The sun’s gravity raises tides in Earth’s oceans through the same basic mechanism as the moon, but with roughly half the strength. Because the sun is so much farther away, its tide-generating force comes out to about 46 percent of the moon’s, despite the sun being vastly more massive. That fraction is large enough to matter: when solar and lunar tides work together, coastal water levels swing dramatically, and when they oppose each other, tides become noticeably smaller. But gravity is not the sun’s only path to influencing tides, and the interplay between solar and lunar forces ripples outward into ocean mixing, solid-earth deformation, and even the biological clocks of marine organisms.
Why the Sun’s Gravity Creates Tides at All
Tides arise from the difference in gravitational pull across the width of the Earth. The side of the planet facing the sun is pulled slightly harder than the center, and the center slightly harder than the far side. That difference stretches the ocean into a subtle bulge on both the near and far sides. Earth rotates through these bulges roughly once a day, creating a solar tidal cycle with two highs and two lows every 24 hours. This particular pattern is called the solar semidiurnal tide, known in oceanography as S2.
The moon produces its own set of bulges on a slightly longer schedule, since it orbits in the same direction Earth rotates and takes a bit more than a day to return to the same overhead position. The lunar semidiurnal tide, called M2, is the dominant tidal signal on most coastlines. The sun’s S2 tide rides on top of it, reinforcing or partially canceling the lunar tide depending on where the sun and moon sit relative to each other.
Spring Tides and Neap Tides
Twice each month, the sun and moon line up on the same side of Earth (new moon) or on opposite sides (full moon). In both cases, their tidal forces pull along the same axis, adding together. The result is spring tides, when high tides are higher and low tides are lower than average. The name has nothing to do with the season; it comes from an old word meaning “to leap.”
About a week after each spring tide, the moon sits at a right angle to the sun relative to Earth. Now the lunar and solar bulges partially cancel, producing neap tides, where the daily tidal range shrinks. This spring-neap cycle repeats roughly every two weeks and is one of the most visible ways the sun shapes the tides you can observe at a beach. Oysters in Normandy, for instance, show valve-opening rhythms that track not just individual tidal cycles but also the spring-neap pattern, with activity intensity rising and falling on a two-week beat that reflects the combined pull of sun and moon.1PubMed. Field chronobiology of a molluscan bivalve: how the moon and sun cycles interact to drive oyster activity rhythms
How Earth’s Orbit Changes Solar Tidal Strength
Earth does not orbit the sun in a perfect circle. Its path is slightly elliptical, bringing the planet closest to the sun (perihelion) around early January and farthest (aphelion) around early July. Because tidal force scales strongly with distance, the sun’s tide-generating pull is a few percent stronger at perihelion than at aphelion. That is a small difference, but it stacks on top of other factors. An analysis of the timing of peak astronomical tides found that the largest tides consistently occur near perihelion, when the sun’s right ascension closely matches the longitude of perihelion, generally within about 15 degrees.2Geophysical Journal International. Times of peak astronomical tides
Over very long timescales, the orientation of Earth’s orbit precesses, slowly shifting when in the year perihelion falls. During the period examined by that same analysis, perihelion drifted from northern-hemisphere autumn deeper into winter. This means the seasonal timing of the year’s highest tides is not fixed; it creeps later into the calendar over millennia. For anyone living on a low-lying coast, the practical upshot is that the biggest astronomical tides tend to cluster in January, and climate-driven sea-level rise on top of those tides creates an especially dangerous combination.
The Sun’s Other Route to Tides
Gravity is the mechanism most people learn about in school, but the sun also drives tides through its heat. Sunlight warms the atmosphere unevenly as Earth rotates, creating a large-scale pressure oscillation that circles the planet roughly twice a day. This atmospheric thermal tide pushes down on the ocean surface wherever air pressure is locally higher, creating a small but measurable contribution to the S2 ocean tide that has nothing to do with gravitational pull.
The solar radiation that powers this process is absorbed at multiple levels of the atmosphere. Water vapor in the lower atmosphere, ozone in the middle atmosphere, and ultraviolet radiation higher up all contribute to exciting these pressure oscillations.3Journal of Geophysical Research: Space Physics. Atmospheric tides: 1. Model description and results for the solar diurnal component The resulting atmospheric pressure wave then loads the ocean surface, creating what oceanographers call a radiational tide.
Globally, this atmospherically driven S2 ocean tide runs about 15 percent as large as the gravitationally driven S2 tide, and it sits roughly 109 degrees out of phase with it.4Geophysical Research Letters. Atmospheric forcing of the oceanic semidiurnal tide That phase offset matters because the two signals do not simply add up; they partially interfere with each other. Depending on location, the radiational component can either boost or reduce the total S2 tide you measure at a tide gauge. In places like the East China Sea, where shallow coastal waters amplify the signal, the radiational S2 amplitude can reach about 14 centimeters and amounts to roughly 12 percent of the local astronomical S2 amplitude.5Continental Shelf Research. Large radiational S2 tides in the East China Sea
Tide predictions made from gauge observations naturally bake in both the gravitational and radiational signals without distinguishing them. This creates a practical headache for storm-surge forecasting: weather models that already account for atmospheric pressure effects on sea level can end up double-counting the radiational portion if the tide prediction they start from already includes it.6Ocean Science. Radiational tides: their double-counting in storm surge forecasts and contribution to the Highest Astronomical Tide Getting this overlap wrong can mean overestimating or underestimating the peak water level during a storm by a meaningful amount.
The S2 Tide Is Not Standing Still
If solar tides were driven purely by celestial mechanics, you would expect them to be extremely stable over human timescales. Yet twentieth-century tide gauge records along the eastern coast of North America and at Bermuda reveal that the S2 tide has been shrinking in amplitude, at rates on the order of 10 percent per century.7Geophysical Research Letters. Secular changes in the solar semidiurnal tide of the western North Atlantic Ocean That is a surprisingly large drift for a signal tied to the sun’s gravitational pull, which has not changed at all on that timescale.
The explanation likely involves changes in how the ocean responds to the forcing rather than changes in the forcing itself. Sea-level rise alters water depth on continental shelves, which in turn shifts how tidal waves propagate and reflect. Changes in ocean stratification from warming can also modify the way tidal energy moves vertically through the water column. The S2 tide is especially sensitive to these shifts because its period exactly matches the 12-hour cycle of atmospheric pressure loading, making it a hybrid signal influenced by both gravitational and radiational components. When the ocean’s physical properties change, the balance between those components shifts as well.
The Solid Earth Flexes Too
Tides are not confined to water. The sun and moon also deform the solid body of Earth itself, producing what geophysicists call earth tides or body tides. These deformations are elastic: the ground beneath you rises and falls by roughly 20 to 30 centimeters over the course of a day in response to the combined lunar and solar pull. You cannot feel it because the entire landscape moves together, but sensitive instruments like gravimeters and GPS stations measure it continuously. The sun’s share of this deformation follows the same proportional split as ocean tides, with lunar forces dominating but solar forces making a substantial contribution.8ScienceDirect. Earth Tides and Tidal Deformations
These earth tides are not just a curiosity. They slightly change the shape of the planet and distort the direction of the local vertical, which means that even the definition of “sea level” at a tide gauge shifts as the crust flexes beneath it. Precision measurements in geodesy, particle physics, and even long-baseline interferometry for radio astronomy must account for earth tides to get accurate results.
Tidal Friction and Earth’s Slowing Spin
Every tidal cycle dissipates energy. As the ocean sloshes back and forth across continental shelves and through narrow straits, friction converts tidal kinetic energy into heat. The moon is the primary driver of this dissipation, but the sun contributes its share. Over geological time, the cumulative effect has been to slow Earth’s rotation. Days were shorter hundreds of millions of years ago, and the evidence shows up in growth patterns of fossil corals and bivalves whose daily and annual bands allow researchers to count how many days fit into a year.
Fossil-based estimates of Earth’s rotational deceleration line up remarkably well with modern satellite measurements. The weighted average rotational deceleration derived from fossil data falls within about 1 percent of the total tidal deceleration measured using precision laser and Doppler ranging from artificial satellites.9The Astronomical Journal. EARTH’S ROTATIONAL DECELERATION: DETERMINATION OF TIDAL FRICTION INDEPENDENT OF TIMESCALES That close agreement, using completely independent methods separated by hundreds of millions of years, is strong evidence that tidal friction has been a steady brake on Earth’s spin throughout its history.
Deep-Ocean Mixing Powered by Tidal Energy
Not all tidal energy dissipates on shallow shelves. A significant fraction transfers into internal tides, waves that propagate along density boundaries within the deep ocean rather than at the surface. When tidal currents flow over rough seafloor features like mid-ocean ridges and seamounts, the interaction generates these internal waves, which then travel and eventually break, stirring the deep ocean much the way wind stirs the surface.
This mixing turns out to be a major driver of the ocean’s large-scale circulation. Research combining models with satellite and in-situ measurements has shown that small-scale internal tides, which had been largely overlooked, account for more than half of all global internal tide generation and the deep mixing that follows.10PubMed Central. Deep-ocean mixing driven by small-scale internal tides The energy feeding these internal tides comes from the surface tidal flows driven by both the moon and the sun. Without it, the deep ocean would stratify more strongly, and the overturning circulation that carries heat and nutrients around the planet would look very different.
The amplification of tides in specific basins also matters here. When the natural resonant period of an ocean basin happens to be close to the tidal period, the tide is amplified well beyond what the gravitational forcing alone would produce. The North Atlantic, for example, has a resonant period close enough to the semidiurnal tidal period that its tides are unusually large.11The Astrophysical Journal Letters. Consequences of Tidal Dissipation in a Putative Venusian Ocean This resonance effect means that the same solar gravitational forcing can produce very different tidal amplitudes depending on the geometry of the coast and ocean floor.
How Marine Life Tracks the Sun’s Tidal Signature
Marine organisms in the intertidal zone live and die by tidal rhythms, and many have evolved internal clocks that anticipate those rhythms rather than merely reacting to them. These biological clocks do not track just one cycle. Circadian clocks follow the 24-hour day. Circatidal clocks follow the roughly 12.4-hour tidal cycle. Circalunar clocks follow the monthly lunar cycle. And seasonal clocks modulate all of the above. The sun’s influence threads through several of these timekeepers.
The spring-neap cycle, which depends directly on the sun-moon alignment, modulates the intensity of tidal forcing that organisms experience. Pacific oysters studied in the field showed that the tidal cycle was the strongest driver of their valve behavior, but the strength of that tidal signal was itself modulated by the spring-neap pattern, which in turn was shaped by the moon’s distance from Earth.1PubMed. Field chronobiology of a molluscan bivalve: how the moon and sun cycles interact to drive oyster activity rhythms Light, a purely solar signal, also played a role in the oysters’ rhythms but was subordinate to the tidal driving force. In other words, the sun shapes these animals’ behavior both through its gravitational contribution to the tides and through the day-night light cycle, but the gravitational pathway dominates.
The interconnections between different types of biological clocks are elaborate. Lunar reproduction in many marine species is restricted to only certain months of the year, a seasonal filter that depends on day length and water temperature, both of which are solar-driven. Species that reproduce on a lunar schedule also tend to spawn within a defined window of the day, linking circadian and circalunar timing. In the most finely tuned cases, reproduction is confined to specific hours of specific days of specific months, requiring multiple clocks to coordinate.12PubMed Central. Another Place, Another Timer: marine species and the Rhythms of Life The molecular machinery behind this coordination is still being worked out, but it appears to involve shared signaling molecules that mediate cross-talk between the different clocks within a single species.
Solar Tides on Other Worlds
Earth is not the only place where solar tides matter, and looking at other planets puts the sun’s tidal influence into sharper perspective. Venus, which orbits much closer to the sun than Earth does, experiences solar tides strong enough to influence the planet’s rotation. On Venus, the competition between gravitational tides that deform the solid body and thermal tides driven by solar heating of the thick atmosphere determines the planet’s spin state. Close to a star, gravitational tides dominate and tend to synchronize a planet’s rotation with its orbit. Farther out, thermal atmospheric tides can push the spin in different directions.13Astronomy & Astrophysics. The rotation of planets hosting atmospheric tides: from Venus to habitable super-Earths
The transition between these two regimes happens to overlap with the habitable zone around low-mass stars, where both prograde and retrograde spin equilibria can exist.14Astronomy & Astrophysics. Spin evolution of Venus-like planets subjected to gravitational and thermal tides A thick atmosphere can shift the balance further, and changes in a star’s luminosity over time add another layer of complexity. For exoplanets orbiting red dwarf stars, which are fainter and require tighter orbits for habitability, solar tidal effects on rotation could determine whether a planet ends up tidally locked with one face permanently baked and the other frozen, or whether atmospheric thermal tides keep it spinning. On Earth, we are far enough from the sun that its tidal torque on our spin is minor compared to the moon’s. But on a planet without a large moon, the sun’s tidal influence on rotation could be the whole story.
Solar Tides in the Upper Atmosphere
The sun’s tidal reach extends well above the ocean surface. In the ionosphere, the electrically charged layer of the upper atmosphere roughly 80 to 300 kilometers up, solar tidal winds drive electric currents. As solar heating creates large-scale atmospheric oscillations at tidal frequencies, those oscillations push charged particles through Earth’s magnetic field, generating what is known as the ionospheric wind dynamo. Different tidal modes, both diurnal and semidiurnal, contribute differently to these currents. Some modes are prominent in the lower ionosphere and produce substantial current systems, while others generate little current despite being energetic in terms of wind speed.
These ionospheric currents are not just academic. They produce daily variations in Earth’s magnetic field that are measurable at ground-based observatories, and they influence the propagation of radio signals. The quiet-day magnetic variation pattern, which magnetometers have tracked for over a century, is largely a product of solar tidal winds driving current loops in the ionosphere. Disruptions to this pattern during geomagnetic storms can degrade GPS accuracy and high-frequency communications. Understanding the baseline solar tidal signal in the ionosphere is essential for distinguishing normal daily fluctuations from space-weather disturbances that require operational responses.