Also called the obliquity of the ecliptic, this term is used to denote the tilt of the earth's axis with respect to the plane of the earth's orbit. This is one of the three main orbital perturbations (the other two being eccentricity and precession) involved in the Milankovitch theory and as such varies from about 22 to 25°. at a period of about 41,000 years. Obliquity perturbations tend to amplify the seasonal cycle in the high latitudes of both hemispheres simultaneously, with the effect small in the tropics and maximum at the poles.