Epicycles and Errors: The Story of the Geocentric Model

Figure 1. Ancient Greek Debate

It begins, as so much else does, with the ancient Greeks and their efforts to explain what they saw in the sky.  The Ancient Greeks were among the first to attempt to scientifically assess the nightly motion of the celestial objects, in part because of a culture that encouraged discussion, disagreement and heated debate! 

Their observations were based on what they could see with their naked eyes – approximately 2000 stars, the Milky Way, Earth’s moon and 5 of the planets – Mercury, Venus, Mars, Jupiter, and Saturn.

Over the course of a night, stars slowly rotate westward across the sky. Over 24 hours, the Sun also moves westward across the sky. Over the course of a year, the sun and other planets move eastward relative to the constellations.
However, the planets observed by the ancient Greeks appeared to move differently over the course of a year. Planets often changed in brightness and sometimes appeared to reverse their direction of motion. Indeed, the word “Planet” is derived from the Greek word for wanderer!

Figure 2. A Geocentric Model

Figure 3. Ancient Celestial Spheres

Greek philosophers primarily believed in a geocentric model of the universe - everything seen in the sky was orbiting an Earth that was fixed in place.  One of the first models of the universe was of the ancient celestial sphere, which explained the nightly motion of the stars. Proposed in about 600 BCE that stars were attached to a spinning celestial sphere, with earth at the center.

The motion of the sun, moon, and planets was explained by placing them on different, offset spheres.  

Figure 4. Retrograde Mars

However, while Greek philosophers were able to explain the motion of stars by using this geocentric model, they were unable to explain the movement of planets as they seemingly reversed their direction, only to reverse back again. This is known as the apparent retrograde motion of planets.  

 Claudius Ptolemy (90-168 C.E.) explained apparent retrograde motion by proposing a geocentric model with the concept of “Epicycles” – mini orbits that planets would follow even as they orbited the Earth.

Figure 5. Claudius Ptolemy

Figure 6. Ptolemy's Epicycles

The idea of Ptolemy’s Epicycles persisted for hundreds of years but by 1500 C.E., astronomers began to his revise his model. The idea of epicycles seemed too complicated and couldn’t easily be proven, which as we know is a disadvantage in science! To better explain the retrograde motion of planets, astronomers began to turn to the heliocentric model, in which the Earth and planets revolve around the sun. Ancient Greek astronomers had considered heliocentric models (indeed, sun-centered models were considered by the Greeks as early as 300 B.C.E.) but dismissed them because of various reasons.

The 150-year period in which the modern heliocentric model began to develop was known as the Copernican Revolution, named after Nicolaus Copernicus (1473 – 1543 C.E.) who proposed a heliocentric model to explain retrograde motion without the need for epicycles.

Figure 7. Planetary Motion Explained!

Other key figures in the Copernican revolution included Johannes Kepler (1571-1630 C.E.), Galileo Galilei (1564- 1642 C.E.), and Isaac Newton (1642-1727 C.E.). Each of these figures refined the heliocentric model.

But how do we actually explain retrograde motion? 

Apparent retrograde motion actually isn’t retrograde motion at all! Today we know that more distant planets orbit the Sun at slower average speeds, meaning that “retrograde motion” is no more than the Earth moving at a different speed than other planets! When planets switch directions in their orbit in respect to fixed background stars (appearing to move from east to west, rather than west to east), what is actually happening is an optical illusion. The Earth makes a complete orbit around the sun in 1 year, while planets father away take a longer time to complete a full orbit. As Earth pulls up to and then passes a planet like Mars in orbit, Mars falls behind and appears to be switching directions, although its orbit actually remains the same. Of course, this explanation of apparent retrograde motion is dependent on both the Earth and Mars orbiting the Sun at different speeds and thus a heliocentric model is necessary.



Images Used: 

http://cf.mp-cdn.net/54/aa/efa2e24344fe58ec66a9243cc6be-in-ancient-greece-debate-played-a-significant-role-in-citizenship-is-this-true-of-american-society.jpg

https://en.wikipedia.org/wiki/Geocentric_model#/media/File:Bartolomeu_Velho_1568.jpg

https://en.wikipedia.org/wiki/Celestial_spheres
http://www.sciography.com/images/ptolemy.gif
http://apod.nasa.gov/apod/ap100613.html

https://en.wikipedia.org/wiki/Deferent_and_epicycle

http://www.splung.com/content/sid/7/page/earlymodels