Obliquity of the ecliptic
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The Obliquity of the ecliptic is the angle between the plane of the Earth's equator 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 23.4 degrees. (Another way of looking at this is to say that the Earth's axis is 23.4 degrees away from a line perpendicular to the plane of the ecliptic.) This inclination changes over time, due to the gravitational influence of the Sun, Moon, and other planets on the equatorial bulge of the earth.