Models of the Universe

Ptolemaic model
Many, many years ago, most ancient Greek astronomers believed that planet Earth was positioned at the centre of the Universe, and the Moon, stars (including the sun) and planets orbited the Earth in perfect circles. This is called the Ptolemaic model. As it has the Earth as the centre of the Universe, it is known as a geocentric model.

The Ptolemaic model was accepted from the time of the ancient Greeks until the 1500s. From the 1600s, the Copernican model started to replace it (see below).

Copernican model
The Copernican model is a model of the Universe which states that the Sun is at the centre of the Universe, and all the planets orbit the Sun in perfect circles.

Although this idea was already circulating for approximately 2000 years, the actual model wasn't introduced until 1543, when Nicolaus Copernicus (1473 - 1543) published it in a book. The book showed that astronomical observations were explainable without having the Earth as the centre of the Universe. The Copernican model is a heliocentric model, as it depicts the Sun as the centre of the Universe.

The Copernican model was supported by strong pieces of evidence discovered by Galileo Galilei (1564 - 1642).

Galileo and the Copernican model
Around 1610, Galileo was observing Jupiter using a telescope (a recently-developed invention back then), when he discovered a line of stars near the planet. He noticed that these stars never moved away from Jupiter and appeared to be moving along with the planet. This suggested that they were not stars; they were actually moons orbiting Jupiter. This proved that not everything orbited the Earth, and is evidence that the Ptolemaic model was incorrect.

In Autumn 1610, Galileo noticed that planet Venus has phases, where the amount of the planet that is illuminated by the Sun changes over time. These changes would have been small if the Ptolemaic model was correct, as it suggested that Venus was always positioned in front of the sun. However, Galileo saw large changes; if the Copernican model was correct, Venus would be able to move in front of and behind the Sun, and the changes in the amount of time venus was lit by the Sun would be large; this matched Galileo's observations.

The Copernican model was controversial at the time it was first devised, as people had believed the Ptolemaic model for many years. The Church also condemned the Copernican model; they claimed the new model contradicted the Bible, which stated that planet Earth was at the centre of the Universe. Thanks to advances in technology, further evidence for the Copernican model was discovered gradually.

The Catholic Church eventually accepted the Copernican model in 1992.

Present day
The current model of the Solar System still states that all of the planets orbit the Sun, however these orbits are elliptical instead of circular and the Sun is not at the centre of the Universe.

Even though the other two models were found to be incorrect, they played an important part in helping us reach the current model.