<P> For an ocean in the shape of a circular basin enclosed by a coastline, the cotidal lines point radially inward and must eventually meet at a common point, the amphidromic point . The amphidromic point is at once cotidal with high and low waters, which is satisfied by zero tidal motion . (The rare exception occurs when the tide encircles an island, as it does around New Zealand, Iceland and Madagascar .) Tidal motion generally lessens moving away from continental coasts, so that crossing the cotidal lines are contours of constant amplitude (half the distance between high and low water) which decrease to zero at the amphidromic point . For a semi-diurnal tide the amphidromic point can be thought of roughly like the center of a clock face, with the hour hand pointing in the direction of the high water cotidal line, which is directly opposite the low water cotidal line . High water rotates about the amphidromic point once every 12 hours in the direction of rising cotidal lines, and away from ebbing cotidal lines . This rotation, caused by the Coriolis effect, is generally clockwise in the southern hemisphere and counterclockwise in the northern hemisphere . The difference of cotidal phase from the phase of a reference tide is the epoch . The reference tide is the hypothetical constituent "equilibrium tide" on a landless Earth measured at 0 ° longitude, the Greenwich meridian . </P> <P> In the North Atlantic, because the cotidal lines circulate counterclockwise around the amphidromic point, the high tide passes New York Harbor approximately an hour ahead of Norfolk Harbor . South of Cape Hatteras the tidal forces are more complex, and cannot be predicted reliably based on the North Atlantic cotidal lines . </P> <P> Investigation into tidal physics was important in the early development of celestial mechanics, with the existence of two daily tides being explained by the Moon's gravity . Later the daily tides were explained more precisely by the interaction of the Moon's and the Sun's gravity . </P> <P> Seleucus of Seleucia theorized around 150 B.C. that tides were caused by the Moon . The influence of the Moon on bodies of water was also mentioned in Ptolemy's Tetrabiblos . </P>

Forces that cause water movements lead to mixing of the entire ocean once every