The Ocean’s Daily Rhythm
Tides represent the regular rise and fall of sea levels, a predictable natural phenomenon observed along coastlines worldwide. They are essentially very long-period waves that move through the ocean. Most ocean beaches typically experience two high tides and two low tides each day, a pattern known as semi-diurnal. This consistent cycle influences coastal environments and human activities, from navigation to marine life.
The Moon’s Dominance
The Moon’s gravitational pull is the primary force responsible for Earth’s tides. As the Moon orbits our planet, its gravity exerts a stronger pull on the side of Earth closest to it, drawing the ocean waters towards it and creating a bulge. Simultaneously, a second bulge forms on the side of Earth directly opposite the Moon, as the Moon’s gravity pulls the Earth itself away from the water on that far side.
Earth rotates through these two tidal bulges over the course of a lunar day. A lunar day, which is the time it takes for a specific point on Earth to return to the same position relative to the Moon, lasts approximately 24 hours and 50 minutes. This results in high tides occurring roughly every 12 hours and 25 minutes.
Solar Influence and Tidal Patterns
The Sun also exerts a gravitational pull on Earth’s oceans, contributing to tidal forces. Although the Sun is far more massive than the Moon, its much greater distance from Earth means its tidal influence is about half that of the Moon. The alignment of the Sun, Moon, and Earth modifies the magnitude of the two daily high and low tides.
When the Sun, Moon, and Earth are aligned, such as during new and full moons, their gravitational forces combine. This combined pull results in “spring tides” with exceptionally higher high tides and lower low tides. Conversely, when the Sun and Moon are at right angles to each other, typically during the first and third quarter moon phases, their gravitational pulls partially counteract each other. This leads to “neap tides” with lower high tides and higher low tides, resulting in a smaller overall tidal range. These spring and neap tidal cycles occur approximately twice each lunar month.
Local Geography and Other Factors
While gravitational forces from the Moon and Sun drive the fundamental tidal patterns, local geographical features significantly modify how tides manifest along coastlines. The shape of landmasses, the configuration of coastlines, and the depth and size of ocean basins can restrict, amplify, or otherwise alter tidal movements. For instance, funnel-shaped bays, like the Bay of Fundy, can dramatically magnify tidal ranges due to tidal resonance.
These local factors can lead to deviations from the standard semi-diurnal pattern. Some areas experience “diurnal tides” (one high and one low tide per day), a pattern common in parts of the Gulf of Mexico due to its shallow waters and unique coastline. Elsewhere, “mixed tides” have two high and two low tides daily, but with notable differences in height between successive waters. The Pacific coast of North America frequently exhibits this mixed tidal pattern. Beyond geography, temporary factors like strong winds and atmospheric pressure systems can also influence local tidal heights.