Early ideas about the solar system

Thales and early models

The history of how our view of the heavens changed is fascinating. This is a reconstruction, identifying some salient points useful for science lessons.

Astronomy is one of the oldest sciences. Ever since the first person had time to stop and gaze at the night sky, people have wondered about the nature of the cosmos and developed models to explain what they observed.

The first models were based on the idea that the Earth is flat and fixed. For instance, Thales, the Greek philosopher (born 624 BCE), believed that the Earth was a flat disc floating on an infinite ocean. The importance of Thales' contribution is that he was the first recorded person who tried to explain phenomena by rational means based on observation and evidence rather than by appealing to the supernatural.

The Celestial Sphere

Early Greek ideas placed all the stars on a large sphere, known as the Celestial Sphere, that surrounded the Earth. There were only seven objects, in addition to the stars, visible to the ancients.

These were the Sun and the Moon, plus the five planets, Mercury, Venus, Mars, Jupiter and Saturn. It was obvious that the planets were not on the Celestial Sphere since the Moon clearly passes in front of the Sun and planets. In addition, Mercury and Venus can be seen to cross in front of the Sun.

Plato proposed that the planets follow perfectly circular orbits around the Earth in what is now called the geocentric solar system model. Later, in about 330 BCE, Heraclides developed that model, apparently placing the planets in order from the Earth (although some historians claim that Heraclides believed the Sun to orbit the Earth with the planets orbiting the Sun).

In 270 BCE, Aristarchus of Samos proposed an alternative system to the geocentric model, placing the Sun at the centre, in the heliocentric system.

While today we know that the Sun is at the centre of the solar system, this did not become at all apparent until the 16th century. In particular, the philosophers of the day ruled out Aristarchus' model for two reasons:

• If the Earth is in orbit around the Sun then the Earth is moving. Before the discovery and recognition of Newton's laws of motion, it was difficult to imagine motion without being able to feel it. As people cannot sense the Earth moving, it was hard for them to believe that it was. If, for example, the wind blew constantly in a given direction, then people might have been more likely to be convinced.
• A geocentric model seemed more natural to the philosophers of the day. Having the Earth at the centre of the universe is a highly egocentric point of view with a strong aesthetic appeal.

The Ptolemaic system

The Ptolemaic system was based on the idea that the Earth is fixed and immovable at the centre. It was developed by a Greek, Ptolemy, who lived in Alexandria between 87–150 CE. Ptolemy set out his ideas in 13 books called the Almagest. The basis of his theory was that everything is fixed on celestial spheres, which were set out like the layers of an onion. These were not intended as theoretical entities but as real crystalline spheres, the outermost of which was the primum mobile (prime mover) that drove the whole system.

Each of the 13 books of the Almagest deals with different aspects of the objects in the solar system and the stars. What made Ptolemy's model so successful, and the feature that led to it dominating all thinking about the solar system for the next 13 centuries, was its comprehensive nature.

The other reason why Ptolemy's theory survived for so long is that it worked. It could be used to predict the motions of the stars and the planets pretty accurately. What Ptolemy's theory lacked though, was simplicity or elegance as it needed 28 epicycles to account for all of the observed motions. From a scientific perspective, this makes the theory less attractive as scientists look for simple solutions to problems.

The Ptolemaic theory was very accurate at predicting the motion of the planets, so why did the Sun-at-centre theory displace it? Confronted with competing theories, science draws on some basic values to make a judgement between them. Despite a general view that science is value free, this is not true.

One of those values is parsimony or simplicity. Given two theories that can explain things equally well, scientists tend to go for the simpler one. Another value in science is explanatory power, such that theories that explain more are generally reckoned to be better.

Those who saw Galileo's theory recognised its inherent simplicity and explanatory power, but unfortunately the church did not. Only 359 years later did the Vatican finally publish an edict admitting that Galileo was right after all!

This is a very good example of the way in which science always operates in a social and cultural setting and is inevitably influenced (and in this case constrained) by that setting.

Retrograde motion

The major challenge for Ptolemy's model was explaining the retrograde motion of the planets. This is the fact (as mentioned earlier) that the planets appear, when observed over a period of time, to loop or wander backwards against the backdrop of the stars.

Ptolemy solved this problem by suggesting that the planets moved in circles about a point on the circumference of a circle, technically known as an epicycle.

The combination of the moving centre and the planet moving around a circle produces a motion that almost exactly models the motion of the planets.

The downfall of the Earth-at-centre world view

It was Galileo's observations (made around 1609) of Jupiter and the orbit of its moons that were of enormous significance in leading to the downfall of the geocentric worldview developed by the Greeks.

These observations were in accordance with, and built upon, the Sun-at-centre model proposed by Copernicus.

Galileo did not invent the telescope but he was perhaps the first person to improve on the original Dutch design and use it to make astronomical observations. Here is a reproduction of his original notes and sketches.

The Church and the society of the time believed that everything went around the Earth. Yet the only way to explain why sometimes there were two, sometimes, three and sometimes four stars besides Jupiter was that these were not stars but moons going around Jupiter and not around the Earth!

If the Sun is at the centre, the explanation of the retrograde motion of the planets becomes simpler. It is the line of sight from the Earth to the planet that appears to move against the background of fixed stars.