Nicolaus Copernicus, Polish-born (1473-1543), suggested a heliocentric model, far less complex than the Ptolemaic model. For this to be accepted -that the Earth is not standing still and the centre of the universe-, a change in the paradigma reigning at that time had to occur.
Consequences of the heliocentric model:
- The order of all planets relative to the Sun could be established.
- Planets are divided between inferior planets (planets whose orbit is closer to the Sun than that of Earth), and superior planets (the opposite).
- Inferior planets have greatest eastern/western elongation (they have a maximum angular separation east or west of the Sun, in the sky).
- Inferior planets can pass in front of the Sun (inferior conjunction).
- The superior planets can be seen as much as 180º from the Sun. Then, they are in opposition.
- Retrograde motion was explained (for retrograde motion see Greek Astronomy).
The relative orbital motions of Earth and the other planets means that the time elapsed between successive oppositions or conjunctions can differ significantly from the amount of time necessary to make a complete orbit.
Synodic period (S): Time interval between oppositions/conjunctions.
Sidereal period (P): Time that one planet takes to make one complete orbit relative to the background stars.
Still, the Copernican model could not predict the planetary motion any more accurately than the Ptolematic model. The assumption that orbits of planets had to be perfect circles was the cause. Copernicus was forced to fix his model by using epicycles, as the Greeks did.