The daily rise and fall of the ocean, known as the tide, often causes confusion about where the water goes during the low-tide cycle. The water does not vanish or drain away; rather, it is constantly being pulled and pushed into different areas of the globe by predictable forces. The low water observed at the shore is a consequence of the ocean shifting its mass to create two enormous, temporary high-water zones elsewhere.
The Driving Force of Tides
The movement of the Earth’s ocean water is a direct result of two forces: the gravitational pull exerted by the Moon and the Sun, and centrifugal force. Although the Sun is far more massive, the Moon’s proximity to Earth makes its tide-generating force approximately 2.25 times stronger. This force is based on the difference in gravity’s strength across the diameter of the Earth.
Centrifugal force arises because the Earth and the Moon revolve around a common center of mass, called the barycenter. As the Earth orbits this point, a corresponding force acts equally on all parts of the planet, directed away from the barycenter. The combination of these two forces creates an imbalance that redistributes the planet’s surface water.
Creating the Global Water Bulges
The interplay of gravity and centrifugal force creates two distinct high-tide bulges on opposite sides of the Earth. On the side of Earth facing the Moon, the Moon’s gravitational pull is strongest, drawing the ocean water toward it to form the first bulge.
The second bulge forms on the side of Earth facing away from the Moon because the centrifugal force is dominant over the Moon’s gravitational pull. Here, the water is essentially lagging behind the solid Earth as the planet is pulled toward the Moon. When a coastal location experiences low tide, it is situated in the area between these two high-water bulges, where the ocean has been drawn away to feed the bulges.
The Earth’s Rotation and Tidal Cycles
A specific location experiences the shift from high to low tide because the Earth rotates underneath these two massive, relatively stable water bulges. The bulges remain aligned with the Moon as it orbits, while the entire planet spins on its axis once a day. A coastline experiences high tide as it passes through the center of a bulge and then moves into low tide as it rotates out from under the raised water.
The full cycle from high tide to high tide takes approximately 12 hours and 25 minutes, resulting in a typical semidiurnal pattern of two high tides and two low tides each day. This timing occurs because the planet must rotate an extra 50 minutes to “catch up” with the Moon, which is also moving in its orbit. The transition from high to low water is a continuous flow, often called a tidal current, moving laterally along the coast and out to the ocean.
Factors Influencing Tidal Range
The difference between high and low tide, known as the tidal range, varies significantly across the globe due to astronomical alignments and local geography. The Sun’s gravitational influence, though weaker than the Moon’s, can either amplify or diminish the tidal range depending on its position.
When the Sun, Moon, and Earth align in a straight line (during the new and full moon phases), their combined forces produce the largest ranges, known as spring tides. Conversely, when the Sun and Moon are positioned at right angles relative to the Earth, their gravitational pulls partially counteract each other, resulting in a smaller tidal range called neap tides.
Local geography also modifies the tidal range. In open ocean areas, the range might only be about one meter. However, in funnel-shaped bays or estuaries, the incoming tide is compressed into a smaller area. This compression can dramatically amplify the water’s vertical movement, leading to extreme tidal ranges that can exceed 15 meters in places like the Bay of Fundy.