Apparent retrograde motion

[1] The term retrograde is from the Latin word retrogradus – "backward-step", the affix retro- meaning "backwards" and gradus "step".

Retrograde is most commonly an adjective used to describe the path of a planet as it travels through the night sky, with respect to the zodiac, stars, and other bodies of the celestial canopy.

Ancient Greek astronomer Ptolemy in 150 AD believed that the Earth was the center of the Solar System and therefore used the terms retrograde and prograde to describe the movement of the planets in relation to the stars.

[3] This apparent retrogradation puzzled ancient astronomers, and was one reason they named these bodies 'planets' in the first place: 'Planet' comes from the Greek word for 'wanderer'.

In the geocentric model of the Solar System proposed by Apollonius in the third century BCE, retrograde motion was explained by having the planets travel in deferents and epicycles.

[4] It was not understood to be an illusion until the time of Copernicus, although the Greek astronomer Aristarchus in 240 BCE proposed a heliocentric model for the Solar System.

Since Neptune was only beginning its yearly retrograde cycle, the motion of the planet was far too slight to be detected with Galileo's small telescope.

In fact, the Moon orbits the Earth from west to east, as do the vast majority of manmade satellites such as the International Space Station.

All other planetary bodies in the Solar System also appear to periodically switch direction as they cross Earth's sky.

Asteroids and Kuiper Belt objects (including Pluto) exhibit apparent retrograde motion.

Then, as Earth swings past the planet in its orbit, it appears to resume its normal motion west to east.