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 23.5 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 changes over time, due to the gravitational influence of the Sun, Moon, and other planets on the equatorial bulge of the earth. Earth, also known as Terra, and (mostly in the 19th century) Tellus, is the third-closest planet to the Sun. ... The plane of the ecliptic is well seen in this picture from the 1994 lunar prospecting Clementine spacecraft. ... The Sun (or Sol) is the star at the center of our Solar system. ... Crust composition Oxygen 43% Silicon 21% Aluminium 10% Calcium 9% Iron 9% Magnesium 5% Titanium 2% Nickel 0. ... A planet is generally considered to be a relatively large mass of accreted matter in orbit around a star that is not a star itself. ...

The periodicity of the obliquity of the ecliptic is approximately 40,000 years. It is also called the "tilt cycle" and changes the length of the seasons.