Tides are the periodic rise and fall of the ocean’s surface, driven primarily by the gravitational forces of the Moon and the Sun. A neap tide is a specific type of tide characterized by the smallest difference between the height of the high water and the low water, a measurement known as the tidal range. These are considered moderate tides because the usual extreme fluctuations are significantly reduced. The phenomenon of neap tides occurs twice monthly as part of the regular lunar cycle.
The Gravitational Alignment
Neap tides are a direct result of a specific geometric arrangement of the Sun, Earth, and Moon. During this event, the three celestial bodies form a right angle, or a 90-degree angle, with the Earth positioned at the vertex. This configuration is known in astronomy as quadrature. The Moon’s gravitational influence is the main driver of tides.
In the neap tide alignment, the Sun’s pull acts at a perpendicular angle to the Moon’s pull. This means the gravitational forces are pulling the ocean’s water in two different, opposing directions. The Sun’s force effectively works against the Moon’s, partially canceling out the size of the Moon’s tidal bulges. This reduction in the combined gravitational effect leads to a smaller overall deformation of the ocean surface.
Neap Tides Versus Spring Tides
Neap tides are best understood by contrasting them with spring tides. Neap tides have a minimal tidal range because the Sun and Moon’s gravitational pulls work against each other in the 90-degree alignment.
In contrast, spring tides are characterized by the largest tidal range, producing the highest high tides and the lowest low tides. Spring tides occur when the Sun, Earth, and Moon are aligned in a straight line, a configuration known as syzygy. This straight-line alignment, which happens during the new moon and full moon phases, causes the gravitational forces of the Sun and Moon to combine or reinforce each other. The combined pull creates a greater oceanic bulge, leading to the more extreme water level fluctuations. Therefore, neap tides represent the weakest tidal force due to the conflicting 90-degree alignment, while spring tides represent the strongest tidal force due to the combined 180-degree alignment.
When They Occur and How They Look
Neap tides occur twice each lunar month, approximately seven days after a spring tide event. This timing corresponds precisely with the Moon’s phases when it appears half-lit from Earth. Specifically, neap tides happen during the first quarter and third quarter (or last quarter) moon phases.
The observable effect of a neap tide is a subdued, or moderate, change in water level throughout the day. During a neap tide, the high tide is noticeably lower than the monthly average, while the low tide is higher than the monthly average. This means that the total vertical distance the water travels between high and low marks is small.