When the sun, the earth and the moon are in one line (full and new moon), the solar and lunar tides reinforce each other. The ellipsoid becomes more pronounced and the tide gets a bigger amplitude and is called spring tide. When the solar and lunar tides are 90° out of phase, their effects cancel each other (first and last quarter). The ellipsoid approaches a circle, and consequently the tide gets a smaller amplitude. This situation is called neap tide (Fig. 3.21).
Figure 3.22 shows an example of these tidal variations.
The spring and neap tide cycle varies with moon phases and therefore with the lunar month of 29.5 days. The ratio of spring and neap tide amplitudes according to equilibrium theory can be estimated from Eqs. 18.104.22.168 and 22.214.171.124 as:
\[(0.515 + 1.13)/(1.13 - 0.515) = 2.7 : 1\]
This ratio is an approximation for the spring to neap tide ratio at open oceans. Anywhere else the ratio is affected by the presence of land masses (see Sect. 3.8).