Tides are the predictable, periodic movement of the ocean’s surface, a constant cycle of rising and falling sea levels. This rhythmic action is primarily governed by the gravitational attraction exerted by the Moon and, to a lesser extent, the Sun. The neap tide represents a period where the difference between the high and low water marks is at its least pronounced.
What Exactly Is a Neap Tide?
A neap tide is defined by having the smallest possible vertical difference between the daily high and low tides, known as the minimum tidal range. The term “neap” derives from an Anglo-Saxon word suggesting a lack of power or strength, accurately describing the subdued nature of the water movement.
During this phase, the gravitational forces of the Sun and Moon do not work in concert, leading to a diminished overall effect on the ocean’s surface. This results in high tides that are lower than the monthly average and low tides that are simultaneously higher than average.
The minimal tidal range means the sea level does not fluctuate dramatically over the course of a day. This reduced variation in sea level distinguishes the neap tide from the more dramatic spring tide, which features the maximum tidal range.
The Astronomical Alignment That Causes Them
The subdued nature of the neap tide is caused by a specific geometric arrangement of the Earth, Moon, and Sun, known as quadrature. During quadrature, the three celestial bodies form a right angle, or 90-degree configuration, as viewed from Earth.
The Moon’s gravitational pull is the dominant factor in generating Earth’s tides due to its close proximity, creating two bulges of water on opposite sides of the planet. However, the Sun’s gravitational force, while weaker in creating tidal bulges, is substantial enough to interfere with the Moon’s action.
In this right-angle alignment, the Sun’s gravitational pull works against the Moon’s to partially cancel out the tidal bulge. The Sun attempts to create its own tidal bulge along a different axis perpendicular to the Moon’s axis.
This perpendicular pull essentially stretches the water in a crosswise direction, dampening the height of the bulge created by the Moon’s stronger force. The opposing nature of these pulls results in the moderate tidal fluctuations characteristic of a neap tide.
The Moon’s gravitational attraction is responsible for approximately 70% of the Earth’s tidal force, with the Sun accounting for the remaining 30%. This counteracting force minimizes the difference between the high and low water marks.
When Neap Tides Occur
Neap tides occur reliably twice a month, following a predictable schedule tied directly to the Moon’s phases. These events happen when the Moon is in its quarter phases: the first quarter moon and the third quarter moon. At these times, exactly a quarter of the Moon’s surface appears illuminated, visually confirming the quadrature alignment.
The predictable timing means a neap tide occurs approximately seven days after a spring tide, establishing a regular two-week cycle between the maximum and minimum tidal ranges. This precise scheduling allows for accurate prediction of the minimal tidal range, which is important for coastal activities.
Knowing when neap tides occur is beneficial for marine navigation and certain fishing practices. The lessened tidal range produces gentler currents and less turbulent water movement in many coastal areas. This reduced fluctuation provides calmer conditions for mariners and can affect the accessibility of certain shorelines.