Definitions for "Obliquity of the ecliptic"
The amount of the tilt in Earth's axis (23.5°), which determines the angle the ecliptic makes with the celestial equator as they intersect in the sky.
The angle between the plane of the ecliptic (the plane of the earth's orbit) and the plane of the celestial equator. The obliquity of the ecliptic is computed from the following formula: 23 degrees 27 minutes 08.26 seconds - 0.4684 (t - 1900) seconds, where t is the year for which the obliquity is desired.
The angle which the ecliptic makes with the plane of the Earth's Equator. Its value is approximately 23.45°.