Tides are primarily caused by the gravitational attraction of the Moon and the Sun acting on Earths oceans, combined with the orbital motion of the Earth Moon system and the rotation of Earth. The Moon is the dominant tidal influence because it is much closer to Earth than the Sun, even though the Sun has vastly greater mass. Gravity is not identical at every point on Earth.
The side of Earth facing the Moon experiences a slightly stronger lunar gravitational attraction than the centre of Earth, while the far side experiences a slightly weaker attraction. This difference across Earth, called the tidal force, contributes to the formation of two broad tidal bulges in the idealised explanation.
One bulge occurs on the side facing the Moon, while another occurs on the opposite side because the Earth Moon system revolves around a common centre of mass and the associated inertial effects produce the second component. As Earth rotates through these tidal patterns, many coastal locations experience alternating high and low water.
The Sun also produces tides through differential gravity. Although solar gravity on Earth is stronger overall than lunar gravity, the solar tidal force is weaker because tidal force depends strongly on distance as well as mass. The Moon therefore has the larger direct tidal effect. The relative alignment of the Sun, Earth and Moon determines whether their tidal effects reinforce or partly oppose one another.
When the Sun and Moon are approximately aligned during new moon and full moon, their tidal forces combine to produce larger tidal ranges called spring tides. When the Sun and Moon are approximately at right angles relative to Earth during the first and third quarter phases, their tidal effects partially counteract one another, producing smaller tidal ranges called neap tides. These patterns repeat with the lunar cycle rather than simply following the length of the solar day.
The simplified picture of two permanent ocean bulges is useful for introducing tidal forces but does not accurately describe the behaviour of real oceans. Continents divide the ocean into basins, and the shape, depth and coastline geometry of each basin strongly affect how tidal waves propagate. Tides in the open ocean can behave as long waves whose movement is modified by Earths rotation and seafloor topography.
As tidal energy travels through ocean basins, the water level response can form complex amphidromic systems in which tidal amplitude is very small around an amphidromic point and increases with distance from that point. The phase of the tide also varies spatially, so high tide does not occur simultaneously around an entire ocean basin. Local geography can greatly amplify tidal range in some areas through resonance and constriction.
Earths rotation is important because the Coriolis effect influences the movement of tidal waves and currents. As tidal energy moves through an ocean basin, the rotating Earth causes moving water to be deflected, producing complex rotating and standing wave patterns. The balance among gravitational forcing, inertia, Coriolis effects, pressure gradients and friction determines the local tidal response.
In some regions, the tidal wave behaves partly like a progressive wave that travels through the basin, while in others it behaves more like a standing wave constrained by boundaries. Coastal geometry can transform relatively modest offshore tidal amplitudes into much larger coastal changes. Narrowing bays and estuaries can concentrate water, while shallow shelves can slow and reshape the tidal wave. These physical processes explain why tidal behaviour differs dramatically from one coastline to another.
The timing of tides is also related to the Moon's orbit. Earth rotates relative to the stars approximately once every 24 hours, but because the Moon moves eastward along its orbit, Earth must rotate for slightly longer before the same location is again oriented toward the Moon. This produces a lunar day of approximately 24 hours and 50 minutes.
Many locations therefore experience two high tides and two low tides approximately every lunar day, with successive high tides separated by roughly 12 hours and 25 minutes in an ideal semidiurnal system. However, real tidal regimes vary. Some locations have predominantly semidiurnal tides, some have predominantly diurnal tides with one high and one low tide per lunar day, and others have mixed tides in which successive high and low tides have unequal heights. Local basin geometry and tidal constituents determine these patterns.
Tides are often described through tidal constituents, which represent periodic components of the tidal signal associated with astronomical cycles. The principal lunar semidiurnal constituent is commonly called M2, while the principal solar semidiurnal constituent is S2. Other constituents arise from the interaction of lunar orbital motion, solar effects and longer astronomical cycles. Harmonic tidal analysis uses observed water levels to determine the amplitudes and phases of these constituents, allowing future astronomical tide predictions to be calculated.
This approach is highly effective because the astronomical forcing is predictable. However, actual water levels can differ from predicted astronomical tides because wind, atmospheric pressure, storm surge, waves, river discharge and other processes modify the observed sea level. Thus a tide prediction is not necessarily an exact prediction of every future coastal water level.
Tides are important far beyond the daily rise and fall of coastal water. They drive tidal currents that transport water through estuaries and straits, influence sediment movement, shape coastal landscapes and contribute to mixing of ocean waters. Tidal mixing can help redistribute heat, nutrients, dissolved gases and suspended material. Intertidal ecosystems depend on the regular exposure and inundation produced by tides. Mudflats, salt marshes, mangroves and rocky shores are strongly controlled by tidal cycles. Tides also affect marine organisms whose feeding, reproduction and movement are synchronised with water levels and currents. Human activities such as navigation, fishing, coastal construction, dredging and tidal energy generation likewise depend on accurate knowledge of tidal behaviour.
A common misconception is that tides are simply caused by the Moon pulling the oceans upward on the side facing it. The actual mechanism involves differential gravitational forces and the dynamics of the entire Earth ocean system. The land itself also experiences tidal deformation, and the solid Earth responds to the same tidal forcing. Ocean tides are therefore a coupled physical response rather than water independently rising toward the Moon.
Another misconception is that high tide always occurs when the Moon is directly overhead. Local tidal phase can differ substantially because tidal waves take time to propagate through ocean basins and are controlled by geography. Tides should consequently be understood as predictable astronomical forcing filtered through the physical structure and dynamics of Earths oceans, rather than as a simple instantaneous gravitational pull.
