Daily tides work primarily because the Earth rotates through two tidal bulges of water created by the gravitational pull of the Moon and, to a lesser extent, the Sun.
Understanding the Daily Tidal Cycle
As the Earth spins, different parts of the globe pass through these bulges and the areas between them, resulting in the rise and fall of sea levels we observe as tides.
Based on scientific understanding, including insights from resources like the provided reference, because the Earth rotates through two tidal “bulges” each lunar day, most coastal areas experience two high and two low tides every 24 hours and 50 minutes. This specific timing relates to the lunar day, which is slightly longer than a solar day because the Moon is also orbiting the Earth.
Here's a breakdown of the typical daily cycle:
- High Tides: Occur approximately every 12 hours and 25 minutes apart.
- Low Tides: Also occur approximately every 12 hours and 25 minutes apart, alternating with high tides.
The transition between high and low tide takes about half of this period. It takes 6 hours and 12.5 minutes for water at the shore to go from high-to-low or from low-to-high.
Why Two High Tides?
While one bulge is easy to understand (it's the water pulled towards the Moon), the second bulge occurs on the opposite side of the Earth. This happens because the Moon's gravity pulls the solid Earth more strongly than the water on the far side, effectively leaving that water behind and creating a bulge there as well. As Earth rotates, a location experiences a high tide as it passes through each bulge.
Tidal Timing Table
To visualize the cycle within a 24-hour and 50-minute period:
| Event | Approximate Time Interval (from previous same event) | Approximate Time Interval (from previous different event) |
|---|---|---|
| High Tide to High Tide | 12 hours 25 minutes | N/A |
| Low Tide to Low Tide | 12 hours 25 minutes | N/A |
| High Tide to Low Tide | N/A | 6 hours 12.5 minutes |
| Low Tide to High Tide | N/A | 6 hours 12.5 minutes |
This consistent pattern is why you can often predict tide times for a specific location.
Factors Influencing Tides
While the Moon is the primary driver, other factors slightly modify the daily cycle:
- The Sun's Gravity: The Sun also creates tidal bulges, although smaller than the Moon's. When the Sun, Moon, and Earth align (during new and full moons), their gravitational pulls combine, causing larger-than-average tides called spring tides. When they are at right angles (during quarter moons), their pulls partially cancel out, resulting in smaller-than-average tides called neap tides.
- Local Geography: The shape of the coastline, ocean depth, and seafloor topography can significantly affect the timing and height of tides in specific locations.
Understanding the daily tidal cycle is crucial for activities like navigation, fishing, and coastal planning.
