Perigean Spring Tide

A perigean spring tide is the largest of the monthly tides, occurring when a spring tide at new or full Moon coincides with the Moon near perigee, its closest approach at about 356,500 km. The inverse-cube dependence of the tide-generating force makes the perigean lunar tide about 20 to 30 percent stronger than the apogean one, so when that maximum lines up with syzygy the M2 and S2 constituents reinforce the larger lunar elliptic N2 to produce the highest astronomical range, often called a king tide. These events recur near the 411.8-day perigean-spring cycle and drive predictable coastal flooding that mariners and harbor authorities plan around.