**Earth radius** is the distance from the center of Earth to a point on its surface. Its value ranges from 6,378 km (3,963 mi) at the equator to 6,357 km (3,950 mi) at a pole. A nominal *Earth radius* is sometimes used as a unit of measurement in astronomy and geophysics, denoted in astronomy by the symbol `R`_{⊕}. In other contexts, it is denoted or sometimes .

The Earth is not a perfect sphere but approximately an oblate spheroid (an ellipse rotated around its minor axis) with a larger radius at the equator than at the poles. When only one radius is stated, the International Astronomical Union (IAU) prefers that it be the equatorial radius.^{[1]} The International Union of Geodesy and Geophysics (IUGG) recommends three values: the arithmetic mean of the radii measured at the equator and the poles (R_{1}); the authalic radius, which is the radius of a sphere with the same surface area (R_{2}); and the volumetric radius, which is the radius of a sphere having the same volume as the ellipsoid (R_{3}).^{[2]} All three values are about 6,371 kilometres (3,959 mi).

There are many other ways to define and measure the Earth radius. Some appear below. A few definitions yield values outside the range between polar radius and equatorial radius because they include local or geoidal topology or because they depend on abstract geometrical considerations.

Earth's rotation, internal density variations, and external tidal forces cause its shape to deviate systematically from a perfect sphere.^{[a]} Local topography increases the variance, resulting in a surface of profound complexity. Our descriptions of Earth's surface must be simpler than reality in order to be tractable. Hence, we create models to approximate characteristics of Earth's surface, generally relying on the simplest model that suits the need.

Each of the models in common use involve some notion of the geometric radius. Strictly speaking, spheres are the only solids to have radii, but broader uses of the term *radius* are common in many fields, including those dealing with models of Earth. The following is a partial list of models of Earth's surface, ordered from exact to more approximate:

- The actual surface of Earth
- The geoid, defined by mean sea level at each point on the real surface
^{[b]} - A spheroid, also called an ellipsoid of revolution, geocentric to model the entire Earth, or else geodetic for regional work
^{[c]} - A sphere

In the case of the geoid and ellipsoids, the fixed distance from any point on the model to the specified center is called *"a radius of the Earth"* or *"the radius of the Earth at that point"*.^{[d]} It is also common to refer to any *mean radius* of a spherical model as *"the radius of the earth"*. When considering the Earth's real surface, on the other hand, it is uncommon to refer to a "radius", since there is generally no practical need. Rather, elevation above or below sea level is useful.

Regardless of the model, any radius falls between the polar minimum of about 6,357 km and the equatorial maximum of about 6,378 km (3,950 to 3,963 mi). Hence, the Earth deviates from a perfect sphere by only a third of a percent, which supports the spherical model in most contexts and justifies the term "radius of the Earth". While specific values differ, the concepts in this article generalize to any major planet.

Rotation of a planet causes it to approximate an *oblate ellipsoid/spheroid* with a bulge at the equator and flattening at the North and South Poles, so that the *equatorial radius* a is larger than the *polar radius* b by approximately aq. The *oblateness constant* q is given by

where ω is the angular frequency, G is the gravitational constant, and M is the mass of the planet.^{[e]} For the Earth 1/*q* ≈ 289, which is close to the measured inverse flattening 1/*f* ≈ 298.257. Additionally, the bulge at the equator shows slow variations. The bulge had been decreasing, but since 1998 the bulge has increased, possibly due to redistribution of ocean mass via currents.^{[4]}

The variation in density and crustal thickness causes gravity to vary across the surface and in time, so that the mean sea level differs from the ellipsoid. This difference is the *geoid height*, positive above or outside the ellipsoid, negative below or inside. The geoid height variation is under 110 m (360 ft) on Earth. The geoid height can change abruptly due to earthquakes (such as the Sumatra-Andaman earthquake) or reduction in ice masses (such as Greenland).^{[5]}

Not all deformations originate within the Earth. Gravitational attraction from the Moon or Sun can cause the Earth's surface at a given point to vary by tenths of a meter over a nearly 12-hour period (see Earth tide).

Given local and transient influences on surface height, the values defined below are based on a "general purpose" model, refined as globally precisely as possible within 5 m (16 ft) of reference ellipsoid height, and to within 100 m (330 ft) of mean sea level (neglecting geoid height).

Additionally, the radius can be estimated from the curvature of the Earth at a point. Like a torus, the curvature at a point will be greatest (tightest) in one direction (north–south on Earth) and smallest (flattest) perpendicularly (east–west). The corresponding radius of curvature depends on the location and direction of measurement from that point. A consequence is that a distance to the true horizon at the equator is slightly shorter in the north/south direction than in the east-west direction.

In summary, local variations in terrain prevent defining a single "precise" radius. One can only adopt an idealized model. Since the estimate by Eratosthenes, many models have been created. Historically, these models were based on regional topography, giving the best reference ellipsoid for the area under survey. As satellite remote sensing and especially the Global Positioning System gain

The Earth is not a perfect sphere but approximately an oblate spheroid (an ellipse rotated around its minor axis) with a larger radius at the equator than at the poles. When only one radius is stated, the International Astronomical Union (IAU) prefers that it be the equatorial radius.^{[1]} The International Union of Geodesy and Geophysics (IUGG) recommends three values: the arithmetic mean of the radii measured at the equator and the poles (R_{1}); the authalic radius, which is the radius of a sphere with the same surface area (R_{2}); and the volumetric radius, which is the radius of a sphere having the same volume as the ellipsoid (R_{3}).^{[2]} All three values are about 6,371 kilometres (3,959 mi).

There are many other ways to define and measure the Earth radius. Some appear below. A few definitions yield values outside the range between polar radius and equatorial radius because they include local or geoidal topology or because they depend on abstract geometrical considerations.

Earth's rotation, internal density variations, and external tidal forces cause its shape to deviate systematically from a perfect sphere.^{[a]} Local topography increases the variance, resulting in a surface of profound complexity. Our descriptions of Earth's surface must be simpler than reality in order to be tractable. Hence, we create models to approximate characteristics of Earth's surface, generally relying on the simplest model that suits the need.

Each of the models in common use involve some notion of the geometric radius. Strictly speaking, spheres are the only solids to have radii, but broader uses of the term *radius* are common in many fields, including those dealing with models of Earth. The following is a partial list of models of Earth's surface, ordered from exact to more approximate:

- The actual surface of Earth
- The geoid, defined by mean sea level at each point on the real surface
^{[b]} - A spheroid, also called an ellipsoid of revolution, geocentric to model the entire Earth, or else geodetic for regional work
^{[c]} - A sphere

In the case of the geoid and ellipsoids, the fixed distance from any point on the model to the specified center is called *"a radius of the Earth"* or *"the radius of the Earth at that point"*.^{[d]} It is also common to refer to any *mean radius* of a spherical model as *"the radius of the earth"*. When considering the Earth's real surface, on the other hand, it is uncommon to refer to a "radius", since there is generally no practical need. Rather, elevation above or below sea level is useful.

Regardless of the model, any radius falls between the polar minimum of about 6,357 km and the equatorial maximum of about 6,378 km (3,950 to 3,963 mi). Hence, the Earth deviates from a perfect sphere by only a third of a percent, which supports the spherical model in most contexts and justifies the term "radius of the Earth". While specific values differ, the concepts in this article generalize to any major planet.

Rotation of a planet causes it to approximate an *oblate ellipsoid/spheroid* with a bulge at the equator and flattening at the North and South Poles, so that the *equatorial radius* a is larger than the *polar radius* b by approximately aq. The *oblateness constant* q is given by

where ω is the angular frequency, G is the gravitational constant, and M is the mass of the planet.^{[e]} For the Earth 1/*q* ≈ 289, which is close to the measured inverse flattening 1/*f* ≈ 298.257. Additionally, the bulge at the equator shows slow variations. The bulge had been decreasing, but since 1998 the bulge has increased, possibly due to redistribution of ocean mass via currents.^{[4]}

The variation in density and crustal thickness causes gravity to vary across the surface and in time, so that the mean sea level differs from the ellipsoid. This difference is the *geoid height*, positive above or outside the ellipsoid, negative below or inside. The geoid height variation is under 110

Each of the models in common use involve some notion of the geometric radius. Strictly speaking, spheres are the only solids to have radii, but broader uses of the term *radius* are common in many fields, including those dealing with models of Earth. The following is a partial list of models of Earth's surface, ordered from exact to more approximate:

In the case of the geoid and ellipsoids, the fixed distance from any point on the model to the specified center is called *"a radius of the Earth"* or *"the radius of the Earth at that point"*.^{[d]} It is also common to refer to any *mean radius* of a spherical model as *"the radius of the earth"*. When considering the Earth's real surface, on the other hand, it is uncommon to refer to a "radius", since there is generally no practical need. Rather, elevation above or below sea level is useful.

Regardless of the model, any radius falls between the polar minimum of about 6,357 km and the equatorial maximum of about 6,378 km (3,950 to 3,963 mi). Hence, the Earth deviates from a perfect sphere by only a third of a percent, which supports the spherical model in most contexts and justifies the term "radius of the Earth". While specific values differ, the concepts in this article generalize to any major planet.

Rotation of a planet

Rotation of a planet causes it to approximate an *oblate ellipsoid/spheroid* with a bulge at the equator and flattening at the North and South Poles, so that the *equatorial radius* a is larger than the *polar radius* b by approximately aq. The *oblateness constant* q is given by