The angle between the Earth’s equator and the ecliptic. It is the same as the Earth’s axial tilt. The mean obliquity, corresponding to the mean equator, is currently just over 23° 26′, but is decreasing slowly at a rate of 47.5″ per century because of planetary perturbations of the Earth’s orbit. The obliquity oscillates between limits of about 22° and 24°.5 with a mean period of some 41 000 years. In the current cycle it was at a maximum of 24°.2 some 9500 years ago and will reach a minimum of 22°.6 in another 10 200 years before starting to increase again. The true obliquity at any epoch is the sum of the mean obliquity and the nutation in obliquity (see Nutation).