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# Heliocentric Model of the Solar System Facts

Updated July 19, 2017

The word heliocentric comes from the Greek "helios," meaning sun. Heliocentrism, an astronomical theory, assumes the sun is the centre of the solar system and all planets orbit the sun. It did not fully emerge as a developed model until the late 16th century, with the work of Polish astronomer Nicolaus Copernicus.

## History

Aristarchus of Samos proposed the first heliocentric model in the third century B.C., but it did not receive much attention until the introduction of the Copernican model. Copernicus' view was in direct opposition to the widely held belief in geocentrism, which assumed that the sun and surrounding planets orbit the Earth. The Greek astronomer Ptolemy developed the most commonly accepted geocentric model in the second century A.D.

When we observe the sky from Earth, the planets appear to move from east to west. Occasionally, the planets will appear to reverse direction--this phenomenon is known as retrograde motion. Ancient astronomers had difficulty explaining this motion. Ptolemy's proposed explanation of this movement was epicycles: "In this case, the planet moved on a little circle, the centre of which rotated on the circumference of the large circle centred on the Earth," according to Rice University's Galileo Project website. Instead, the heliocentric model solves this problem by assuming planets farther away from the sun orbit slower than closer planets.

One problem with the Copernican model is that Copernicus posited that planets moved in perfect circular orbit around the sun. Because of this, the model was not accurate in predicting the position of planets. Using the calculations of fellow astronomer Tyco Brahe and his own mathematical skill, Johannes Kepler refined the model in the early 1600s by proposing that planets are in an elliptical orbit around the sun.

## Copernicus

Copernicus introduced his model of a heliocentric universe in his work "On the Revolutions of the Heavenly Bodies." Copernicus hypothesised that the Earth was the third planet in orbit around the sun, and that stars do not orbit the Earth, but the Earth's rotation makes it appear that way. Copernicus noted, "Finally we shall place the Sun himself at the centre of the Universe. All this is suggested by the systematic procession of events and the harmony of the whole Universe, if only we face the facts, as they say, 'with both eyes open.'"

## Church Views

In 1616, the Catholic Church examined the legitimacy of the heliocentric model. Church concern stemmed from the fact that the heliocentric model contradicted passages in the Bible, and they summoned Galileo to Rome for his expertise on the proposed model. The Pope Urban VIII encouraged Galileo to publish his work on the subject, in hopes that Galileo would favour the geocentric model. Galileo did publish work on the matter; however, he advocated the heliocentric model, something the church could not accept. In 1633, Galileo was put on trial for his views, found guilty and sentenced to house arrest.

## Later Development

Acceptance of heliocentrism was gradual, and by the 18th and 19th century, it became clear that the sun was not the centre of the universe, but one star among many. In fact, the sun is not even the centre of the solar system, but rather the focus of the elliptical orbits traversed by the planets.