Spring-neap tides
In Figure 9 the tide-generating forces of the sun and the moon are acting in the same directions, and the solar and lunar equilibrium tides coincide. The tidal range produced is larger than the average. This happens at full and new moon and such tides are known as spring tides.

Figure 9: The earth-moon-sun system at spring tides.

 

 

 

In Figure 10 the sun and the moon act at right angles to each other, and the solar and lunar tides are out of phase. The tidal range is smaller than the average. This happens when the moon is said to be in quadrature and such tides are known as neap tides. The spring and neap tides are also illustrated in Figure 11.

Figure 10: The earth-moon-sun system at neap tides.

 

 

 

 

 

Figure 11: Predicted tides for Tromsø for December 1996.