Throughout history there have been many questions relating to the creation and arrangement of the universe. After all, the answers to these questions provide the basis for explaining everything we know. Understanding the universe allows us to make sense of the world that we live in.

As you can imagine, trying to discover the truth about the universe is not an easy task. As the universe was the beginning of time and all that we know, it is only possible to assume information based on scientific and historical knowledge.

Many different theories about the universe have existed over time. One of the oldest and most popular theories suggests that a god exists beyond the physical world and that this god created the entire universe. This theory is based on the idea that something beyond our comprehension must have been responsible for the beginning of everything. Versions of this theory still form the basis of many different religions practised around the world today.

What is a geocentric model?

The early Greeks were among the first to study the night sky. They observed that objects in the sky were moving. From this, they assumed that the Earth was motionless and all other objects they could see were moving around the Earth. This is understandable because they could not feel the Earth itself moving but could see the stars moving around it. From this they concluded that the universe must all be centred around the Earth. This theory is called a geocentric model of the universe, which means 'Earth-centred'. The Roman Catholic Church even believed in this theory because certain Bible passages suggested that the sun was in constant motion around the Earth.

Who was Ptolemy?

One of the most influential Greek astronomers, geographers and mathematicians of the time, Claudius Ptolemy, supported the geocentric theory of the universe. Around 140 AD Ptolemy documented the most detailed explanation of the structure of the universe of that time. His detailed theory was known as the Ptolemy System.

This Ptolemy System was based on the belief that all heavenly bodies, including the stars and planets, were attached to spheres which rotated around the Earth. It was believed that the order of the planets from the Earth were as follows: the Moon, Mercury, Venus, the Sun, Mars, Jupiter, Saturn and the stars.

How was Ptolemy's model disproved?

Galileo disproved Ptolemy's model while using his telescope to investigate the planets. During his observations he discovered that the planet Venus goes through phases, just like our moon, which causes it to appear to change shape.

Galileo realised that this would not be possible under the Ptolemaic system. This is because the position of Venus, in relation to the sun and the Earth, could not possibly explain the different reflections that would cause phases.

The only way that the phases could be explained was if Venus were orbiting the sun instead of the Earth.

What is a heliocentric model?

Several others found faults with Ptolemy's model as time went on. Astronomer Nicolaus Copernicus was convinced that the Earth was a planet just like all the other planets. He believed, therefore, that the Earth also rotated like other planets.

Copernicus proposed a new model with the sun in the middle and all the other planets moving around it in the same direction. This model of the universe is called a heliocentric model. Heliocentric means sun-centred.

This heliocentric model remains the model of the universe that we know and accept today.

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