<P> In the M plot above, each cotidal line differs by one hour from its neighbors, and the thicker lines show tides in phase with equilibrium at Greenwich . The lines rotate around the amphidromic points counterclockwise in the northern hemisphere so that from Baja California Peninsula to Alaska and from France to Ireland the M tide propagates northward . In the southern hemisphere this direction is clockwise . On the other hand, M tide propagates counterclockwise around New Zealand, but this is because the islands act as a dam and permit the tides to have different heights on the islands' opposite sides . (The tides do propagate northward on the east side and southward on the west coast, as predicted by theory .) </P> <P> The exception is at Cook Strait where the tidal currents periodically link high to low water . This is because cotidal lines 180 ° around the amphidromes are in opposite phase, for example high water across from low water at each end of Cook Strait . Each tidal constituent has a different pattern of amplitudes, phases, and amphidromic points, so the M patterns cannot be used for other tide components . </P> <P> Because the moon is moving in its orbit around the earth and in the same sense as the Earth's rotation, a point on the earth must rotate slightly further to catch up so that the time between semidiurnal tides is not twelve but 12.4206 hours--a bit over twenty - five minutes extra . The two peaks are not equal . The two high tides a day alternate in maximum heights: lower high (just under three feet), higher high (just over three feet), and again lower high . Likewise for the low tides . </P> <P> When the Earth, moon, and sun are in line (sun--Earth--moon, or sun--moon--Earth) the two main influences combine to produce spring tides; when the two forces are opposing each other as when the angle moon--Earth--sun is close to ninety degrees, neap tides result . As the moon moves around its orbit it changes from north of the equator to south of the equator . The alternation in high tide heights becomes smaller, until they are the same (at the lunar equinox, the moon is above the equator), then redevelop but with the other polarity, waxing to a maximum difference and then waning again . </P>

When does low tide and high tide occur