129CHAPTER 9TIDES AND TIDAL CURRENTSORIGINS OF TIDES900. IntroductionTides are the periodic motion of the waters of the seadue to changes in the attractive forces of the Moon and Sunupon the rotating Earth. Tides can either help or hinder amariner. A high tide may provide enough depth to clear abar, while a low tide may prevent entering or leaving aharbor. Tidal current may help progress or hinder it, may settheshiptowarddangersorawayfromthem.Byunderstandingtidesandmakingintelligentuseofpredictions published in tide and tidal current tables anddescriptions in sailing directions, the navigator can plan anexpeditious and safe passage through tidal waters.901. Tide and CurrentThe rise and fall of tide is accompanied by horizontalmovement of the water called tidal current. It is necessaryto distinguish clearly between tide and tidal current, for therelation between them is complex and variable. For the sakeof clarity mariners have adopted the following definitions:Tide is the vertical rise and fall of the water, and tidalcurrent is the horizontal flow. The tide rises and falls, thetidal current floods and ebbs. The navigator is concernedwith the amount and time of the tide, as it affects access toshallow ports. The navigator is concerned with the time,speed, and direction of the tidal current, as it will affect hisship’s position, speed, and course.Tidesaresuperimposedonnontidalrisingandfalling water levels, caused by weather, seismic events,or other natural forces. Similarly, tidal currents aresuperimposed upon non-tidal currents such as normalriver flows, floods, and freshets.902. Causes of TidesThe principal tidal forces are generated by the Moonand Sun. The Moon is the main tide-generating body. Dueto its greater distance, the Sun’s effect is only 46 percent ofthe Moon’s. Observed tides will differ considerably fromthe tides predicted by equilibrium theory since size, depth,and configuration of the basin or waterway, friction, landmasses, inertia of water masses, Coriolis acceleration, andother factors are neglected in this theory. Nevertheless,equilibrium theory is sufficient to describe the magnitudeand distribution of the main tide-generating forces acrossthe surface of the Earth.Newton’s universal law of gravitation governs both theorbits of celestial bodies and the tide-generating forceswhich occur on them. The force of gravitational attractionbetween any two masses, m1and m2, is given by:where d is the distance between the two masses, and G is aconstant which depends upon the units employed. This lawassumes that m1and m2are point masses. Newton was ableto show that homogeneous spheres could be treatedaspoint masseswhendeterminingtheir orbits.