quadrature of moon— Position of the Moon when its longitude differs by 90° from the 

 longitude of the Sun. The corresponding phases are known as first quarter and last 

 quarter. 



quadrature tide— Same as neap tide. 



range of tide— The difference in height between consecutive high and low waters. The mean 

 range is the difference in height between mean high water and mean low water. The great 

 diurnal range or diurnal range is the difference in height between mean higher high water 

 and mean lower low water. Where the type of tide is diurnal the mean range is the same 

 as the diurnal range. For other ranges see spring, neap, perigean, apogean, and tropic 

 tides. 



reference station— A tide or current station for which independent daily predictions are 

 given in the NOS tide tables, and from which corresponding predictions are obtained for 

 subordinate stations by means of differences and ratios. 



Sa (solar annual constituent)- This constituent, with Ssa, accounts for the nonuniform 

 changes in the Sun's declination and distance. In actuality, tlie changes mostly reflect 

 yearly meteorological variations influencing sea level. See Ssa. 



saros- A period of 223 synodic months corresponding approximately to 19 ecUpse years or 

 18.03 Julian years, and is a cycle in which solar and lunar eclipses repeat tliemselves 

 under approximately the same conditions. 



sea level datum (SLD)— Use of tliis term is discouraged. See National Geodetic Vertical 

 Datum. 



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

 made over a minimum period of 1 year but less than a 19-year metonic cycle. The series 

 is reduced by comparison with simultaneous observations from a primary control tide 

 station. This station provides for a 365-day harmonic analysis including the seasonal 

 fluctuation of sea level. See tide station, primary control tide station, subordinate tide 

 station (1), and tertiary tide station. 



semidiurnal- Having a period or cycle of approximately one-half of a tidal day. The 

 predominating type of tide throughout the world is semidiurnal, with two high waters 

 and two low waters each tidal day. The tidal current is said to be semidiurnal when there 

 are two flood and two ebb periods each day. A semidiurnal constituent has two 

 maximums and . two minimums each constituent day, and its symbol is usually 

 distinguished by the subscript 2. See type of tide. 



shallow-water constituent— A short-period harmonic term introduced into the formula of 

 tidal (or tidal current) constituents to take account of the change in tlie form of a tide 

 wave resulting from shallow-water conditions. Shallow-water constituents include the 

 overrides and compound tides. 



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