°. The maximum and minimum of a harmonic constituent 

 have phase values of 0° and 180°, respectively. 



primary control tide station— A tide station at which continuous observations have been 

 made over a minimum of a 19-year metonic cycle. It provides data for computing 

 accepted values of the harmonic and nonharmonic constants essential to tide predictions 

 and to the determination of tidal datums for charting and coastal boundaries. The data 

 series from this station serves as a primary control for the reduction of relatively short 

 series from subordinate tide stations through the method of comparison of simultaneous 

 observations, and for monitoring long-period sea level trends and variations. See tide 

 stations, subordinate tide stations (1), secondary control tide station, and tertiary tide 

 station. 



primary tidal bench mark— See bench mark. 



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