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Axial tilt of eight planets and two dwarf planets, Ceres and Pluto. All four of the innermost, rocky planets of the Solar System may have had large variations of their obliquity in the past. Since obliquity is the angle between the axis of rotation and the direction perpendicular to the orbital plane, it changes as the orbital plane changes due ...
Earth's rotation axis moves with respect to the fixed stars (inertial space); the components of this motion are precession and nutation. It also moves with respect to Earth's crust; this is called polar motion. Precession is a rotation of Earth's rotation axis, caused primarily by external torques from the gravity of the Sun, Moon and other bodies.
In addition, the rotational tilt of the Earth (its obliquity) changes slightly. A greater tilt makes the seasons more extreme. Finally, the direction in the fixed stars pointed to by the Earth's axis changes (axial precession), while the Earth's elliptical orbit around the Sun rotates (apsidal precession).
The image shows the relations between Earth's axial tilt (or obliquity), rotation axis, and orbital plane. The celestial equator is the great circle of the imaginary celestial sphere on the same plane as the equator of Earth. By extension, it is also a plane of reference in the equatorial coordinate system.
The rotation axis of the Earth describes, over a period of 25,700 years, a small blue circle among the stars near the top of the diagram, centered on the ecliptic north pole (the blue letter E) and with an angular radius of about 23.4°, an angle known as the obliquity of the ecliptic. The direction of precession is opposite to the daily ...
The tilt of the Earth's polar axis remains constant but describes a circular path in space during a period known as The Great Year. The term Great Year has more than one major meaning. It is defined by scientific astronomy as "The period of one complete cycle of the equinoxes around the ecliptic , or about 25,800 years".
In 1966, Peter Goldreich published a classic paper on the evolution of the Moon's orbit and on the orbits of other moons in the Solar System. [8] He showed that, for each planet, there is a distance such that moons closer to the planet than that distance maintain an almost constant orbital inclination with respect to the planet's equator (with ...
Unlike the stars, the Sun and Moon do not have a fixed declination. Since Earth's rotational axis is tilted by about 23.5° with respect to a line perpendicular to its orbital plane (the ecliptic), the Sun's declination ranges from +23.5° at the June solstice to −23.5° at the December solstice, as the Earth orbits the Sun once every ...