Have you ever wondered how the ancient people think of the Universe? Without advanced technology, what they could do was to look at the sky, record the changes and try to figure out what is going on with our Earth and the Universe. Simply by observations, they came up with two ingenious models in explaining the Universe, which are famously known as the geocentric model and heliocentric model.
Geocentric model suggests that the Earth is located at the center of the Universe while other planets and stars are circling around it. The origin of Geocentric model can be traced back to the ideas of ancient Greek philosophers Plato (423 - 347 BC) and Aristotle (384 - 322 BC). It became the predominant cosmological system of the Western world thanks to the modification made by Claudius Ptolemy (AD 90 - 168).
The geocentric model:
Ancient people were inclined to the model due to two easily-made observations. First of all, they discovered that the stars, the Sun and the planets appear to revolve around the Earth each day. The easiest way to discover this is that the Sun rises at the East and sets at the West, as well as other stars. However, they were not satisfied with this observation, they imagined that every star is located on a celestial sphere. Every day, the celestial sphere rotates around the Earth, resulting in the rise and set of stars. Secondly, as they couldn't experience any movement of the Earth, they concluded that the Earth is at rest while other stars and the sky is moving.
The geocentric model seems to be plausible to explain the movement of stars; however, how about the movements of planets? In fact, the former geocentric model cannot explain the retrograde motion of planets. Relative to the background stars, the planet normally move eastward. But they would occasionally move westward for a short period of time. This is known as retrograde motion.
The retrograde motion of planets under the geocentric model :
Claudius Ptolemy’s geocentric model solves this problem. He proposed that there are two circles involved in the movement of planets, rather than one in the movement of stars, namely the deferent and the epicycle. The larger circle is called the deferent, which can be said as the movement of the celestial sphere. The smaller circle is called the epicycle and the center of it moves along the deferent.The epicycle can be said as the motion of planets. When the circular motion of celestial sphere and planet is mixed together, we may observe a change of direction and speed of planet’s movement, which can be said as the retrograde motion of planet.
Explaining retrograde motion in terms of geocentric model:
Ptolemy's model successfully reproduced the retrograde motion of planets. Thus, ancient people were satisfied with the geocentric model. The geocentric model has dominated the western world for over 1500 years, until the rise of the heliocentric model.
As opposed to the geocentric model, heliocentric model states that the sun is the center of the universe, but not the Earth. In fact, its origin is nothing later than geocentrism. Heliocentrism is first proposed by ancient Greek astronomer Aristarchus of Samos (310 - 230 BC ) but had become obsolete with the rise of the Ptolemic model. Heliocentrism became more popular after the elaboration made by Nicolaus Copernicus(1473 - 1543) in the 16th century.
The heliocentric model:
Copernicus made four innovations to the Aristarchus’s heliocentrism. Firstly, he identified seven planets in the Solar system in which the Earth is one of them. Secondly, he discovered that there are three motions of the Earth, namely daily rotation, annual revolution and annual tilting of its axis. Thirdly, he explained the retrograde motion in terms of the motion of the Earth but not that of other planets. This is a simple and elegant idea as planets move in a single direction, but not forward and backward. Finally, he concluded that the planets are far more closer to the sun than other stars on the sky.
Explaining retrograde motion in terms of heliocentric model:
Galileo Galilei (1564 - 1642) is one of the supporters of heliocentrism and he proposed important evidence to support it. Galileo is the first astronomer using telescope to observe the sky.
By using the telescope, Galileo made some important discoveries which defend heliocentrism. Firstly, he discovered that there are four satellites orbiting around the Jupiter. This implies that the Earth is not the only center in the universe; and if this is true, it seems to be possible that the Earth is in fact orbiting around other stars as well. Another discovery related to heliocentrism is that Venus goes through a full set of phases. The phases of Venus is similar to lunar phases, where different variation of lighting are seen on Venus surface, providing that it must be orbiting around the sun but not the Earth. If they orbited the Earth (geocentric), we would never see a full phase when they are in the direction of the Sun, as illustrated below.
Although the heliocentric model explains many of our observations of the Universe, we do not believe in it anymore. Nowadays, we know that the Sun is approximately the center of the Solar system while the Solar system is only one of the billion systems orbiting the center of the Milky Way galaxy. Again, our Milky Way Galaxy is one among the billions of galaxies in our Universe. After all, thanks to the technology advancement, the Universe is becoming less mysterious to us than before. Let's stay tuned to the discoveries on Astronomy and get a better understanding to our Universe.
