Gravity of Earth

Gravity on Earth is the net acceleration experienced by objects due to two combined effects. These effects are gravitation, which comes from the distribution of mass within the planet, and centrifugal force, which results from the Earth's rotation. Gravity is a vector quantity, meaning it has both a specific magnitude and a direction. The direction of gravity coincides with the path of a plumb bob, which is a weight hanging from a string. In the International System of Units, this acceleration is expressed in meters per second squared (m/s²). Near the Earth's surface, the acceleration is approximately 9.8 m/s². This means a falling object's vertical velocity increases by about 9.8 meters per second every second, if air resistance is ignored.

While we often think of gravity as a constant, its precise strength varies depending on your location. A perfect, non-rotating sphere with uniform mass density would produce a uniform gravitational field. However, the Earth is an oblate spheroid, meaning it is not a perfect sphere. It is slightly flatter at the poles and bulges at the equator due to its rotation. This shape, combined with the rotation itself, causes deviations in gravity across the surface. The total variation in magnitude across the Earth is around 0.7%. For example, gravity is 9.7639 m/s² on Nevado Huascarán in Peru, but it is 9.8337 m/s² at the surface of the Arctic Ocean.

Latitude plays a major role in these variations. At latitudes closer to the equator, the outward centrifugal force from Earth's rotation is larger. This force counteracts gravity by up to a maximum of 0.3% at the equator, reducing the apparent downward acceleration. Additionally, the equatorial bulge means objects at the equator are further from the planet's center. Because gravitational attraction varies inversely with the square of the distance between two masses, this distance reduces the pull. Consequently, sea-level gravity increases from about 9.780 m/s² at the equator to about 9.832 m/s² at the poles. This means an object weighs approximately 0.5% more at the poles than at the equator.

Altitude also changes the strength of the gravitational pull. As you rise above the surface, you increase your distance from the Earth's center, which weakens the force. An increase in altitude from sea level to 9,000 meters results in a weight decrease of about 0.29%. It is a common misconception that astronauts in orbit are weightless because they have escaped Earth's gravity. In reality, at the altitude of the International Space Station, gravity is still nearly 90% as strong as it is on the ground. Astronauts feel weightless because they are in a state of free-fall, not because gravity has disappeared.

In 1901, the third General Conference on Weights and Measures established a standard gravitational acceleration. This conventional value is defined as 9.80665 m/s². It was based on measurements taken at the Pavillon de Breteuil near Paris in 1888. To create this standard, scientists applied a theoretical correction to convert the measurements to a latitude of 45° at sea level. This value is not an average of all locations, but a formal agreement to use when a specific local value is not required. This standard is also used to define the units known as kilogram force and pound force.

Local geology and topography create what are known as gravity anomalies. These are regional differences in the gravitational field caused by the density of the ground. Denser rocks, such as those containing mineral ores, create higher than normal local gravitational fields. Conversely, less dense sedimentary rocks cause a weaker field. Scientists use highly sensitive instruments called gravimeters to measure these fluctuations. By subtracting the effects of known factors like topography, researchers can use these anomalies to find oil and mineral deposits. These anomalies can even be large enough to cause bulges in sea level.

Understanding gravity also requires looking deep beneath the surface. According to the Shell theorem, proven by Isaac Newton, the gravitational force at a certain radius depends only on the mass inside that sphere. All the mass outside that radius cancels out. This means that if you were inside the Earth, the gravity you feel would depend on the density and mass of the layers beneath you. The Earth's density is not uniform; it changes as you move from the crust through the mantle to the core. Models like the Preliminary Reference Earth Model (PREM) help scientists map how this density distribution affects the overall gravitational field.