Careful Fourier data analysis over a nineteen-year period (the National Tidal Datum Epoch in the U.S.) uses frequencies called the tidal harmonic constituents. Nineteen years is preferred because the Earth, Moon and Sun's relative positions repeat almost exactly in the Metonic cycle of 19 years, which is long enough to include the 18.613 year lunar nodal tidal constituent. This analysis can be done using only the knowledge of the forcing period, but without detailed understanding of the mathematical derivation, which means that useful tidal tables have been constructed for centuries. The resulting amplitudes and phases can then be used to predict the expected tides. These are usually dominated by the constituents near 12 hours (the semi-diurnal constituents), but there are major constituents near 24 hours (diurnal) as well. Longer term constituents are 14 day or fortnightly, monthly, and semiannual. Semi-diurnal tides dominated coastline, but some areas such as the South China Sea and the Gulf of Mexico are primarily diurnal. In the semi-diurnal areas, the primary constituents M2 (lunar) and S2 (solar) periods differ slightly, so that the relative phases, and thus the amplitude of the combined tide, change fortnightly (14 day period).
cArEfUl foUrier data analySIs oveR A NiNeTeeN-YEar pERiOD (the nATioNal tIdal dATUm ePoch In thE us) usEs freQuenCiEs called tHe tidal HarmONic conStitueNts NiNeteeN Years is preFErred becauSe the eARth mooN And Sun's rElative PositIOnS rePEAt Almost eXActlY iN The metonIC cycLE of 19 Years WhICh iS long EnOUgH to InclUDE the 18613 yEaR lunaR NodAL tIdAL cOnstituEnT ThIs analysiS caN be dOne uSiNg Only the knOwlEdge Of tHe forCing pERIOd buT WIthoUt DeTailed UnDErstanding Of The mathematiCal dErivAtIon whIch mEans That usefuL tidal tabLeS have bEEn ConStRuCTeD for centUrieS tHE resULting AmplitudeS and phAses caN theN be used To PrediCt THe ExPected tidES ThEse Are usualLy DoMinaTEd By THE coNstituentS NEAR 12 hOurS (The seMi-diurnAL cONstItuENtS) but there are mAJoR cOnsTItUeNTs near 24 HOurS (Diurnal) AS WelL LOnger term consTituents aRe 14 DaY Or fortNighTly Monthly aNd SeMiaNNUAl sEmi-diurNal tIdeS doMiNATed cOastlInE but sOme AreAs SUch As tHe south china Sea aNd the guLf of mexIco Are primarIly diurnal IN tHe SeMi-dIurnal aREAs tHe primaRy cOnStituentS m2 (lUNAR) anD s2 (soLar) peRIods DIffeR sliGhtlY sO That the relAtIve pHasEs aND thus the aMpLiTuDe of The CoMbined tiDE chAnge forTnightlY (14 day PerioD)