The heliocentric model and its influence on the current astronomical model

The heliocentric model is an astronomical model that puts the Sun at the center of the universe. This is opposite to the geocentric model, which puts the Earth at the center of the universe. 

But what does being at the center of the universe mean? The heliocentric model implies that all the other planets, stars, etc., describe an orbit around the Sun. In the geocentric model, all these celestial bodies revolve around the Earth. 

The heliocentric model was introduced in the 16th century by Polish astronomer and mathematician Nicolaus Copernicus. The paradigm shift from geocentrism to heliocentrism is called the Copernican Revolution.

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Who proposed the heliocentric theory?

Although Nicolaus Copernicus is credited with the proposal of the heliocentric theory, he admitted to being influenced by the work of ancient Greek astronomer and mathematician Aristarchus of Samos. 

Aristarchus developed an early heliocentric theory of the universe around 250 B.C.E., after the influence of Philolaus of Croton, who wrote about a fire at the center of the universe. Most of Aristarchus’s work has not survived, but his ideas are known from references by other Greek philosophers, including Archimedes, Plutarch, and Sextus Empiricus.

Aristarchus identified the central fire with the Sun and built a model where all the planets in the solar system described an orbit around it. He also (more or less) correctly calculated the order and the distance to the Sun of the known planets.

In contrast, most ancient Greek philosophers held that the heavens were arranged in perfectly round circles around the Earth. One problem with this theory was that the planets sometimes appeared to stop in their orbit of Earth and move retrograde across the sky. 

In the second century A.D., the Alexandrian geographer and astronomer Ptolemy proposed a solution to this problem by arguing that the sun, planets, and moon all move in small circles, called epicycles, around much larger circles, called deferents, that revolve around the Earth. By proposing that the different epicycles rotate at different speeds, the Ptolemaic model could conform to existing observations.

The geocentric model proposed by Aristotle and Ptolomy overshadowed Aristarchus' astronomical ideas in the West until Copernicus revived them in the 1500s. In Copernicus’ time, astronomers were increasingly struggling with mathematical and observational inconsistencies in the Ptolemaic system and on the order of the planets from Earth.

Copernicus' theory had several issues. One of which was the question of why objects would fall to the ground if the Earth was not the center of the universe. Another was that he believed that the planets moved in circular orbits, even though evidence showed otherwise. Because of these issues, Copernicus delayed the publication of his major work, De revolutionibus orbium coelestium libri vi (Six Books Concerning the Revolutions of the Heavenly Orbs) until the year of his death - 1543.

It was not until the early 17th century that Galileo and Johannes Kepler would further develop and prove Copernicus's theory. 

Formation of the heliocentric theory

Thanks to the invention of the telescope in 1608, Galileo could make astronomical observations that validated the heliocentric theory developed by Copernicus. 

In 1609 Galileo observed that the Moon was rough and cratered, which contradicted Aristotle’s notion that all heavenly bodies were perfect spheres. He also observed that Venus goes through phases, the nature of which could only be explained if Venus orbited the Sun, not the Earth. In 1610, Galileo's discovery that Jupiter also had moons proved that it was possible for celestial bodies to orbit objects other than the Earth.

In 1610, he published his findings in a short pamphlet in New Latin called Sidereus Nuncius (The Starry Messenger), where he defended Copernicus’ ideas. The pamphlet made Galileo a celebrity. He was appointed a mathematician and philosopher to the ruling Medicis.

The problem was that the heliocentric model contradicted the geocentric description of the universe accepted by most scientists and treated as dogma by the Catholic Church. Copernicus hadn’t suffered any consequences because he died shortly after the publication of his magnum opus, De revolutionibus orbium coelestium (1543). Although Galileo was not the only one making these discoveries, his work had spread further.

In 1616, the Catholic Church banned Copernicus’s De Revolutionibus, and Pope Paul V summoned Galileo to Rome to warn him that he could no longer publicly support Copernicus and heliocentrism. 

However, in 1632, Galileo published a work presenting hypothetical arguments both for and against heliocentrism called “Dialogue Concerning the Two Chief World Systems.” The supposed balance of the pamphlet did not fool the Church and angered the Pope, who felt Galileo had gone back on his word not to promote Copernican theory.

Galileo was summoned to appear before the Roman Inquisition in 1633 and was eventually convicted of “vehement suspicion of heresy,” forced to recant his theories, and placed under house arrest for the last nine years of his life. While under house arrest, he wrote a summary of his early motion experiments that became his outstanding scientific work, Discourses and Mathematical Demonstrations Concerning Two New Sciences.

By that time, the Church had already banned several heliocentric books in the Index Librorum Prohibitorum (List of Prohibited Books). The ban was lifted in 1758, except for Copernicus’ and Galileo’s works. These didn’t disappear from the Index until its 1835 edition. 

By then, the heliocentric model had become the accepted model, mainly thanks to Johannes Kepler’s Epitome of Copernican Astronomy (1618-1621) and Newton’s theoretical foundation of Keppler's laws of planetary motion, and through his laws on motion and universal gravitation laws published in Philosophiæ Naturalis Principia Mathematica (1687).  

Principles of the heliocentric model

Copernicus's Commentariolus, a forty-page manuscript written in 1514, sets out the seven main principles of his heliocentric model:

1. Celestial bodies do not all orbit around a particular point.
   
   
2. The center of Earth is the center of the moon's orbit around the Earth.
   
   
3. All the spherical bodies rotate around the Sun (Which is near the center of the universe).
   
   
4. The distance between the Earth and the Sun is a minor fraction of the distance from the Earth and the Sun to the stars. Thus, parallax is not perceptible in the stars.
   
   
5. The stars do not really move. The Earth does.
   
   
6. The Earth describes an orbit around the Sun, causing the apparent annual migration of the Sun. The Earth has more than one kind of motion.
   
   
7. The Earth’s orbital motion around the Sun causes the seeming reverse in direction of the motions of the planets.
   

Difference between geocentric and heliocentric models

As explained above, the main difference between geocentric and heliocentric models is that the first one puts the Earth at the center of the universe and the second one puts the Sun at the center of the universe.

This implies that in the geocentric model, all astronomical objects rotate around the Earth. In the heliocentric model, planets, natural satellites (such as the Moon), stars, etc., revolve around the Sun. It also implies that the Earth spins over its own axis, which causes the illusion of movement in the stars.

There is also a difference in the kind of motion that each model describes. The geocentric model state that celestial bodies move in a circular motion around the Earth, while the heliocentric model stipulates that celestial bodies move in an elliptical motion. 

Johannes Kepler is the astronomer who is responsible for introducing the idea of elliptical orbits through his three laws of planetary motion:

1. The orbit of a planet is an ellipse with the Sun at one of the two foci.
2. A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time.
3. The square of a planet's orbital period is proportional to the cube of the length of the semi-major axis of its orbit.

Before Kepler’s intervention, heliocentric models used the same circular orbits that are described in the geocentric model, but these circular orbits couldn’t explain why planets revolved around the Sun at different velocities at different times. 

Current astronomical model

In the early 20th century, heliocentrism was replaced by galactocentrism, which put the Milky Way at the center of the universe. 

The galactocentric model was mainly developed by American astronomers Harlow Shapley and Heber Doust Curtis. Shapley demonstrated that the stars in the Milky Way revolved around a galactic center near the Sagittarius constellation, and Curtis argued that there were other galaxies in the universe besides the Milky Way. 

In 1925, Edwin Hubble confirmed that Andromeda and other “clouds of gas and cosmic dust” previously classified as nebulae were actually other galaxies. Then, in 1929, Hubble developed what is now known as Hubble’s Law, which states that galaxies are moving away from Earth at a velocity proportional to their distance from it. This observation supports the Big Bang cosmological model of an acentric universe that is in constant expansion.

All of this eventually led to the current acentric model called Lambda-Cold Dark Matter, which builds on the Big Bang Theory to include the existence of dark matter.