The Geocentric Model of the Universe

3 minutes ago4 min read

By Shubham Pant

Introduction

For someone to propose today that the Earth is flat like a pancake would likely inspire incredulity. Yet if you were to take a trip back to ancient-day Greece, you would be spoiled for choice by philosophers fuelled by this belief. Pythagoras was the first to introduce the concept of a spherical Earth at the turn of the 5th Century BCE. His idea, however, was primarily inspired by aesthetics rather than tangible physical evidence. Aristotle later improved upon his visualization, providing several arguments for a spherical Earth, such as how if you’ve ever had the opportunity to observe a ship at sea, you would have noticed that they disappear hull first while sailing over the horizon, or how the visibility of varying constellations at different latitudes changes as a result of the Earth’s round shape.

Over a hundred years later, Eudoxus of Cnidus (c. 390–337 BCE), an ancient Greek mathematician and astronomer, presented the first mathematical theory of the universe. He stipulated that it was this spherical Earth that featured at the center of the universe, encompassed by 27 concentric spheres.

Why did Greek philosophers assume a stationary Earth?

If we take a walk in the shoes of the Greeks, it’s evident how this notion arose. If we had not been taught about science and the solar system, then as a stationary observer on Earth, it seems pretty obvious that it is indeed the sky around us that shifts while we lay stranded on our immobile rocky home.

Geocentric theorists were also affirmed by another persuasive argument that concerns itself with parallax. Let’s first delve into what parallax is. Parallax is a term used to refer to the apparent shift in the position of an object, due to a change in the position of observation. An easy way to understand this is to hold your thumb out in front of you. Start by looking at your thumb with your right eye shut. Make a mental note of where your thumb aligns with any object in the background. Now close your left eye, and peer at it with your right. You’ll notice a shift in the position of your thumb, i.e. apparent movement. As our eyes occupy different positions, we observe parallax in the position of our thumb. The ancient Greeks theorized that if the Earth were indeed moving around the sun, then contrary to our current observations, we would note a shift in the position of stars. Fast forward to the present day, we now know that the stars do in fact shift, they’re just too far away for it to be noticeable.

In Eudoxus’ model, the Sun, the moon, the planets and the fixed stars were given spheres, where each was attached to a larger sphere through a pole. Every 24 hours, the spheres rotate on their poles, which account for the daily rotation of our planet.

In the 4th century BCE, Aristotle, a Greek philosopher and astronomer, expanded upon Eudoxus’ ideas. Aristotle believed the universe existed eternally in time but occupied a finite amount of space. He retained the geocentric model of the universe, with the Sun, moon, planets and stars orbiting the Earth from within Eudoxus’ spheres. His conception of the universe unfolded in the form of 53 concentric, crystalline and transparent spheres, wherein stars were affixed to the outermost sphere. These spheres were all centered on the Earth, which allowed them to be simultaneously geocentric and heliocentric.

The Eudoxan–Aristotelian model was based on a few fundamental principles:

The center of the universe is the stationary Earth
The sun, stars and planets revolve around Earth.
As objects are attached to spherical shells, all motions in the sky follow circular paths
Objects orbited Earth at the same speed.

Enter Ptolemy in around 150 CE. He noted that the movement of planets around the sun, when viewed from the Earth, did not appear in the form of perfect circles. In fact, some planets like Mars even seemed to move backward before moving forward. He sought to resolve this contradiction, by introducing his concept of irregular orbits. Ptolemy suggested that planets like Mars moved in a combination of several regular circular motions, as viewed from the Earth. These circular motions were termed epicycles and were first introduced by Apollonius.

Conclusion

Ptolemy’s work was revolutionary at the time, with his model allowing the prediction of the position of a planet within 2°, and it could account for the observed planetary motions, retrograde motion and variations in brightness. His model of the universe went on to retain its influence until it was challenged by Copernicus in the mid-1500s.