Tilt of the Earth's polar axis as measured from the perpendicular to the plane of the Earth's orbit around the sun. The angle of this tilt varies from 22.5 to 24.5° over a 41,000 year period. Current obliquity is 23.5°.
in general the angle between the equatorial and orbital planes of a body or, equivalently, between the rotational and orbital poles. For the Earth the obliquity of the ecliptic is the angle between the planes of the equator and the ecliptic.
of the Ecliptic: The difference between the plane of the earth's equator and the plane of her orbit. These have a mean value of 23°26'40".16 in 1960 and diminishes 0".48 per year. (See Observations Using the Gnomon)