Ocean tides are the rhythmic fluctuations in sea level that occur across the globe. These movements are primarily the result of gravitational forces exerted by the Moon and, to a lesser degree, the Sun. While the daily cycle of high and low water is constant, the maximum height and minimum depth of the tides change over the course of the lunar month. This cyclical variation creates periods of more extreme tides and periods of moderate tidal activity.
The Name for Moderate Tides
The specific term for tides that are not very high and not very low is Neap Tides. This phenomenon is characterized by having the smallest difference between the high and low water marks, known as the minimum tidal range. During a neap tide, the high water level is noticeably lower than the monthly average, while the low water level is concurrently higher. The word “neap” comes from an Anglo-Saxon term meaning “scanty” or “without power,” reflecting the reduced intensity of the tidal movement.
The Gravitational Cause
Neap tides occur because of a specific geometric arrangement of the Earth, Moon, and Sun. The gravitational forces that create tides are partially canceled out when the Sun and Moon are positioned at a 90-degree angle relative to Earth. The Moon’s gravitational pull is the primary driver of tides, but during a neap tide, the Sun’s tidal force pulls the ocean water in a perpendicular direction. This perpendicular alignment means the Sun’s influence works against the Moon’s, preventing the formation of the largest possible tidal bulges and resulting in the diminished tidal range.
The Full Tidal Range
Neap tides represent the minimum end of the monthly tidal scale and occur twice during the lunar cycle, specifically during the first and third quarter moon phases. In contrast, the maximum end of the tidal scale is known as a Spring Tide, which produces the highest high tides and the lowest low tides. Spring Tides occur when the Sun, Earth, and Moon are nearly in alignment, during the new and full moon phases. At these times, the gravitational forces of the Sun and Moon combine and reinforce each other, resulting in the most extreme tidal range. Neap Tides occur roughly seven days after a Spring Tide, providing a regular and predictable alternation between the maximum and minimum tidal ranges.