248 Principal Features of Tidal Phenomena 



At these times perigean and apogean tides occur. The cause is the oscillations 

 of the parallax during the period of the lunitidal interval which is an 

 anomalistic month (27-55 days). This anomalistic month is only about 2 days 

 shorter than the synodic month. The earth lags more and more behind the 

 full moon and the tidal curves change accordingly. The extent of the parallax 

 inequality is generally smaller than that of the semi-monthly inequality; 

 however, there are localities (e.g. at the east coast of North America) where 

 both are equally large, or even where the parallax inequality is larger. In 

 that case, the tides are more governed by the parallax of the moon than by 

 the phase of the moon. 



(b) Declinational inequality is caused when the moon moves from the 

 celestial equator into the northern and southern hemispheres. The oscillation 

 of the declination of the moon is completed in a tropical month of 27-32 

 mean solar days. The declinational inequality is dependent only upon the 

 absolute amount of the declination, and not upon its sign, so that its period 

 is only one-half tropical month = 13-66 days. Soon after the moon is upon 

 the equator, the greatest semi-daily range of tide will occur and, soon after 

 the moon's greatest declination, the smallest. Besides this declinational 

 inequality caused by the moon there is also a solar one, which depends 

 upon the sun's declination; declinational inequality is smallest at the time 

 of solstices and greatest at the time of the equinoxes. At that time, the full 

 and new moon are about upon the equator and the range of the spring tide 

 is extremely large and the range of the neap tide extremely small. 



(c) Diurnal inequality in height is the difference in height between two 

 consecutive high waters or low waters. Diurnal inequality in time or interval 

 is the difference in length of the consecutive high- or low-water intervals. 

 These inequalities, which make the tide curves unsymmetrical, have a period 

 of a half tropical month for the moon and of a half tropical year for the sun. 

 The largest diurnal inequality occurs with a certain lag after the moon's 

 extreme north and south declination and disappears with a same lag when 

 the moon crosses the equator. 



The diurnal inequality influences strongly the picture of the tides. Some- 

 times this influence is so great that the semi-diurnal tides are almost entirely 

 suppressed. The tides then degenerate into an oscillation with a period of 

 one day (see p. 307). In the Atlantic waters, especially in the North Sea and 

 in the English Channel, however, the diurnal inequality is insignificant. 



This brief review of the main phenomena of the tides already shows that 

 the tides are an extraordinarily intricate phenomenon obeying definite laws. 

 The tides depend mainly upon the moon and the sun and upon their position 

 relative to each other. A certain position between sun and moon does not 

 recur until a Saros period (18 years) has elapsed. It would be necessary to 

 have a series of observations extending over 19 years to compute exactly 

 the tides for a certain locality. But even a one-year period of observations 



