Tides represent the regular rise and fall of sea levels, caused primarily by the gravitational forces of the Moon and, to a lesser extent, the Sun. The mass of these celestial bodies causes the oceans to bulge, resulting in the daily cycle of high and low water. The exact height and depth of the tides change continually based on the shifting positions of the Earth, Moon, and Sun. The most extreme tidal events, featuring the deepest high tides and the shallowest low tides, occur only under specific astronomical circumstances.
Understanding the Basic Tidal Cycles
The primary variation in tidal height results from the changing alignment of the Sun, Earth, and Moon, creating two main tidal cycles. Spring Tides occur when the Sun, Earth, and Moon align in a nearly straight line, a configuration known as syzygy. This alignment happens twice each lunar month during the new moon and full moon phases, combining the gravitational pulls of the Sun and Moon. When their forces are combined, the gravitational effect is maximized, creating the largest tidal range with the highest high tides and the lowest low tides of the month.
Conversely, Neap Tides occur when the Sun and Moon are positioned at right angles to the Earth, during the first and third quarter moon phases. In this arrangement, the Sun’s gravitational pull partially works against the Moon’s pull, reducing the tidal effect. This opposition leads to a reduced tidal range, characterized by lower-than-average high tides and higher-than-average low tides.
The Influence of Lunar Distance on Tidal Height
The Moon’s orbit around the Earth is an ellipse, meaning its distance from our planet constantly changes and introduces another layer of tidal variation. When the Moon reaches the point in its orbit closest to Earth, known as Perigee, the gravitational force it exerts is strongest. This increased pull causes the entire tidal range to be noticeably exaggerated. This stronger gravitational influence happens approximately once every 27.5 days, coinciding with the Moon’s orbital period.
About two weeks after Perigee, the Moon reaches Apogee, the point farthest from Earth. At this maximum distance, the Moon’s gravitational pull is at its weakest, resulting in a diminished tidal force and a smaller overall tidal range than average.
Naming the Absolute Extremes: Perigean Spring Tides
The most extreme tidal events are scientifically termed Perigean Spring Tides. This name describes the combination of the two major tidal factors: a Spring Tide (maximum gravitational alignment of the Sun, Earth, and Moon) occurring simultaneously with the Moon being at its closest point to Earth (Perigee). This synchronization of maximum alignment and minimum distance significantly amplifies the resulting tidal bulge, leading to the largest tidal range of the year.
These exceptional events typically occur only a few times annually, often between three and eight times per year, when the new or full moon phase closely aligns with the Moon’s Perigee. While Perigean Spring Tides is the accurate scientific designation, the phenomenon is widely known in coastal communities by the colloquial term King Tide. King Tide is the popular phrase used to describe the highest predicted tides of the year, which result from this Perigean Spring Tide alignment. These events are important for coastal management as the extra-high water levels can lead to minor flooding in low-lying areas, offering a preview of what permanent sea-level rise may look like.