Ocean tides are the regular rise and fall of water levels along coastlines. While predictable, their specific height and timing vary depending on various factors.
The Basic Mechanics of Tides
The primary force behind ocean tides is the gravitational pull of the Moon. As the Moon orbits Earth, its gravity creates bulges of water on both the side of Earth facing the Moon and the side directly opposite it. The Moon’s gravitational attraction is strongest on the near side, pulling the water towards it. On the far side, the entire Earth is pulled towards the Moon, leaving the water on the opposite side to bulge outward due to inertia.
Earth continuously rotates through these two tidal bulges. This rotation is why most coastal areas experience two high tides and two low tides daily. Although the Sun is much larger than the Moon, its greater distance from Earth means its gravitational influence on tides is about half that of the Moon. The Sun’s gravity still contributes, modifying the Moon’s tidal bulges and influencing tidal patterns.
Spring Tides: Powerful Peaks and Troughs
Spring tides are characterized by the highest high tides and the lowest low tides, resulting in the greatest tidal range. This phenomenon occurs when the Sun, Earth, and Moon align in a straight line, a configuration known as syzygy. This alignment happens twice each lunar month, during the New Moon and Full Moon phases.
During a spring tide, the gravitational pulls of the Sun and Moon combine, reinforcing each other. This combined force causes the ocean’s water to bulge more. The term “spring” refers to the water “springing forth” or increasing in range, not the season. These tides can lead to higher water levels and stronger currents in coastal areas.
Neap Tides: Milder Highs and Lows
Neap tides are periods of moderate tides, meaning they have a smaller difference between high and low water levels. These tides occur when the Sun and Moon are positioned at right angles (90 degrees) to the Earth. This perpendicular alignment happens during the First Quarter and Third Quarter Moon phases.
In this configuration, the gravitational pulls of the Sun and Moon work against each other. The Sun’s force partially cancels the Moon’s tidal force, reducing the overall tidal bulge. As a result, high tides are lower than average, and low tides are higher, leading to the smallest tidal range. The word “neap” means “without power,” reflecting the reduced tidal range.