As our planet spins in space, the gravitational forces of the moon and sun cause predictably massive movements of its waters.
Tides and tidal currents have been around ever since the Earth’s oceans first formed. No doubt our ancient ancestors must have watched the shifting seas with awe and studied them in relation to fishing and migration.
Playing a key role in controlling the tides, the moon dictates their daily cycle. Every 12 hours and 25 minutes tides repeat themselves – that’s twice in every lunar day – with different factors influencing the speed and movement of a tide.
Of particular interest are the effects of the moon’s orbit on tidal events. During a “perigean spring tide,” the moon travels closest to the Earth (perigee). That’s when the greatest tide movement and fastest currents occur, as the gravitational pull of the sun and moon align in the spring.
By contrast, the “apogean neap tide” has the slowest currents and lowest tide movement. The moon in its elliptical orbit is farthest from the Earth (apogee). During an apogean neap tide, the sun and moon are separated by 90 degrees, so the gravitational forces partially cancel each other out.
In addition to the positions of the sun and moon, numerous non-astronomical factors are involved in the predicting of when tides will occur and how high they will be. The timing and size of tides is affected by the depth of the water, the ocean floor topography and the geography of the coastline, to name a few.
Tides can be further classified into four broad categories. “Diurnal tides” have just one daily episode of high water and one of low water; they usually occur when the moon is distant from the equator. “Semi-diurnal tides” are more common and have two highs and lows each day; they are prevalent when the moon is directly above the equator.
“Mixed tides” are like semi-diurnal ones, but the water levels on each rise and fall differ in height; they happen when the moon is extremely far north or south of the equator. And “meteorological tides” represent all atmospherically influenced tidal events, including those affected by wind, barometric pressures, rainfall, melting ice and drying land.
Tides and tidal currents are critical to the circulation of ocean waters. They influence wave patterns, help shape shorelines and greatly affect sea life. Of particular interest to future generations, the power of tides can be harnessed, too. Electricity can be generated not only by the up and down motion of sea level, but also by the sideways flow of tidal currents, thus providing a potential source of reliable, renewable energy.
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