Tides, the regular rise and fall of ocean waters, are a predictable consequence of gravitational mechanics. They result from the interplay between the Earth, the Moon, and the Sun. The Moon’s gravitational pull is the primary force driving ocean motion, establishing a reliable cycle for coastal water levels along with the Earth’s rotation. This cycle is slightly longer than the 24-hour solar day, which ensures the timing of high and low tides shifts consistently.
The Semidiurnal Cycle
The most common tidal pattern globally is the semidiurnal cycle, characterized by two high tides and two low tides within a single tidal day. These four events are typically of comparable height in a truly semidiurnal system. This repeating pattern is a physical manifestation of the two bulges of ocean water created by the Moon’s gravity.
One bulge forms on the side of Earth facing the Moon, where gravitational attraction is strongest. A second, nearly equal bulge forms simultaneously on the opposite side. This second bulge occurs because the Moon’s pull is weakest there, causing the water to lag behind the bulk of the Earth. As the Earth rotates, any point passes through both bulges during the tidal cycle. The interval between one high tide and the next is approximately 12 hours and 25 minutes.
Understanding the Tidal Day
The reference period for tidal prediction is the tidal day, also known as the lunar day. This is the time it takes for a specific point on Earth to return to the same position relative to the Moon. Unlike the 24-hour solar day, the lunar day is approximately 24 hours and 50 minutes long because the Moon orbits the Earth in the same direction the Earth spins.
While the Earth completes one full rotation in 24 hours, the Moon has moved forward slightly in its orbit. To return a location directly under the Moon’s influence, the Earth must rotate for an additional 50 minutes to “catch up.” This extra rotation time causes the timing of high and low tides to advance by about 50 minutes each calendar day. The 24-hour and 50-minute lunar day establishes the full period for the complete cycle of two high and two low tides.
The Mechanism Linking the Two
The relationship between the semidiurnal period and the tidal day is a precise mathematical division based on the physical reality of the ocean bulges. Since the semidiurnal pattern involves two complete tidal cycles (two high and two low waters) within one lunar day, the duration of a single semidiurnal period is half the length of the tidal day. The Earth rotates through both the near-side and far-side tidal bulges during one revolution relative to the Moon.
This direct relationship means that dividing the 24-hour and 50-minute tidal day by two yields the 12-hour and 25-minute semidiurnal period. This value represents the average time elapsed between successive high tides at a given location. The predictable nature of this astronomical mechanism allows coastal communities and maritime navigation to rely on accurate tide tables. The Earth’s rotation relative to the Moon dictates the 24-hour and 50-minute cycle, which defines the 12-hour and 25-minute interval between successive high tides.