3. Interaction of Solar and Lunar Tides. 



The most prominent modulation of the tidal range is that associated with the phase of 

 tlie Moon as shown in Figure 10. When the Sun and Moon are generally in hne as at new 

 Moon or fuU Moon, the tide-generating forces of the Sun and Moon are in phase. The two 

 results are additive and the resulting tides are called spring tides. When the Sun and Moon 

 are separated by about 90° with respect to the Earth, as at the first and third quarters of the 

 Moon, the tide-generating force of the Sun is in opposition to that of the Moon. The 

 resulting smaller tides are called neap tides. Spring and neap tides are sometimes referred to 

 as syzygy tides and quadrature tides, respectively. 



Spring or Syzygy Tides 



Earth 



New Moon 



Sun 



iiJ Neap or Quadrature Tides 



First Quarter 



IT 



Moon in Quadrature 

 Earth 



Sun 



-HJ Spring or Syzygy Tides 



Earth gun 



Full Moon 



Neap or Quadrature Tides 

 Earth Sun 



Last Quarter 



s 



Moon in Quadrature 



Figure 10. Spring and neap tides during a lunar month (after Schalowitz, 1964). 



4. Net Variation in Tidal Range. 



The amphtudes of diurnal or semidiurnal tides are modulated by several factors: 



(a) Distance between Earth and Moon (apogee) (perigean or apogean tides). 



(b) DecUnation of Moon (equatorial or tropical tides). 



(c) DecUnation of the Sun (equinoctial or solstitial tides). 



(d) Phase of the Moon (spring (syzyg)) or neap (quadrature) tides). 



The astronomical tides are higher tlian normal when perigean and spring tides are 

 approximately in phase. The highest astronomical tides at many locations occur when there 

 is a near coincidence of spring, perigean, and tropical tides. Wood (1978) discusses the 

 coincidence of coastal flooding with perigean-spring tides and presents an extensive 

 collection of data to show that flooding and coastal erosion tend to be more than normally 

 severe when spring and perigean tides are in near coincidence. Wood suggests that tlie tide 

 prediction equations should be expanded to a higlier order of accuracy for those dates in 

 which there is a near coincidence of spring and perigean tides. 



32 



