Apsidal precession

In celestial mechanics, apsidal precession (or apsidal advance)[1] is the precession (gradual rotation) of the line connecting the apsides (line of apsides) of an astronomical body's orbit. The apsides are the orbital points closest (periapsis) and farthest (apoapsis) from its primary body. The apsidal precession is the first time derivative of the argument of periapsis, one of the six main orbital elements of an orbit. 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]

Each planet orbiting the Sun follows an elliptic orbit that gradually rotates over time (apsidal precession). This figure illustrates positive apsidal precession (advance of the perihelion), with the orbital axis turning in the same direction as the planet's orbital motion. The eccentricity of this ellipse and the precession rate of the orbit are exaggerated for visualization. Most orbits in the Solar System have a much lower eccentricity and precess at a much slower rate, making them nearly circular and stationary.
The main orbital elements (or parameters). The line of apsides is shown in blue, and denoted by ω. The apsidal precession is the rate of change of ω through time, dω/dt.
Animation of Moon's orbit around Earth - Polar view
  Moon ·   Earth

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