The ocean tides, the rhythmic rise and fall of sea level, are a fundamental feature of our planet’s coastal environments. The most dramatic range between high and low tide, known as a spring tide, occurs consistently around the time of the New Moon and the Full Moon. This maximum tidal effect is caused by the specific astronomical configuration of the Earth, Moon, and Sun.
The Moon’s Role in Creating Tidal Bulges
The primary driver of Earth’s ocean tides is the Moon’s gravitational pull. Gravity is a differential force that weakens with distance. This difference in gravitational strength across our planet creates the tidal bulges.
The side of Earth facing the Moon experiences the strongest pull, drawing the water toward the Moon and forming one tidal bulge. On the opposite side, the Moon’s gravitational pull is weakest. The solid Earth is pulled toward the Moon more strongly than the water on the far side, leaving that water behind to form a second, opposing bulge.
This mechanism results in two high tides and two low tides occurring roughly every 24 hours and 50 minutes at most coastal locations. As Earth rotates, any point on the surface passes through both water bulges. The Moon’s close proximity makes its differential gravitational force the dominant factor in the daily tidal cycle.
The Sun’s Influence on Tidal Strength
Although the Moon is the main influence, the Sun also exerts a measurable gravitational force on the Earth’s oceans. The Sun is vastly more massive than the Moon, but its distance is approximately 390 times greater. Because tidal force depends on the difference in gravity across the Earth, the Sun’s effect is diluted by its immense separation.
The Sun’s tidal influence is significant, contributing about 44% of the Moon’s tide-generating force. The solar gravitational pull creates its own set of tidal bulges on Earth, separate from the lunar bulges. The Sun acts as a modifier, either amplifying or diminishing the Moon’s bulges based on the geometric alignment of the three celestial bodies.
When Lunar and Solar Forces Combine
The highest high tides and lowest low tides, known as spring tides, occur when the Earth, Moon, and Sun align in a nearly straight line. This specific configuration is known as syzygy. Syzygy happens twice during the lunar cycle: once during the New Moon and once during the Full Moon.
During the New Moon, the Moon is positioned between the Earth and the Sun. Both the Sun and the Moon pull the ocean water in the same direction. Their gravitational forces combine, resulting in an exaggerated tidal bulge.
The same additive effect happens during the Full Moon, when the Earth is positioned between the Moon and the Sun. The Moon pulls the water toward it, and the Sun pulls the water on the opposite side. Since both the lunar and solar bulges are aligned along the same axis, their forces reinforce one another. This combined gravitational pull creates the maximum tidal range characteristic of spring tides.
How Alignment Differs During Lesser Tides
The contrast to spring tides is the neap tide, which features the smallest difference between high and low water levels. Neap tides occur when the gravitational pulls of the Moon and the Sun work against each other. This happens during the First Quarter and Last Quarter Moon phases.
During these quarter phases, the Sun and Moon are positioned at a 90-degree angle relative to the Earth. The Moon creates its primary tidal bulges along one axis, while the Sun pulls the water along a perpendicular axis. This orthogonal alignment causes the Sun’s tidal force to partially counteract the Moon’s pull.
Instead of combining to create a single, large bulge, the two forces compete, pulling the water in different directions simultaneously. This results in the high tides being lower than average and the low tides being higher than average, minimizing the overall tidal range.