Theory of the Tides 



275 



Springtide 



6 hr 



Between spring 

 and neaptide 



Between neap and 

 spring tide 



12 



6 



hr 



12 



Fig. 118. Interaction between principal lunar and solar tides for different positions of the 

 sun and moon. Schematic apparent course of the sun (Q) and moon( ])) in the sky. At 

 the start of the tide curve (left) both sun and moon occupy the indicated positions in the 

 apparent orbit. They have travelled 180° at the right end of the curves. Indicated time given 



the position of the sun at that particular time. The line shows the principal lunar 



tide, the , the principal solar tide, the , the combination of both tides. Sun 



and moon are assumed to occupy during 12 h the same position with respect to each other 



(Hann-Bidlingmaier). 



in 28 C S. latitude. During the diurnal rotation of the earth around its axis, a point at the equator 

 has no daily inequality, as can be readily seen from Fig. 120; at 10'N. latitude a zenith tide of 24 cm 

 corresponds with a nadir tide of 14 cm. At 30°S. lat. conditions are still more extreme, the zenith 

 tide having only a range of 5 cm, whereas that of the nadir tide is 28 cm; the former remains per- 

 manently below the mean water level. 



The equilibrium theory of the tides is able to explain many facts related 

 to the ocean tides but, on the other hand, a comparison with the observa- 

 tions shows its insufficiency. There is agreement insofar as spring tide always 

 appears at full moon and new moon, neap tide at the quadratures; further- 

 more, the height of spring tide is at least three times that of neap tide. How- 



