1 Edmond Halley: This is a most marvellous opportunity. We can ask Copernicus how he concluded that the planets move around the sun.
2 Edmond Halley: In fact, if we travel back far enough in time, we could give him the idea!
3 Edmond Halley: It would be a case of circular logic. Ha ha ha!
4 Isaac Newton: Mr Halley, stick to science, not comedy.

Nicolaus Copernicus lived in the late 15th and early 16th centuries, and is best known for being the first person to seriously propose a complete model of the solar system in which the Earth and all the other planets moved around the sun.

Unfortunately Copernicus was still stuck with the semi-mystical notion that the motions of the celestial bodies had to be in the perfect shape of circles. This notion plagued not only the previous geocentric models of the universe, but also Copernicus' new heliocentric model, because circular orbits could not adequately describe the observed movements of the planets, even to within the accuracy of naked eye observations made at the time.

This led to a system called deferents and epicycles, which have been mentioned before. Basically, a planet did not move in a simple circle around the sun in Copernicus' model (nor in a simple circle around the Earth in the older geocentric model). To account for the observed motions of the planets, it was proposed that the planet moved in a smaller circle around another point, and that point moved in a circle around the sun (or the Earth). The smaller circle was called the epicycle, and the larger circle that it moved around the deferent.

This ad hoc complexity was eventually solved by... but wait. That is a story for another time.