Sea Level, Gravity, and the True Definition of Sea Level

Overview

MinutePhysics explains that sea level is not simply the average height of the oceans. It shows how gravity, Earth's rotation, and continental mass distort the sea surface into a geoid that varies around the globe. The talk highlights calculations using the Earth Gravitational Model to predict sea level and to imagine what sea level would be beneath mountains if their gravity were present.

• Sea level is defined by gravity and the geoid, not just ocean average
• Earth's spin causes equatorial bulge and polar flattening
• Continents and mountains affect nearby sea level via gravity
• Earth Gravitational Model enables accurate predictions and GPS calibration

Introduction: redefining sea level

The video opens by challenging the simplistic idea that sea level is just the average height of the oceans. In reality, sea level is a surface defined by the gravitational field of the entire planet. Even in areas without oceans, the height of the sea would be determined by gravity pulling water toward denser regions and by the way the planet’s mass distribution molds the sea surface into a complex shape called the geoid. This geoid is not a flat ellipse but a wavy, uneven surface that varies by location due to the Earth's internal density structure and surface features.

Non-sphericity and rotation

The narrator explains that the Earth is not a perfect sphere. Because the planet spins, centrifugal forces are strongest at the equator, causing an equatorial bulge and a flattening at the poles. This means the Earth is actually about 42 kilometers wider at the equator than end-to-end pole to pole, and if one assumed a spherical Earth, distant points could appear much farther from sea level than expected. This non-sphericity also explains why Chimborazo in Ecuador is farther from the Earth’s center than Mount Everest, despite Everest being higher above sea level.

Gravity, density, and the geoid

Gravity is not uniform across the globe. The inside of the Earth has varying density, which makes gravity stronger in some places and weaker in others. Oceans tend to puddle more over dense regions, raising the sea locally. Conversely, valleys and lighter regions pull less water. The combined effect is that sea level can deviate by up to about 100 meters from a simple ellipsoid model due to the Earth’s internal density structure and surface mass distribution.

Continents, mountains, and ocean distribution

The video emphasizes a paradox: the very presence of mountains and continents changes sea level. The gravity of land masses attracts nearby water, raising sea level around them, while ocean basins and valleys exert different gravitational pulls. This interplay makes the sea level under a mountain different from the height you would predict by ignoring the mountain’s gravitational influence. The question geodetic scientists grapple with is how to define sea level in a world with mass concentrations such as continents and mountains, and how to measure the height of a mountain above sea level when gravity is involved rather than a purely topographic reference.

The Earth Gravitational Model and GPS

To resolve these questions, researchers developed an extremely detailed model of Earth’s gravitational field, known as the Earth Gravitational Model. This model is used to interpret GPS data so that devices can estimate sea level accurately, even in regions of weak gravity. By using this gravity-based reference for sea level, scientists can also define what sea level would be like beneath mountains if those mountains’ gravity were present but their topographic height were removed. This lays a consistent foundation for comparing sea levels across different terrains and for precise geodetic measurements globally.

Conclusion

The transcript closes by noting that the gravity-based sea level definition enables accurate global predictions and calibrates technologies like GPS. It also highlights how gravity, density variations, and landmasses interplay to shape the sea surface, illustrating why a simple average ocean height cannot fully describe sea level in our planet’s complex, dynamic system.