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What Makes High & Low Tides in the Ocean?

Hundreds of years ago, sailors believed tides to be the ebb and flow of an earth monster's breathing. It wasn't until the 17th century that tides were revealed to be the effects of planetary bodies on our oceans. In the 21st century, increasingly precise knowledge of tidal science, such as accurate prediction of tides, continuously plays a role in the success of bridge and dock construction, large vessel navigation and commercial fishing.

Bulge Theory
- The occurrences of high and low tides are largely tied to gravity. The bulge theory, developed in 1687 by Sir Isaac Newton, states that the gravitational pull of the moon causes two "bulges," or swells, in Earth's ocean waters: one in the direction of the moon and the other on the opposite side of the planet. Water is pulled from the remaining sides of the planet to create these bulges, leaving lowered sea levels from where it was drawn. Thus, two sides of Earth experience high tide together as they simultaneously "bulge;" meanwhile, the other planetary sides experience low tide.
Daily Occurrences
- As Earth rotates on its axis every 24 hours, the bulges created by the moon's gravitational pull remain aligned with the position of the moon. Each area of Earth experiences high tide as it rotates into the bulge and low tide when it rotates out again. With two of each low and high tide zones on the planet, it takes a quarter of a 24-hour rotation for a single point on Earth to fully complete its pass through one zone; therefore, in a single day, any particular location experiences two alternating high and low tides with six hours to transition from one to the other.
Spring Tides and Neap Tides
- When the moon and the sun are positioned on the same or opposite sides of Earth as one another, the pull from the sun enhances that of the moon; consequently, spring tides occur worldwide causing greater extremes in both high and low tides. During the first and third quarter phases of the moon, the sun and moon are positioned at a 90-degree angle from one another. At these times, the gravitational pull from the sun is in opposition with that of the moon, creating weaker tides all over the planet. These are known as neap tides.
Other Factors
- Besides the moon and sun, high and low tides can be affected by several factors. The shapes of shorelines, bays and estuaries can all play roles in the extremities of tides that occur in those particular locations. In Nova Scotia's funnel-shaped Bay of Fundy, normal patterns are amplified, resulting in 15 meter high tides, the highest in the world. Wind can directly alter tides, pushing water in one direction or the other to amplify the effects of a high or low tide, and while high-pressure systems may result in unusually low tides for an area, low-pressure systems -- like those characteristic of a hurricane -- may cause dramatic rises in tides.

Nature