From very ancient times, the five naked eye planets were known to be different to the stars. Whereas the stars appeared to be fixed onto the sky (which was thought to resemble a crystal sphere), the planets moved around relative to the stars. Indeed, the word planet is from a Greek word meaning "wanderer".
The planets were originally thought to revolve around the Earth. Casual observation showed that a planet like Jupiter would travel slowly across the sky in a West to East direction (right to left in the Northern Hemisphere). The planet takes about 12 years to complete a circuit of the sky. Mars completes a similar circuit in just over 2 years.
By the time of the ancient Greeks, it was thought that all motion in the heavens was circular with a constant speed. The planetary orbits were called geocentric (meaning "centered on the Earth"). An example is shown below where a planet (P), orbits the Earth (E) in an anticlockwise direction (as seen from above the Earth's North Pole).
As observations became more accurate, it was noted that the planets would sometimes change direction in the sky and travel from East to West for a short while before resuming their general Easterly motion. The normal West to East motion is described as direct motion while the rarer East to West motion is called retrograde. During the 2 year movement of Mars around the sky it will spend a couple of months moving in a retrograde direction. An example of this motion is shown below.
The ancient Greeks had great problems explaining this retrograde motion. However, they did not want to give up the idea of circular motion at a constant speed. To explain this complex motion they invented the notion of epicycles. These are circles on circles. In the diagram below, the point C (which is empty) orbits the Earth (E) at a constant speed. This point C is the centre of another smaller circle (an epicycle). The planet (P) orbits the moving point C at a constant speed. This combination of motions explains the complex paths of the planets.
The system became more complicated as more accurate observations were made. Epicycles had to added to the epicycles! The Greek thinker, Aristarchus, suggested around 250 BC that the Earth was moving around the Sun. This would explain the retrograde motions. If the Earth was on the same side of the Sun as a planet, it would appear to overtake it and leave it behind, causing it to appear to move "backwards" as seen from Earth. It took 1800 years before this idea became accepted. In 1543 the Polish astronomer, Nicholas Copernicus, published a book explaining the new system mathematically.
This heliocentric system ("sun centred") was better at explaining the motion of the planets and could be used for predicting their positions in the future. However, the planets were still considered to move in circular orbits with constant speed.
At that time the ideas of force and motion were hazy. For example it was thought that heavier objects fell towards the Earth more quickly. It was also assumed that the planets had to be continually pushed around through space so that they could orbit the Sun.
The Italian mathematician, physicist and astronomer, Galileo Galilei, performed a series of experiments in the early 1600s. These experiments showed that all objects dropped from the same height fell to Earth at the same time regardless of their weight. They also indicated that objects accellerated (speeded up) as they fell to Earth. His work also showed that if objects were moving at right angles to the ground, the horizontal and vertical movements were independent of each other. In other words, there could be a constant speed in the horizontal direction but a varying speed in the vertical direction. This explained the curved paths that objects like cannon balls took when fired into the air.
