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Tidal Constituent, also known as a Constituent Tide

One of the harmonic elements in a mathematical expression for the tide-producing force and in corresponding formulas for the tide or tidal current. Each constituent represents a periodic change or variation in the relative positions of the Earth, Moon and Sun. A single constituent is usually written in the form y = A cos(at+µ), in which y is a function of time as expressed by the symbol t and is reckoned form a specific origin. The angle (at+µ) changes uniformly and its value at any time is called the phase of the constituent. The speed of the constituent is the rate of change in its phase and is represented by the symbol a in the formula. The quantity µ is the phase of the constituent at the initial instant from which the time is reckoned. The period of the constituent is the time required for the phase to change through 360 degrees and is the cycle of the astronomical condition represented by the constituent.

In the case of tides the timing and amplitude of high and low water at the reference station are used to obtain the timing and amplitude of the corresponding events at the subordinate station by applying time differences and height ratios and/or differences which are unique to the subordinate station. The reference and subordinate stations must have similar tidal characteristic so that the reference station may serve as an accurate template in predicting the tide at the subordinate station. Each reference station may have many subordinated stations associated with it; however, each subordinate station has only one reference station.

Revised: 10/15/2013
NOAA / National Ocean Service
Web site owner: Center for Operational Oceanographic Products and Services