<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> <P> The tides' influence on current flow is much more difficult to analyse, and data is much more difficult to collect . A tidal height is a simple number which applies to a wide region simultaneously . A flow has both a magnitude and a direction, both of which can vary substantially with depth and over short distances due to local bathymetry . Also, although a water channel's center is the most useful measuring site, mariners object when current - measuring equipment obstructs waterways . A flow proceeding up a curved channel is the same flow, even though its direction varies continuously along the channel . Surprisingly, flood and ebb flows are often not in opposite directions . Flow direction is determined by the upstream channel's shape, not the downstream channel's shape . Likewise, eddies may form in only one flow direction . </P> <P> Nevertheless, current analysis is similar to tidal analysis: in the simple case, at a given location the flood flow is in mostly one direction, and the ebb flow in another direction . Flood velocities are given positive sign, and ebb velocities negative sign . Analysis proceeds as though these are tide heights . </P>

What are tides and how do they work