Ancient Greek philosophers such as Pythagoras believed the Earth was the centre of everything. As the rotation of the Earth cannot be felt, they believed that it was a solid and unmoving sphere, which all the planets and star revolved around. The Ancient Greeks deduced that the Earth was a sphere by observing the world around them. For example, as a ship disappeared over the horizon, the crow's-nest was the last to disappear. It was also noticed that the further north one is, the lower the sun appears in the sky and the higher the North Star.
The Ptolemaic Model
In Ancient Greek, it
was believed that all the celestial objects were inside spheres that rotated around Earth. This theory, however,
could not explain the irregular patterns of the planets. In the second century
BCE, the Greek astronomer, Hipparchus, used a pattern of circles to justify the movements of the
planets. His model shows every planet rotating around a circle called an epicycle. The epicycle's centre orbits a larger circle called the deferent. At the centre of the deferent is Earth.
In the 2nd century CE, the astronomer Ptolemy improved Hipparchus' model to more accurately explain the patterns in the sky. He moved Earth out of the centre of the deferent and into an eccentric position. Like Hipparchus, he based the patterns of the planets on a 'perfect circle'. The Ptolemaic system explained the appearance of the planets speeding up and slowing down and also moving backwards. This occurs when Earth passes a planet in its rotation around the Sun. Mars, for example, passes Earth roughly every 26 months. Using his model, Ptolemy predicted Mars would take 26 months to complete one rotation of its epicycle. |
The Copernican heliocentric System
The Ancient Greek Aristarchus suggested that the sun was the centre of the Solar System and all the planets revolved around it. This was later referred to as the heliocentric system. It was a simpler explanation for the backwards, or retrograde, motion of the planets. However, Ptolemy's model was favouredAS no one was willing to believe that the Earth was moving.
During the Middle Ages the Ptolemaic system was increasingly questioned. Philosophers believed that nature could be explained by simple systems and Ptolemy's model was not based on a principal that could explain every planet's motion. |
Nicolaus Copernicus, a polish mathematician, investigated Aristarchus's heliocentric model. It enabled him to explain retrograde motion and why Venus and Mercury can never be seen at midnight. These are inferior planets as they are closer to the sun than Earth, and can only be seen within three hours of sunset and sunrise. Copernicus used trigonometry to find the radius of each planet’s rotation around the sun. He could not calculate the Earth's radius so he used astronomical units (AU) where one AU equalled the radius of the Earth's rotation around the Sun. His estimates of the radiuses have been found to be very accurate (see table on right).
The Copernican heliocentric system could not correctly predict the movement of celestial bodies. Copernican first found that his predictions were worse than the Ptolemaic model. He added epicycles that were smaller than Ptolemy's, to achieve more accurate results. They sped up and slowed down the planets to fit his observations.
Copernicus only published his research on his deathbed as the Church opposed the heliocentric model. However, he started the 'Copernican Revolution', which included Astronomers such as Kepler and Galileo.
The Copernican heliocentric system could not correctly predict the movement of celestial bodies. Copernican first found that his predictions were worse than the Ptolemaic model. He added epicycles that were smaller than Ptolemy's, to achieve more accurate results. They sped up and slowed down the planets to fit his observations.
Copernicus only published his research on his deathbed as the Church opposed the heliocentric model. However, he started the 'Copernican Revolution', which included Astronomers such as Kepler and Galileo.
Galileo's Theory
Galileo Galilei was born in 1564, in Pisa, Italy. He was very interested in using mathematics to describe the logistics of the world. In 1609, Galileo used the telescope, originally developed by Dutch opticians, to study space. When examining the moon, he noticed what seemed to be mountains and seas, as found on Earth. Galileo took measurements of the terrain using the shadows cast by the mountains and concluded that the Moon was not a heavenly body. He discovered spots rotating around the Sun, changing in size and disappearing. Like the Moon, the Church considered the Sun heavenly. The idea that the creations of God were blemished was unthinkable. Galileo created more controversy when he discovered four Moons orbiting Jupiter. It clashed with the Church's view that everything revolved around Earth. Galileo also studied the different phases of Venus. He realised that its change in size and shape corresponded with its motion around the Sun. The further Venus was from Earth, the fuller it was and the smaller its diameter seemed. This
was an important finding that supported the heliocentric model, which Galileo
was convinced made more sense.
Cycle of Venus Sunspots