The predictable movements of ocean water known as tides are primarily governed by the gravitational pull of the Moon, causing the water level to rise and fall in a continuous, rhythmic cycle along coastlines worldwide. The most important concept for understanding these fluctuations is the tidal range, which is the vertical difference in height measured between a high tide and the subsequent low tide. This range is not constant but varies significantly throughout the lunar month, leading to distinct tidal types.
Defining Neap Tides
Neap tides are characterized by the smallest tidal range of the entire lunar cycle. The word “neap” comes from an Old English term meaning “scanty” or “without power,” describing the reduced movement of the water. This minimal range means that the high tides are noticeably lower than the average high tide for that location. Conversely, the low tides are higher than the average low tide, meaning the water never fully retreats from the shore. Neap tides occur approximately twice a month, and they are sometimes referred to as quadrature tides.
The Astronomical Cause
The reduced tidal range during a neap tide is a direct consequence of the specific geometric alignment of the Earth, Moon, and Sun. This phenomenon occurs when the three celestial bodies form a right angle, or 90-degree angle, relative to the Earth, an alignment known as quadrature. This configuration happens twice a month, specifically during the first quarter and third quarter moon phases.
In this perpendicular arrangement, the gravitational pull from the Sun works in a direction that partially counteracts the gravitational pull from the Moon. The Moon’s influence is the dominant factor in tides due to its closer proximity to Earth, but the Sun’s force is still substantial. The Sun’s gravity attempts to create a tidal bulge along one axis, while the Moon’s gravity creates a bulge along a perpendicular axis. This conflict of forces results in the diminished tidal range that characterizes a neap tide.
Contrasting Neap and Spring Tides
The best way to understand neap tides is by contrasting them with their opposite: spring tides. Spring tides are defined by the maximum tidal range, featuring the highest high tides and the lowest low tides of the month. Spring tides occur when the Sun, Earth, and Moon are nearly in a straight line, an alignment called syzygy.
During this time, which happens at the new moon and the full moon, the gravitational pulls of the Moon and the Sun combine and amplify each other. The timing of these two types of tides is predictable based on the lunar cycle. Neap tides occur during the quarter moon phases, while spring tides occur approximately seven days later, during the new and full moons. Both neap and spring tides happen twice each lunar month.