Heliocentrism

Heliocentrism is an astronomical model that presents the Sun as the center of the solar system with Earth and the rest of the planets revolving around the Sun. Although heliocentrism was first proposed as the model of the solar system in ancient Greece, geocentrism was the generally accepted theory of the solar system's structure for much of human history.

Geocentrism placed Earth, rather than the Sun, at the center of the solar system and indeed the universe. Polish astronomer Nicolaus Copernicus devised the first early modern heliocentric model of the solar system in the early sixteenth century. One hundred years later, the Roman Catholic Church notably forced Italian astronomer Galileo Galilei to recant his support for heliocentrism, as it conflicted with the church's geocentric view. The global scientific community accepts heliocentrism as fact in the modern era.

Background

Elements of heliocentrism first appeared as astronomical theories in ancient Greece. In the fourth century BCE, the astronomer Heraclides of Pontus proposed that the appearances of regular "movements" of celestial bodies across the sky could in fact be due to the daily spinning of Earth on an imaginary axis. Heraclides's theory was roundly rejected at the time, since reality appeared to suggest to most people that it was the planet Earth, not the heavenly bodies, that was immobile in the universe.

Still, astronomers continued to wonder about the true nature of the universe throughout ancient times. In the third century BCE, the Greek astronomer Aristarchus of Samos became the first known individual to propose a heliocentric model of the solar system (referred to generally as the entire universe at the time). Aristarchus suggested that the Sun was stationary at the center of the universe. He also argued that Earth spun daily on its axis to produce day and night and revolved around the Sun every year. These revolutions explained the Sun's yearlong "journey" to different positions across the sky throughout a given year.

Like the Earth-axis proposal of Heraclides, this heliocentric theory was widely dismissed by Aristarchus's contemporaries. To them, the idea simply did not align with apparent reality and therefore did not make sense. Critics of the theory wondered how Earth could both spin on an axis and revolve around the Sun without causing all objects on the planet to be sent flying through the air. Furthermore, scientists argued that a moving Earth would violate the philosopher Aristotle's physics of natural motion, which claimed that all matter on Earth always oriented itself to the center of the universe. Any of this material that was displaced—by the movement of Earth through the cosmos, for example—would simply leave Earth and quickly return to the universe's center. To ancient people, Aristarchus's heliocentric model of the universe violated generally known laws of physics and was consequently not to be believed.

Overview

Heliocentrism did not return to mainstream astronomy until the early sixteenth century CE. At this time, the Polish astronomer and mathematician Nicolaus Copernicus began researching universal models, intending to determine finally whether the Sun or Earth was the center of the known universe. In Copernicus's time, most people still held a geocentric view, as Earth truly appeared to humans to be stationary while the Sun and planets revolved around it.

After about a decade of naked-eye observations and mathematical calculations, Copernicus recorded elements of his heliocentric theory in the unrefined work Little Commentary (Commentariolus) in 1514. In it, he claimed the center of the universe was near the Sun, not Earth. Additionally, Earth's axis rotation explained why the stars seemed to change positions in the sky daily, while Earth's revolutions around the Sun accounted for why the Sun appeared to traverse a path through the stars every year. These revolutions also explained the regular retrograde, or backward, movements of the planets in the sky, as these bodies simply appeared in different places relative to human observers as Earth passed them while revolving.

Copernicus continued studying astronomy for another three decades. He published his heliocentric theory more fully in the 1543 book On the Revolutions of the Heavenly Spheres. He died shortly thereafter. Copernicus had waited until his later life to publish the theory because he was never fully satisfied with his work on it. Additionally, as a Catholic, he did not believe heliocentrism opposed the Bible's claims about how the universe was organized. The church, however, believed the idea of a Sun-centered universe was heretical, or opposed to the generally accepted belief about Earth being the center of the universe. The church banned On the Revolutions of the Heavenly Spheres in 1616.

Around this time, the Italian astronomer Galileo Galilei found himself at odds with the Catholic Church for agreeing with the Copernican view of the universe. Galileo had built his own telescope and observed some of the revolving moons of Jupiter. His published works about this observation and others challenged the commonly held geocentric view of the time. In 1616, Pope Paul V banned Galileo from publicly supporting Copernicus. Galileo continued writing about heliocentrism, however, and in 1633, during the Roman Inquisition, the church threatened him with torture if he did not retract his support for the idea. Galileo did this and was then placed under house arrest for the rest of his life. He died in 1642, still privately believing Earth revolved around the Sun.

Heliocentrism is regarded as an unquestioned fact in the twenty-first century. The Sun is the medium-sized star at the center of the solar system. Eight planets orbit the Sun in elliptical, or oval, paths due to the star's gravitational pull. Each planet is located at a different distance from the Sun and so has its own set orbit that it completes at a different rate than every other planet. This is the reason that people can observe the retrograde motion of the planets that Copernicus discussed in Little Commentary. For instance, from Earth, Mars will appear to reverse its motion through the night skies during the period when the two planets are passing each other in orbit.

Earth is at the exact distance from the Sun to benefit optimally from the star's heat and light, without which life on Earth would not be possible. The constant spinning of Earth on its axis makes the Sun appear to move across the sky during the day and then disappear to produce night. Earth's yearly revolutions around the Sun create the four seasons of spring, fall, winter, and summer.

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