Early models of the universe (A level Physics)
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Throughout civilisation, various models of the Universe have been proposed. The two that feature in this area of the syllabus are the geocentric (the Earth is the centre of the Universe) and heliocentric (the Sun is the centre) models.
These two models often go by the names of Ptolemaic and Copernican respectively.
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Ptolemaic Model
This model of the Universe was the widely accepted model for almost 1400 years until the work of Kepler and Galileo.
Features
The main feature of this model is the idea that everything in the Universe orbits the Earth. It is important to remember that the planets we know today were not regarded as planets, but 'wandering stars'. Also, stars themselves were not seen as being distant equivalents to our sun.
Other elements of the original model include the notion that the perfect 3D shape is a sphere, everything fell to Earth unless supported (therefore all heavenly bodies are supported by crystalline spheres) and that unlike the imperfect Earth, everything else was perfect and unchangeable.
To match his observations Ptolemy had to modify this model, namely introduce epicycles to explain retrograde motion (planets appearing to change direction). As the planet orbits Earth in a circle (the deferent), it also moves in smaller circles along this path, called epicycles. He also introduced the idea of equants. He had noticed that planets sometimes orbited with different-sized retrograde loop and so suggested that there was a point near the centre of the orbit at which all observed retrograde motion would be at a uniform speed.
While equants may seem a minor issue, its inclusion into the model was what brought about Nicolaus Copernicus' suggestion of a heliocentric model, an idea that had already been proposed by Aristarchus in the third century AD. The model had become over-complicated.
Copernican Model
Copernicus is often erroneously credited with devising this model - actually he just revived the idea. The main reason he did this is because Ptolemy's model was based on the idea that objects orbited the Earth with uniform circular motion, but Ptolemy's additions no longer seemed to conform to this ideal. Copernicus wanted 'a more reasonable arrangement of circles'.
Features
As it was a heliocentric model, everything in space now orbited the sun. Unlike today's model of the Universe, Copernicus stuck with circular orbits.
The heliocentric model could explain retrograde motion without using epicycles. If everything orbits the Sun, then objects further away will take longer to orbit. Therefore when an object on the inside overtakes an outer object, the outer object appears to change direction (avoid using "moves backwards" if asked to define retrograde motion - Venus moves backwards already when compared to the orbits of other planets). The idea of retrograde motion can be likened to travelling on a motorway. When you overtake a car, it appears to move backwards.
Acceptance of the Copernican Model
Why had the Ptolemaic model of the Universe lasted so long? There are several reasons for this, usually concerning the desires of the church. Ptolemy's model worked for the Church mainly because it put the Earth at the centre of the Universe and included the notion that the heavens were perfect. There are, however, several good scientific arguments against a heliocentric model, sometimes referred to as 'Ptolemaic objections'.
Ptolemaic Objections
- If the Earth was moving, why does a dropped object fall in a vertical line down? This was before Newton’s laws of motion, so this view was understandable.
This diagram shows how the angle of observation between two stars should change as the Earth orbits the Sun. In fact it does, but because the stars are so far away, the difference is not observable. Astronomers in Copernicus' day failed to comprehend the true scale of the Universe. - As the Earth orbits the Sun, the positions of the stars should appear to change in relation to each other, known as parallax. In other words, 'the angle subtended by two stars should change during orbit'. Parallax does actually exist, but is hardly noticeable due to the sheer distances involved. Astronomers around at the time of Copernicus didn't realise this, though he used it as an argument.
- Epicycles had to be introduced so that the model would fit the observed positions of the planets. This was because Copernicus used circular orbits in the model.
- The model initially (until the work of Kepler) gave less accurate predictions of planetary positions than the Ptolemaic model.
Kepler
In trying to make the model match the facts, Kepler abandoned his personal belief that the orbit of a planet was circular, and came up with three radical ideas, known as Kepler's Laws:
- Each planet has an elliptical orbit (this allowed the model to fit the data without need for epicycles).
The area of the blue shape is equal to the area of the green one - In any given period of time, a line connecting the Sun and a planet will always sweep out an equal area.
- The square of the orbital period is proportional to the distance from the Sun cubed.
That is to say, 
Galileo
Had been casted by the Church as a Heretic. Had observed 3 out of 4 moons of Jupiter, hence known as the Galileo Satelites; Had observed sunspots. Has used lens to create a telescope ( this had been done before but not used the same way) He was very proud and is reffered to as the Father of Modern Astronomy. Later in his life went blind due to viewing stars with his naked eye(not that important for exam).
