82 



As the length of the anomahstic month is approximately 21 ){ days 

 while that of the synodic month is 29 K days, lunar perigee gains 2 

 days a month on the moon's phases. It follows therefore that perigean 

 tides will most nearly coincide with spring tides at intervals of 7 

 months. Similarly, at stations having tides of the semidiurnal type, 

 an exceptionally small tidal range is to be anticipated once during 

 the month at 7 month intervals, occuring half way between the 

 months at which exceptionally high and low waters occur. 



EFFECT OF THE PEINCIPAL DIURNAL COMPONENTS 



149. The Ki and Oi com'ponents — tropic tides. — Since the difference 

 between the speeds of these components is relatively small, they 

 combine to form a diurnal tidal fluctuation with an amplitude ranging 

 from a minimum of Ki — Oi to a maximum of Ki + d. The period 

 from maximum to maximum is : 



3607(ki-Oi) = 3607(15.041,068,6-13. 943,035,6) 

 = 360°/1.098033 = 327.859 hours. 



This period is one-half of the tropical month (par. 62) . 



When the amplitide of the resultant of these two diurnal compo- 

 nents is a minimum, the tides are called equatorial tides, since the 

 moon is then near the Equator. When it is a maximum, the tides are 

 called tropic tides, since this maximum results from the maximum in- 

 equality of the two daily tidal impulses, and therefore occurs when 

 the moon has its greatest declination, near the celestial Tropics (par. 

 40). 



150. Effect of the diurnal com'ponents on high and low waters. — Since 

 the diurnal part of the tide rises once and falls once daily, it has a zero 

 elevation (at mean sea level) at semidaily intervals approximating the 

 period of the semidiurnal components. If the epochs of the Ki, Oi, 

 and Ma components are such that the resultant ordinate of the diurnal 

 components is nearly zero at the two daily low waters of the M2 com- 

 ponent, the diurnal part of the tide evidently increases one of the two 

 daily high waters and decreases the other, producing a diurnal in- 

 equality of the high waters, as may perhaps be seen more clearly by 

 turning back to figure 13, page 22. Similarly, if these epochs 

 are such that the diurnal part of the tide is nearly zero at the two daily 

 Iiigh waters of the M2 component, a diurnal inequality of the low waters 

 is produced. Obviously, both the high and the low waters usually 

 will show an inequality because of the diurnal components, but the 

 inequality of the high waters is not, in general, the same as that of the 

 low waters. As has been shown, these inequalities in the two daily 

 tides vary from a minimum at the time of equatorial tides to a maxi- 

 mum at the time of tropic tides. 



