Most coastal locations across the globe typically experience two low tides and two high tides each day. This rhythmic pattern of rising and falling sea levels is governed by complex forces, involving the interplay of celestial mechanics and Earth’s dynamic systems.
The Moon’s Gravitational Pull
The primary force driving Earth’s tides is the gravitational attraction exerted by the Moon. The Moon, despite its smaller size compared to the Sun, has a more significant influence on Earth’s tides because it is much closer to our planet. The Moon’s gravitational pull is strongest on the side of Earth directly facing it. This stronger pull draws the ocean waters on that side towards the Moon, creating a high tide bulge.
Why Two Tides Occur Daily
Two high tides and two low tides occur daily due to the Moon’s gravitational influence and Earth’s rotation. As the Moon pulls water on the closest side, a high tide bulge forms. A second high tide bulge forms on the opposite side because the Moon’s gravitational pull is weaker on the far side of Earth.
On the far side, inertia causes water to lag behind as the solid Earth is pulled more strongly towards the Moon. This differential pull results in water bulging outwards, away from the Moon, creating the second high tide.
With two high tide bulges on opposite sides of Earth, the planet rotates through them. As a coastal location moves into a bulge, it experiences a high tide. Conversely, as it moves out of a bulge into areas between them, where water levels are lower, it experiences a low tide. Since Earth completes one rotation approximately every 24 hours and 50 minutes relative to the Moon (a “lunar day”), most areas pass through both bulges and lower water areas, leading to two high tides and two low tides within this period. High tides occur about 12 hours and 25 minutes apart.
Other Influences on Tidal Patterns
While the Moon’s gravity is the primary driver, other factors modify the timing and height of tides. The Sun also exerts a gravitational pull on Earth’s oceans, though its effect is about half that of the Moon due to its much greater distance.
When the Sun, Moon, and Earth align, during new and full moon phases, their gravitational forces combine, resulting in higher high tides and lower low tides. These are known as spring tides, and they produce the largest tidal ranges. Conversely, when the Sun and Moon are at right angles to each other, during the first and third quarter moon phases, their gravitational pulls partially counteract each other. This leads to neap tides, characterized by lower high tides and higher low tides, resulting in a smaller tidal range.
Beyond celestial alignments, geographical features significantly influence local tidal patterns. The shape of coastlines, the depth of ocean basins, and the presence of landmasses can channel or restrict water movement. This can cause variations from the standard two low tides per day, leading to patterns such as diurnal tides (one high and one low tide per day) or mixed tides (two high and two low tides of unequal heights).