What is the Heliocentric Universe Model?

The 16th and 17th centuries saw unparalleled learning and discoveries during the Scientific Revolution. The foundations of contemporary science were formed during this time, courtesy of achievements in physics, mathematics, chemistry, biology, and astronomy. In astronomy, the most significant academic was undoubtedly Nicolaus Copernicus, the man credited with developing the Heliocentric model of the cosmos.

Copernicus postulated a model of the cosmos in which the Earth, planets, and stars all circled around the sun based on continuous measurements of planet motions as well as prior conceptions from ancient antiquity and the Islamic World. In doing so, he eliminated the mathematical issues and contradictions that arose from the original geocentric model, laying the groundwork for contemporary astronomy.

While Copernicus was not the first to propose a solar system model in which the Earth and planets revolved around the sun, his heliocentric universe model was both novel and timely.

While Copernicus was not the first to propose a solar system model in which the Earth and planets revolved around the sun, his heliocentric universe model was both novel and timely. For one, it came at a time when European astronomers were struggling to resolve the mathematical and observational problems that arose out of the then-accepted Ptolemaic model of the universe, a geocentric model proposed in the 2nd century CE.

Furthermore, Ptolemy’s model was the first astronomical system to provide a complete and detailed account of how the universe functioned. Not only did his model overcome problems with the Ptolemaic system, but it also provided a simpler vision of the world that did away with the intricate mathematical devices required for the geocentric model to operate. And, over time, the model gained influential supporters who helped it become the accepted astronomical convention.

Since ancient times, the geocentric model, in which the Earth is the center of the universe and is surrounded by the sun and all the planets, has been the accepted cosmological model. By late antiquity, this concept had been codified by ancient Greek and Roman astronomers such as Aristotle (384-322 BCE), whose physics ideas established the foundation for planet motion, and Ptolemy (ca. 100 – ca. 170 CE), who gave the mathematical answers.

The geocentric concept was mainly based on two observations. To begin with, ancient astronomers saw the stars, sun, and planets to rotate around the Earth on a daily basis. Second, the Earth did not appear to move from the perspective of the Earth-bound observer, making it a fixed point in space.

This system encompassed the concept that the Earth was spherical, which had become an acknowledged truth by the 3rd century BCE. As a result, by Aristotle’s time, the geocentric model of the cosmos had evolved into one in which the Earth, sun, and all planets were spheres, and the sun, planets, and stars all traveled in perfect circular movements.

However, it wasn’t until the 2nd century BCE that Egyptian-Greek astronomer Claudius Ptolemaeus (aka. Ptolemy) published his treatise Amalgest that the details became standardized. Ptolemy maintained, drawing on millennia of astronomical traditions stretching from Babylonian to current times, that the Earth was at the center of the universe and the stars were all a reasonable distance away.

Each planet in this system is also propelled by a two-sphere system – a deferent and an epicycle. The deferent is a circle with a center point that is not on Earth, and it was employed to account for seasonal length changes. The epicycle is nested in the deferent sphere and functions as a “wheel within a wheel.” The epicycle was created to account for retrograde motion, which occurs when planets in the sky appear to slow down, move backward, and then forward again.

Unfortunately, these explanations did not account for all of the observed planet behaviors. The extent of a planet’s retrograde loop (particularly Mars) was sometimes lower and sometimes bigger than projected. Ptolemy devised the equant – a point near the center of a planet’s orbit – to solve the dilemma. A planet’s epicycle would always appear to move at a uniform speed to an observer stationed at this position, but it would appear to move at a non-uniform speed from all other locations.

While this system was the accepted cosmological model for over a thousand years in the Roman, Medieval European, and Islamic worlds, it was cumbersome by modern standards. According to the Ptolemaic model, each planet required an epicycle rotating on a deferent that was offset by an equant that was likewise unique to each planet.

It did, however, anticipate planetary motions with reasonable accuracy, and it was used to produce astrological and astronomical charts for the following 1500 years. This paradigm was progressively displaced by the heliocentric model of the cosmos, as advocated by Copernicus, then Galileo and Kepler, by the 16th century.

Nicolaus Copernicus began developing his version of the heliocentric model in the 16th century. Copernicus, like others before him, is based on the work of Greek astronomer Aristarchus, as well as paying tribute to the Maragha school and numerous famous Islamic scholars (see below). Copernicus’ theories were presented in a brief work named Commentariolus (“Little Commentary”) by the early 16th century.

Copernicus began distributing copies to his acquaintances, many of whom were fellow astronomers and philosophers, by 1514. The heliocentric theory, which was based on seven main principles, was outlined in this forty-page paper. According to these principles:

Celestial bodies do not all orbit around the same location.
The center of the Earth is also the center of the lunar sphere—the moon’s orbit around the Earth.
All spheres revolve around the sun, which is located at the center of the universe.
Because the distance between Earth and the sun is a negligible fraction of the distance between Earth and the stars, parallax is not observed in the stars.
The stars are immobile; their apparent daily motion is generated by Earth’s daily rotation.
The sun appears to migrate annually because Earth moves in a sphere around it. There are several motions on Earth.
The Earth’s orbital motion around the sun seems to reverse the direction of the planets’ movements.

After that, he kept compiling information for a more in-depth book, and by 1532, he was almost finished writing the manuscript for his most famous work, De revolutionibus orbium coelestium (On the Revolutions of the Heavenly Spheres). He presented his seven main points there, but in more depth and with thorough calculations to support them.

Copernicus was able to account for variations in the appearance of Mercury and Venus by positioning their orbits between the Earth and the sun. In brief, when they are on the other side of the sun from Earth, they seem smaller but fuller. They seem bigger and “horned” (crescent-shaped) when they are on the same side of the sun as the Earth.

It also explained the retrograde motion of planets like Mars and Jupiter by demonstrating that Earth astronomers had a shifting frame of reference rather than a fixed one. This explains why Mars and Jupiter appear to be much bigger at certain periods than others. In essence, when they are in opposition, they are far closer to Earth than when they are in conjunction.

However, due to concerns that publishing his theories would result in church condemnation (as well as, possibly, concerns that his theory contained some scientific flaws), he withheld his research until a year before his death. Only when he was near death in 1542 did he send his dissertation to Nuremberg to be published?

Impact of the Heliocentric Model:

Despite Copernicus’s fears that his arguments would be met with scorn and controversy, the publication of his theories elicited only mild condemnation from religious authorities. Many religious scholars attempted to refute his model over time. However, within a few generations, Copernicus’ theory became more widely accepted and gained many influential defenders.

These included Galileo Galilei (1564-1642), whose telescope-based investigations of the heavens allowed him to resolve flaws in the heliocentric model while also discovering aspects of the heavens that supported heliocentrism. Galileo, for example, found moons around Jupiter, sunspots, and faults on the moon’s surface, all of which tended to disprove the idea that the planets were flawless spheres rather than planets like Earth. While Galileo’s support for Copernicus’ theories led to his house arrest, others quickly followed.

With the invention of elliptical orbits, German mathematician and astronomer Johannes Kepler (1571-1630) also contributed to the refinement of the heliocentric model. Previously, the heliocentric model relied on circular orbits, which failed to explain why planets orbited the sun at different speeds at different times. Kepler answered this by demonstrating how the planets raced up at certain times in their orbits and slowed down at others.

Furthermore, Copernicus’ theory about the Earth’s ability to move would inspire a rethinking of the entire field of physics. Whereas previous concepts of motion relied on an outside force to initiate and sustain them (for example, the wind pushing a sail), Copernicus’ theories helped to inspire the concepts of gravity and inertia. Sir Isaac Newton, whose Principia created the foundation of modern physics and astronomy, would elucidate these notions.

Despite its gradual growth, the heliocentric paradigm finally displaced the geocentric approach. In the end, its debut had nothing short of a revolutionary influence. Humanity’s knowledge of the cosmos and our position in it would never be the same again.