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Flattening is a measure of the compression of a circle or sphere along a diameter to form an ellipse or an ellipsoid of revolution (spheroid) respectively. Other terms used are ellipticity, or oblateness. The usual notation for flattening is f and its definition in terms of the semi-axes of the resulting ellipse or ellipsoid is
The compression factor is b/a in each case. For the ellipse, this factor is also the aspect ratio of the ellipse.
There are two other variants of flattening (see below) and when it is necessary to avoid confusion the above flattening is called the first flattening. The following definitions may be found in standard texts ^{[1]}^{[2]}^{[3]} and online web texts^{[4]}^{[5]}
In the following, a is the larger dimension (e.g. semimajor axis), whereas b is the smaller (semiminor axis). All flattenings are zero for a circle (a=b).
The flattenings are related to other parameters of the ellipse. For example:
where e is the eccentricity.
For the Earth modelled by the WGS84 ellipsoid the defining values are^{[7]}
from which one derives
so that the difference of the major and minor semi-axes is 21.385 km (13 mi). (This is only 0.335% of the major axis so a representation of the Earth on a computer screen could be sized as 300px by 299px. Because this would be virtually indistinguishable from a sphere shown as 300px by 300px, illustrations typically greatly exaggerate the flattening in cases where the image needs to represent the oblateness of the Earth.)
Other values in the Solar System are Jupiter, f=1/16; Saturn, f= 1/10, the Moon f= 1/900. The flattening of the Sun is less than 1/1000.
In 1687 Isaac Newton published the Principia in which he included a proof that a rotating self-gravitating fluid body in equilibrium takes the form of an oblate ellipsoid of revolution (a spheroid).^{[8]} The amount of flattening depends on the density and the balance of gravitational force and centrifugal force.
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