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The retrograde motion of a hypothetical extremely distant (and nearly non-moving) planet would take place during a half-year, with the planet's apparent yearly motion being reduced to a parallax ellipse. The center of the retrograde motion occurs at the planet's opposition which is when the planet is exactly opposite the Sun.
In the Hipparchian, Ptolemaic, and Copernican systems of astronomy, the epicycle (from Ancient Greek ἐπίκυκλος (epíkuklos) 'upon the circle', meaning "circle moving on another circle") [1] was a geometric model used to explain the variations in speed and direction of the apparent motion of the Moon, Sun, and planets.
Retrograde motion in astronomy is, in general, orbital or rotational motion of an object in the direction opposite the rotation of its primary, that is, the central object (right figure). It may also describe other motions such as precession or nutation of an object's rotational axis .
This retrograde motion created the foundation for why these particular pathways became known as epicycles. [ 7 ] Ptolemy's unique contribution to this theory was the equant —a point about which the center of a planet's epicycle moved with uniform angular velocity, but which was offset from the center of its deferent.
Retrograde motion is an apparent change in the movement of a planet in the sky, but that doesn’t mean the planet is physically moving backward in its orbit. Retrogrades are visual illusions that ...
The geocentric model was eventually replaced by the heliocentric model. Copernican heliocentrism could remove Ptolemy's epicycles because the retrograde motion could be seen to be the result of the combination of the movements and speeds of Earth and planets. Copernicus felt strongly that equants were a violation of Aristotelian purity, and ...
Mercury retrograde is an astrological phenomenon where the planet Mercury appears to move backward in its orbit from Earth’s perspective. ... when Mercury goes into retrograde motion, any of ...
Between Hipparchus's model and Ptolemy's there was an intermediate model that was proposed to account for the motion of planets in general based on the observed motion of Mars. In this model, the deferent had a center that was also the equant, that could be moved along the deferent's line of symmetry in order to match to a planet's retrograde ...