Obliquity of the ecliptic
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The Obliquity of the ecliptic is the angle between the axis of rotation of the Earth and the ecliptic plane in which the Earth rotates around the Sun. The "obliquity" refers to the fact that this is an oblique angle, i.e. not a right angle or multiple thereof. The actual angle at the present phase of the Earth's history is about 66.5 degrees. (Another way of looking at this is to say that the Earth's axis is 23.5 degrees away from a line perpendicular to the plane of the ecliptic.) This inclination of the Earth's axis with respect to its orbital plane changes over time, due to the gravitational influence of the Sun, Moon, and other planets.
