Retrograde motion

Retrograde motion is the apparent westward reversal of a planet’s usual eastward drift against the background stars, caused by changing viewing geometry as Earth and the planet orbit the Sun.

Apparent motion: A change in an object's observed position that may result from motion by the object, the observer, or both. Retrograde motion is an apparent shift produced by the relative motion of Earth and another planet.
Ecliptic: The apparent annual path of the Sun across the celestial sphere, corresponding to Earth's orbital plane. Planets' usual eastward drift and retrograde loops are tracked against this sky reference.
Ancient Greek astronomy: The study of the heavens in the Greek-speaking world from antiquity through late antiquity. Greek astronomers developed influential geometric accounts of planetary reversals.
Mars: The fourth planet from the Sun, with a reddish appearance caused largely by iron minerals on its surface. Its conspicuous retrograde loop near opposition was a central puzzle for early planetary astronomy.
Geocentric model: A model of the cosmos that places Earth at its center and describes celestial bodies as moving around it. In geocentric accounts, planetary reversals required added motions rather than Earth's changing viewpoint.
Celestial sphere: An imagined sphere centered on an observer, onto which celestial positions are projected. Retrograde motion describes a change in a planet's position on this apparent sky.
Eudoxus of Cnidus: A fourth-century BCE Greek mathematician and astronomer who modeled celestial motions with nested spheres. His concentric-sphere system was an early attempt to represent complex planetary paths.
Mercury: The smallest planet in the Solar System and the one closest to the Sun. As an inferior planet, it reverses apparent direction around its passages between Earth and the Sun.
Heliocentric model: A model that places the Sun near the center of the planetary system, with planets orbiting it. Earth's orbit around the Sun provides the changing viewpoint that produces apparent retrograde loops.
Orbital period: The time an object takes to complete one orbit around another body. Different orbital periods make Earth periodically overtake outer planets and be overtaken by inner ones.

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