The Antarctic Circle's latitude depends on Earth's axial tilt which fluctuates within a margin of 2 degrees over a 40,000 year period.
 The normal at a point on the surface of an ellipsoid does not pass through the centre, except for points on the equator or at the poles, but the definition of latitude remains unchanged as the angle between the normal and the equatorial plane
=== Parametric latitude (or reduced latitude) === The parametric latitude or reduced latitude, , is defined by the radius drawn from the centre of the ellipsoid to that point Q on the surrounding sphere (of radius a) which is the projection parallel to the Earth's axis of a point P on the ellipsoid at latitude 

