Earth radius | |
---|---|
13th century illustration of Earth in De sphaera mundi. | |
General information | |
Unit system | astronomy, geophysics |
Unit of | distance |
Symbol | R_{?} or , |
Conversions | |
SI base unit | ^{[1]} |
Metric system | |
English units |
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. Earth radius is a term of art in astronomy and geophysics and a unit of measurement in both. It is symbolized as R_{?} in astronomy. In other contexts, it is denoted or sometimes .
Earth's radius can be defined in different ways because Earth is not a perfect sphere. The surface to which a radius extends is commonly chosen to be on an ellipsoid representing the shape of Earth. Like the surface, what point gets used for the center of Earth is also a matter of definition and therefore contributes to the diverse ways of defining Earth's radius.
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) gives three global average radii: the arithmetic mean of the radii of the ellipsoid (R_{1}); the authalic radius, which is of a sphere having the same surface area as the ellipsoid (R_{2}); and the volumetric radius, which is of a sphere having the same volume as the ellipsoid (R_{3}).^{[2]} All three of those radii are about 6,371 kilometres (3,959 mi).
Many other ways to define Earth radius have been described. 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:
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 many 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 ? is the angular frequency, G is the gravitational constant, and M is the mass of the planet.^{[e]} For the Earth ? 289, which is close to the measured inverse flattening ? 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. The gravity of the Moon and Sun cause the Earth's surface at a given point to undulate by tenths of meters 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 gained importance, true global models were developed which, while not as accurate for regional work, best approximate the Earth as a whole.
The following radii are fixed and do not include a variable location dependence. They are derived from the World Geodetic System 1984 (WGS-84) standard ellipsoid.^{[6]}
The value for the equatorial radius is defined to the nearest 0.1 m in WGS-84. The value for the polar radius in this section has been rounded to the nearest 0.1 m, which is expected to be adequate for most uses. Refer to the WGS-84 ellipsoid if a more precise value for its polar radius is needed.
The radii in this section are for an idealized surface. Even the idealized radii have an uncertainty of ±2 m.^{[7]} The discrepancy between the ellipsoid radius and the radius to a physical location may be significant. When identifying the position of an observable location, the use of more precise values for WGS-84 radii may not yield a corresponding improvement in accuracy.
The symbol given for the named radius is used in the formulae found in this article.
The Earth's equatorial radius a, or semi-major axis, is the distance from its center to the equator and equals 6,378.1370 km (3,963.1906 mi).^{[8]} The equatorial radius is often used to compare Earth with other planets.
The Earth's polar radius b, or semi-minor axis, is the distance from its center to the North and South Poles, and equals 6,356.7523 km (3,949.9028 mi).
The distance from the Earth's center to a point on the spheroid surface at geodetic latitude ? is:
where a and b are, respectively, the equatorial radius and the polar radius.
There are two principal radii of curvature: along the meridional and prime-vertical normal sections.
In particular, the Earth's radius of curvature in the (north-south) meridian at ? is:
This is the radius that Eratosthenes measured.
If one point had appeared due east of the other, one finds the approximate curvature in the east-west direction.^{[f]}
This radius of curvature in the prime vertical which is perpendicular (normal or orthogonal) to M at geodetic latitude ? is:^{[g]}
This radius is also called the transverse radius of curvature. At the equator, N = R.
The Earth's meridional radius of curvature at the equator equals the meridian's semi-latus rectum:
The Earth's polar radius of curvature is:
The Earth's radius of curvature along a course at an azimuth (measured clockwise from north) ? at ? is derived from Euler's curvature formula as follows:^{[10]}^{:97}
It is possible to combine the principal radii of curvature above in a non-directional manner.
The Earth's Gaussian radius of curvature at latitude ? is:^{[10]}
The Earth's mean radius of curvature at latitude ? is:^{[10]}^{:97}
The Earth can be modeled as a sphere in many ways. This section describes the common ways. The various radii derived here use the notation and dimensions noted above for the Earth as derived from the WGS-84 ellipsoid;^{[6]} namely,
A sphere being a gross approximation of the spheroid, which itself is an approximation of the geoid, units are given here in kilometers rather than the millimeter resolution appropriate for geodesy.
In geophysics, the International Union of Geodesy and Geophysics (IUGG) defines the mean radius (denoted R_{1}) to be^{[2]}
For Earth, the mean radius is 6,371.0088 km (3,958.7613 mi).^{[11]}
In astronomy, the International Astronomical Union denotes the nominal equatorial Earth radius as , which is defined to be 6,378.1 km (3,963.2 mi).^{[1]}^{:3} The nominal polar Earth radius is defined as = 6,356.8 km (3,949.9 mi). These values correspond to the zero tide radii. Equatorial radius is conventionally used as the nominal value unless the polar radius is explicitly required.^{[1]}^{:4}
Earth's authalic ("equal area") radius is the radius of a hypothetical perfect sphere that has the same surface area as the reference ellipsoid. The IUGG denotes the authalic radius as R_{2}.^{[2]}
A closed-form solution exists for a spheroid:^{[12]}
where e^{2} = and A is the surface area of the spheroid.
For the Earth, the authalic radius is 6,371.0072 km (3,958.7603 mi).^{[11]}
Another spherical model is defined by the volumetric radius, which is the radius of a sphere of volume equal to the ellipsoid. The IUGG denotes the volumetric radius as R_{3}.^{[2]}
For Earth, the volumetric radius equals 6,371.0008 km (3,958.7564 mi).^{[11]}
Another mean radius is the rectifying radius, giving a sphere with circumference equal to the perimeter of the ellipse described by any polar cross section of the ellipsoid. This requires an elliptic integral to find, given the polar and equatorial radii:
The rectifying radius is equivalent to the meridional mean, which is defined as the average value of M:^{[12]}
For integration limits of [0,], the integrals for rectifying radius and mean radius evaluate to the same result, which, for Earth, amounts to 6,367.4491 km (3,956.5494 mi).
The meridional mean is well approximated by the semicubic mean of the two axes,
which differs from the exact result by less than 1 ?m (4×10^{-5} in); the mean of the two axes,
about 6,367.445 km (3,956.547 mi), can also be used.
The mean curvature in all directions at all points on the surface is given by the weighted mean Gaussian curvature:
For the WGS 84 ellipsoid, the mean curvature equals 6,370.994 km (3,958.752 mi).
Most global mean radii are based on the reference ellipsoid, which approximates the geoid. The geoid has no direct relationship with surface topography, however. An alternative calculation averages elevations everywhere, resulting in a mean radius larger than the IUGG mean radius, the authalic radius, or the volumetric radius. This average is 6,371.230 km (3,958.899 mi) with uncertainty of 10 m (33 ft).^{[13]}
The best local spherical approximation to the ellipsoid in the vicinity of a given point is the osculating sphere. Its radius equals the Gaussian radius of curvature as above, and its radial direction coincides with the ellipsoid normal direction. The center of the osculating sphere is offset from the center of the ellipsoid, but is at the center of curvature for the given point on the ellipsoid surface. This concept aids the interpretation of terrestrial and planetary radio occultation refraction measurements and in some navigation and surveillance applications.^{[14]}^{[15]}
This table summarizes the accepted values of the Earth's radius.
Agency | Description | Value (in meters) | Ref |
---|---|---|---|
IAU | nominal "zero tide" equatorial | ^{[1]} | |
IAU | nominal "zero tide" polar | ^{[1]} | |
IUGG | equatorial radius | ^{[2]} | |
IUGG | semiminor axis (b) | ^{[2]} | |
IUGG | polar radius of curvature (c) | ^{[2]} | |
IUGG | mean radius (R_{1}) | ^{[2]} | |
IUGG | radius of sphere of same surface (R_{2}) | ^{[2]} | |
IUGG | radius of sphere of same volume (R_{3}) | ^{[2]} | |
IERS | WGS-84 ellipsoid, semi-major axis (a) | ^{[6]} | |
IERS | WGS-84 ellipsoid, semi-minor axis (b) | ^{[6]} | |
IERS | WGS-84 ellipsoid, polar radius of curvature (c) | ^{[6]} | |
IERS | WGS-84 ellipsoid, Mean radius of semi-axes (R_{1}) | ^{[6]} | |
IERS | WGS-84 ellipsoid, Radius of Sphere of Equal Area (R_{2}) | ^{[6]} | |
IERS | WGS-84 ellipsoid, Radius of Sphere of Equal Volume (R_{3}) | ^{[6]} | |
GRS 80 semi-major axis (a) | |||
GRS 80 semi-minor axis (b) | |||
Spherical Earth Approx. of Radius (R_{E}) | ^{[16]} | ||
meridional radius of curvature at the equator | |||
Maximum (the summit of Chimborazo) | ^{[9]} | ||
Minimum (the floor of the Arctic Ocean) | ^{[9]} | ||
Average distance from center to surface | ^{[13]} |
The first published reference to the Earth's size appeared around 350 BC, when Aristotle reported in his book On the Heavens^{[17]} that mathematicians had guessed the circumference of the Earth to be 400,000 stadia. Scholars have interpreted Aristotle's figure to be anywhere from highly accurate^{[18]} to almost double the true value.^{[19]} The first known scientific measurement and calculation of the circumference of the Earth was performed by Eratosthenes in about 240 BC. Estimates of the accuracy of Eratosthenes's measurement range from 0.5% to 17%.^{[20]} For both Aristotle and Eratosthenes, uncertainty in the accuracy of their estimates is due to modern uncertainty over which stadion length they meant.