Ocean tides are the predictable, periodic rise and fall of sea levels, primarily governed by the gravitational forces exerted by the Moon and, to a lesser extent, the Sun. The difference between high and low water marks can vary dramatically over the lunar cycle. Understanding this cycle requires specific terms for the most subdued and the most exaggerated tidal events.
Defining Neap Tides
The periods that produce the shallowest high tides and the least overall change in water level are known as neap tides. A neap tide is characterized by a minimum tidal range, meaning the vertical difference between high water and low water is the smallest during the month. The high tides during this time do not reach as far up the shore as they typically do, and the subsequent low tides are not as far out.
Neap tides occur twice during the lunar month, coinciding with the first quarter and third quarter moon phases. During these times, the high water mark is notably lower, or shallower, than the average high tide. The term “neap” itself comes from an Anglo-Saxon word meaning “without the power,” which aptly describes the reduced strength of the tidal forces.
This reduced effect happens when the gravitational pull of the Sun and the Moon are working against each other. The result is a high tide that is not very high and a low tide that is not very low, creating a much more moderate coastal experience. For instance, the mean high water neaps (MHWN) is a reference level representing the average height of these lowest high tides.
Defining Spring Tides
The most extreme tidal events, which include the deepest low tides, are called spring tides. Despite the name, these tides have no connection to the spring season but instead refer to the water “springing forth” or surging. Spring tides are defined by the maximum tidal range, where the difference between the high and low water marks is the greatest of the entire lunar cycle.
During a spring tide, the high water reaches its peak height, and consequently, the low water mark drops to its absolute lowest point. This extremely low water level exposes large areas of the intertidal zone that remain covered at other times of the month.
These powerful tides occur twice each month, aligning with the new moon and the full moon phases. The large tidal range is caused by the combined, reinforcing gravitational pull of the Sun and the Moon. This combined force creates an exaggerated tidal bulge, leading to both the highest high tides and the lowest low tides.
How the Sun and Moon Create Tidal Extremes
The celestial alignment of the Earth, Moon, and Sun dictates whether a neap or spring tide will occur. The Moon is the dominant force because its proximity to Earth gives it the strongest differential gravitational effect on the water. However, the Sun’s immense mass, despite its distance, contributes a significant portion of the total tidal force.
Spring tides result from a configuration known as syzygy, where the Sun, Earth, and Moon are arranged in a straight line. This alignment occurs during the new moon (Moon between Earth and Sun) and during the full moon (Earth between Sun and Moon). In both cases, the gravitational pulls of the two celestial bodies are acting in concert, reinforcing one another. This constructive interference maximizes the tidal bulge, leading to the largest possible tidal range.
Conversely, neap tides occur during a configuration called quadrature, which means the Sun and Moon are positioned at a 90-degree angle relative to the Earth. This happens during the quarter moon phases, roughly a week after each spring tide. The Sun’s gravitational force works perpendicular to the Moon’s primary tidal force, causing a partial cancellation of the overall effect.
The two gravitational pulls are engaged in a celestial tug-of-war, with the Moon’s pull being partially counteracted by the Sun’s perpendicular influence. This dampening effect reduces the size of the tidal bulge, resulting in the minimum tidal range characteristic of neap tides.