1.2 Observed modulation of the semidiurnal and diurnal tidal amplitudes.1.1 Modulation of semidiurnal and diurnal tides.The effect of shorter periods of the Earth's and Moon's orbital motion is discussed in the article Ocean and shelf tides. This article considers the effect of the lunar node cycle and the lunar perigee cycle on ocean's tides. (perigee = point in the Moon's elliptical orbit nearest to the earth, lunar nodes = points at which the Moon's orbit cuts the ecliptic, perihelion precession = drift of Earth's closest approach to the Sun) 1/f_5= period of Earth's perihelion precession = 20940 years.1/f_4 = period of lunar node precession = 18.613 years.1/f_3= period of lunar perigee rotation = 8.847 years.1/f_2 = period of solar declination = 365.242199 days.1/f_1= period of lunar declination = 27.321582 days.The Earth's and Moon's orbital motion are characterized by a limited number of fundamental periods: The ocean tidal oscillation in each point can therefore be represented by a linear superposition of sinusoidal tidal components with periods derived from the various astronomical cycles. The amplitude of the tidal oscillations is very small compared to ocean depths. The frequencies of these oscillations are determined by the cycles in the motions of moon, sun and earth. Tidal motion is the oscillation of ocean waters under the influence of the attractive gravitational forces of the moon and the sun.
