# Why don't spring tides occur at full/new moon (age of tide)?

Tides observations everywhere show that spring tides occur not at full or new moon, at the time of maximum driving forces, but typically a day or two later.

What causes this delay?

The tidal forces generate waves of very long wavelength that travel all around the ocean with a velocity determined by the shallow water progressive wave velocity equation, $c=\sqrt(gH)$, where $H$ is water depth. The delay between the moon phases and their effect on the tide is the time that it takes for the wave to travel around the ocean.
The inclusion of friction is necessary to explain the observed time lags. Proudman (1941) showed that by including frictional effects on coastal areas, a positive age of tide appear in the tidal solution. If there is some resonant frequency in an ocean basin close to the semi-diurnal frequencies ($M_2$, $S_2$...), friction enhances the age of the tide resulting in higher delay values. Garrett & Munk (1971) and Webb (1973) provided a methodology for the calculation and estimates around the oceans. The longest delays (longer than 60 hours) were estimated in areas with the largest continental shelf, as they represent areas of strong tidal dissipation: the North Sea, the Ross Sea, Hudson Strait, the shelf off Argentina, the Bering Sea, and the Sea of Okhotsk.
The age of tide can be obtained from the harmonic constants by dividing the difference in phases of the $S_2$ and $M_2$ constituents by frequency separation between both constituents: $Age\_of\_tide = {Phase\_of\_M_2 - Phase\_of\_S_2 \over 1.0163 }$, where the phases are measured in degrees and the age of tide is in hours.