Flattening

Under flattening refers to the deformation of a planet or other celestial body by its rotation.

The geometric flattening of ellipsoid body is the relative difference of the radius at the equator and at the pole.

The geometrical flattening is defined by

Where b is the polar radius and a is the equatorial radius.

One also speaks of flattening irregular at a body, such as when its meridian section is no ellipse but a so-called spheroid. This has a significant anomalies in the gravity field, which cause also perturbations of satellites or moons.

The flattening is caused by the centrifugal force of the rotation, which yields as a plastic body of the heavenly bodies. It decreases with decreasing rotation time, but is also dependent on the density of the body.

For Jupiter and Saturn, it is due to the rapid rotation of about 10 hours already in the small telescope clearly visible ( 1:15 or 1:10). Even in the earth, the diameter from pole to pole is less than about 42 km at the equator ( flattening 1:298 ), which is not yet visible but from space with the naked eye.

The sun is because of their long rotation period (about 1 month) almost spherical. Larger or very rapidly rotating stars ( red giants, pulsars, etc.), however, are strongly flattened. The strongest known oblate object is Achernar, which is flattened by its rapid rotation to the limit of the theoretically possible.

In our galaxy ( Milky Way ), a similar effect has performed since its formation to form a lens of about 1:4, although their revolution takes about 250 million years.