Gravity is the fundamental force of attraction that exists between any two masses, and it is the mechanism that keeps us grounded. This gravitational pull, experienced as weight, is not a constant value across the planet. Gravity definitively changes with altitude. Every meter gained in elevation, whether climbing a mountain or ascending in an airplane, results in a measurable, though often imperceptible, reduction in the Earth’s gravitational force.
The Inverse Square Law of Gravity
The physical principle governing this change is the inverse square law of gravitation. This law states that the force of gravity between two objects is directly proportional to the product of their masses, but inversely proportional to the square of the distance separating their centers. The squared relationship with distance is the key concept.
If the distance between the center of the Earth and an object were to double, the gravitational force would be reduced to one-quarter of its original strength. This rapid fall-off means that even small changes in distance can affect the force. The force is always calculated from the center of mass of the Earth to the center of mass of the object, which is why distance is the controlling factor as altitude increases.
Calculating Gravity Decrease with Height
When discussing altitude, the distance measured is the height above sea level, which directly increases the distance from the Earth’s center of mass (averaging about 6,371 kilometers). The standard gravitational acceleration at sea level is approximately 9.81 meters per second squared (\(9.81 text{ m/s}^2\)). As an object moves upward, the acceleration due to gravity systematically decreases.
The magnitude of this change is quantifiable even at moderate elevations. For example, at the summit of Mount Everest (8,848 meters above sea level), the gravitational acceleration drops to roughly \(9.77 text{ m/s}^2\). This change translates to a reduction in weight of about 0.4% compared to sea level. A person weighing 200 pounds at the base would weigh about 199.2 pounds at the peak.
The change is more pronounced at higher altitudes, though the force remains significant. The International Space Station (ISS), orbiting at about 400 kilometers, is still well within the Earth’s gravitational field. At that height, the acceleration due to gravity is still around \(8.7 text{ m/s}^2\), which is approximately 90% of the surface gravity. Astronauts experience weightlessness because they are in a constant state of free fall around the planet, not because gravity is absent.
Influence of Earth’s Shape and Spin
Beyond the simple vertical distance, two global factors influence the gravitational pull measured at the Earth’s surface: the planet’s shape and its rotation. The Earth is an oblate spheroid, meaning it is flattened at the poles and bulges at the equator. This equatorial bulge means that any point on the equator is physically farther from the Earth’s center than a point at the poles.
Because of this greater distance, gravity is naturally weaker at the equator than it is at the poles, even at sea level. This difference is compounded by the Earth’s spin, which creates a centrifugal force that acts against the pull of gravity. The centrifugal force is maximum at the equator, where the rotational speed is highest, and diminishes to almost zero at the poles.
Combining these two effects results in a measurable range of gravitational acceleration values across the globe. At the equator, the value is lowest, at about \(9.78 text{ m/s}^2\), while at the poles, it peaks at about \(9.83 text{ m/s}^2\). These variations demonstrate that the Earth’s geometry plays a major role in local gravity.
Impact of Local Mass Density
The final factor influencing a gravity reading is the distribution and density of the materials directly beneath the measurement location. Gravity measurements are sensitive to variations in local geological structures, which are known as gravitational anomalies. Denser materials in the Earth’s crust, such as iron ore deposits or volcanic rock, can exert a slightly stronger gravitational pull on the surface.
Conversely, areas underlain by less dense materials, like underground water reservoirs or sedimentary basins, show a minor reduction in the local gravity value. These anomalies are typically very small, but they are precise enough to be used by geophysicists for mineral exploration and geological mapping. The mass of a large mountain range can create a subtle, localized increase in gravity, though the overall distance effect still results in a net gravity decrease at the summit.