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Axial precession is the trend in the direction of the Earth's axis of rotation relative to the fixed stars, with a period of about 25,700 years. Also known as the precession of the equinoxes, this motion means that eventually Polaris will no longer be the north pole star.
Apsidal precession is considered positive when the orbit's axis rotates in the same direction as the orbital motion. An apsidal period is the time interval required for an orbit to precess through 360°, [2] which takes the Earth about 112,000 years and the Moon about 8.85 years. [3]
Precession of the equinoxes, perihelion precession, changes in the tilt of Earth's axis to its orbit, and the eccentricity of its orbit over tens of thousands of years are all important parts of the astronomical theory of ice ages. (See Milankovitch cycles.)
The rate of precession depends on the inclination of the orbital plane to the equatorial plane, as well as the orbital eccentricity. For a satellite in a prograde orbit around Earth, the precession is westward (nodal regression), that is, the node and satellite move in opposite directions. [1] A good approximation of the precession rate is
Apsidal precession, where the major axis of an elliptical orbit cycles its orientation within its orbital plane. Nodal precession, where non-spherical objects cause orbiting objects to change their orbits. Not to be confused with: Axial precession, where the rotation axis of a spinning object traces a conical path like a top.
This gives an average cycle length now of 25,676 years. Precession will be greater than p + by the small amount of +0.135052"/a between +30 Ma and +130 Ma. The jump to this excess over p + will occur in only 20 Ma beginning now because the secular decrease in precession is beginning to cross a resonance in Earth's orbit caused by the other planets.
An orbit will be Sun-synchronous when the precession rate ρ = dΩ / dt equals the mean motion of the Earth about the Sun n E, which is 360° per sidereal year (1.990 968 71 × 10 −7 rad/s), so we must set n E = ΔΩ E / T E = ρ = ΔΩ / T , where T E is the Earth orbital period, while T is the period of the spacecraft ...
Hipparchus (c. 120 BC) is the first Greek credited with discovering axial precession roughly two hundred years after Plato's death (see below). Cicero (1st century BC) followed Plato in defining the Great Year as a combination of solar, lunar and planetary cycles. [6] [7] Plato's description of the perfect year is found in his dialogue Timaeus: