Tides are the daily, cyclical rise and fall of sea level, a phenomenon driven primarily by the gravitational pull of the Moon and the Sun. These forces create massive bulges of water in the ocean that Earth rotates through, resulting in the regular change in water height at the coast. Predicting the timing of the next high or low water mark is important for anyone whose activities depend on the state of the water. The basic calculation relies on a standard astronomical rhythm, though real-world locations often introduce slight variations.
Calculating the Next Tide
In locations that experience a semi-diurnal tide, the most common type globally, the cycle involves two high tides and two low tides occurring approximately every 24 hours and 50 minutes. This period is known as a lunar day. To find the interval between any two consecutive tidal phases, such as from a high tide to the next low tide, this total lunar day is divided by four.
The standard interval between a high tide and the subsequent low tide is approximately 6 hours and 12.5 minutes. Therefore, if high tide is at noon (12:00 PM), the theoretical time for the next low tide would be around 6:12 PM. The next high tide would follow this low tide, arriving at about 12:25 AM the following morning.
Why the Tidal Cycle Shifts Daily
The reason a tide does not occur at the exact same time every 24 hours is due to the Moon’s orbit around the Earth. The gravitational force of the Moon is the main driver of the tides, creating the two large bulges of water on opposite sides of the planet. As the Earth rotates beneath these bulges, a coastal location experiences high and low tides.
While the Earth completes one rotation in 24 hours, the Moon is simultaneously moving in its orbit in the same direction. For a specific point on Earth to be directly beneath the Moon again, the planet must rotate for an extra 50 minutes to “catch up” to the Moon’s new position. This means the full cycle of two high tides and two low tides takes 24 hours and 50 minutes, causing a daily lag in the timing of the tides.
How Local Geography and Astronomy Modify Predictions
While the 6 hour and 12.5 minute interval serves as a strong baseline, real-world geography and other celestial mechanics modify the actual timing and height of tides. The semi-diurnal cycle is not universal. Some regions, like the Gulf of Mexico, experience diurnal tides with only one high and one low water per day. Others have mixed semi-diurnal tides with two high and two low waters of unequal height. The standard calculation applies best to regions with a consistent semi-diurnal pattern, such as the east coasts of North America and Australia.
Local Geographic Factors
Local features play a major role in the manifestation of tides, a concept known as bathymetry. The depth of the ocean floor, the shape of the coastline, and the presence of narrow bays or estuaries can accelerate, delay, or amplify the arrival of the tidal wave. For instance, a funnel-shaped bay acts like a natural amplifier, squeezing the incoming water and leading to significantly higher tidal ranges compared to an open coastline.
Astronomical and Weather Factors
Astronomical factors beyond the Moon also introduce variability, most notably the Sun’s gravitational influence. When the Sun, Moon, and Earth align during a new or full moon, their gravitational forces combine to produce the largest difference between high and low tide, known as spring tides. Conversely, when the Sun and Moon are at right angles, their pulls partially cancel each other out, resulting in the smaller-range neap tides. Weather conditions, such as strong winds or changes in barometric pressure, can also temporarily affect the precise time and height of a tide by pushing water toward or away from the shore.