The Heliocentric Solar System

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The Birth of Modern Astronomy

Heliocentrism is an astronomical theory that posits that sun is at the centre of the Solar System.

The word Heliocentric is derived from the Greek words “helios,” or sun and “kentron,” or centre. Heliocentrism existed as an alternate to geocentrism, which claimed that the Earth was at the centre of the universe. The notion that the Earth revolved around the sun was first proposed by the Greek astronomer Aristarchus of Samos in the 3rd Century BC. However, it was not until the 16th Century that a fully predictive heliocentric model emerged.

Origins of Heliocentrism

Anyone observing the sky on a clear night will notice that the Earth seems to be standing still while the stars and planets rotate around it. However, careful observation of the night sky over succeeding nights reveals a more complicated set of movements. As the sun makes its slow and steady way across the horizon over the course of the year, the planets exhibit a phenomenon known as retrograde motion, in which they appear to orbit backwards. As the motions of the planets became better understood, it became necessary to develop more complicated theories describing their movements.

The most famous of these was the Ptolemaic, or geocentric, model of the solar system. Ptolemy synthesized the writings of a number of other ancient astronomers and published his findings in the Algamest. Ptolemy argued that any model used to describe the motions of the planets is merely a mathematical device. He also suggested that since there was no way to be certain whether or not a given model was accurate, it was best to use the simplest model possible. Ptolemy chose to calculate the orbits of the planets based on the geocentric model. In his Planetary Hypothesis, which was a companion to the Algamest, Ptolemy argued that his geocentric model was sufficiently real that he could calculate the distance between the sun, Earth, moon and planets using the celestial spheres, which he believed held them in place.

The first person to present an argument for a heliocentric universe was Aristarchus of Samos in 270 BC. His writings on heliocentrism have not survived, but some information has been passed down to modern historians and scientists through his contemporaries, such as Archimedes. Archimedes believed that the sun was five or six times wider than the Earth and several hundred times larger than the Earth in volume. Some modern researchers have suggested that because of this size difference, it made more sense for the Earth to orbit the sun.

During late antiquity and the Middle Ages a number of astronomers experimented with heliocentrism. In the 5th Century AD, the Roman astronomer Martianus Capella wrote that the planets orbit the sun, not the Earth. Capella’s model of the Solar System was discussed extensively in the 9th Century by a number of astronomers whose names have been lost to history.

Heliocentrism and the Catholic Church

By the 16th Century, the book On the Revolution of Heavenly Spheres by Nicolas Copernicus presented a fully realized heliocentric universe for the first time. As with Ptolemy’s Algamest, On the Revolution of Heavenly Spheres laid out Copernicus’s hypothesis in full geometric detail. In doing so, he gave astronomers the first fully predictive model of the Solar System.

Heliocentrism had been in conflict with religion before Copernicus. Plutarch had argued that Aristarchus should be charged with impiety for, “moving the hearth of the world.” However, until the advent of Copernicus, the ideas of Aristarchus received little attention.

In 1616, Church officials were prompted to decide whether or not to accept heliocentrism. Accordingly, Galileo was summoned to Rome, to examine the pros and cons of heliocentric astronomy. The Church was willing to accept heliocentrism as a calculating device, but not as literal truth.

Pope Urban VIII initially encouraged Galileo to publish the pros and cons of heliocentric astronomy. However, Urban quickly regretted this decision when Galileo began to advocate for heliocentrism. Galileo was arrested ostensibly for making fun of the Pope, in 1633. He was put on trial for supporting heliocentrism, found guilty and sentenced to house arrest.

Papal opposition to heliocentrism, did not imply opposition to astronomy. The Church needed astronomy to maintain its liturgical calendar. In 1664 Pope Alexander VII published an Index of Prohibited Books, which included all books supporting heliocentrism. In 1742, an annotated copy of Sir Isaac Newton’s Mathematical Principles of Natural Philosophy was published by a pair of Catholic priests. The book included a preface which stated that Newton’s theories assumed heliocentrism to be fact, and could not be explained without it. In 1758, the Catholic Church removed all prohibitions against heliocentrism.

Modern thinking, that heliocentrism is an over simplification, was arrived at in steps. Over the course of the 18th and 19th Centuries, it became increasingly obvious that the sun is just one of many star s in the universe.

Today astronomers use a number of co-ordinate orientation systems, depending on the application and the desired outcome.

Sources

Kuhn, Thomas S. (1957). The Copernican Revolution. Cambridge: Harvard University Press

Dreyer, J.L.E. (1953). A History of Astronomy from Thales to Kepler. New York, NY: Dover Publications.

Walker, Christopher, ed. (1996). Astronomy before the telescope. London: British Museum Press.

Heath, T.L. (1913). Aristarchus of Samos, the ancient Copernicus: a history of Greek astronomy to Aristarchus, Oxford, Clarendon.

Hoyle, Sir Fred (1973). Nicolaus Copernicus. Heinemann Educational Books Ltd., London.

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