The geocentric model

1. The earliest widely held picture of the Universe was geocentric, with a stationary Earth at the centre and everything else carried around it on turning spheres.
2. It was set out in detail by the Greek astronomer Ptolemy in about $150AD$ and stayed the accepted model in Europe for well over a thousand years.
3. The model matched everyday observation, because the Sun really does appear to rise in the east and set in the west while the ground under your feet feels perfectly still.
4. Its weakness was the motion of the planets, which sometimes appear to slow, stop and travel backwards against the fixed stars before moving forwards again.
5. To keep the Earth at the centre, astronomers added small extra circles called epicycles, so each planet ran round a small loop that itself ran round the Earth.
6. Every fresh set of measurements demanded more epicycles, and the model grew steadily more complicated without ever fitting the observations exactly.

The heliocentric model

1. In $1543$ the Polish astronomer Copernicus published a heliocentric model that placed the Sun at the centre and made the Earth one of the planets going round it.
2. In this model the Earth turns on its own axis once a day, and that spin, rather than a moving Sun, is what causes day and night.
3. The Moon was still treated as circling the Earth, so the Earth was no longer unique in being orbited.
4. Backwards planetary motion came out of the model automatically, because the faster inner Earth overtakes a slower outer planet and the outer planet appears to slip backwards as it is passed.
5. The new model needed no epicycles to do this, so it explained the same observations far more simply.
6. Acceptance was slow, partly because it conflicted with long established religious teaching and partly because no direct evidence for a moving Earth existed at the time.

Evidence that changed the model

1. The first strong evidence came from the telescope, which let astronomers see detail that the unaided eye cannot resolve.
2. In $1610$ Galileo found four points of light beside Jupiter that changed position from night to night, and they were moons in orbit around Jupiter rather than around the Earth.
3. That single observation destroyed the central claim of the geocentric model, since it proved that not everything in the sky circles the Earth.
4. Galileo also watched Venus run through a full set of phases from thin crescent to nearly full, which can only happen if Venus travels right around the Sun.
5. Later work by Kepler showed that the orbits are slightly squashed circles rather than exact circles, which removed the last mismatch between the heliocentric model and careful measurement.
6. Newton then supplied the missing physics by showing that a single force, gravity, holds every planet in orbit and explains why the model works.
7. Better instruments kept extending the picture, with Uranus found in $1781$ and Neptune in $1846$ after its position was predicted from the wobble it produced in the orbit of Uranus.

How scientific models change

1. A scientific model is kept only for as long as it accounts for the observations, and it is judged on how well it predicts as well as on how well it describes.
2. When new observations cannot be explained, the model is either patched, as the geocentric model was with epicycles, or replaced by one that fits without patching.
3. Replacement is not immediate, because other scientists first repeat the observations and check the reasoning before a new model is accepted.
4. Improvements in technology drive the process, since each better telescope reveals detail that the previous model was never tested against.