What are geocentric and heliocentric theories?

Answer 1

The two theories differ in what occupies the centre of the solar system. Is it the Earth (the geocentric theory) or the Sun (the heliocentric theory)?

We now know the Sun is at the centre because in the 18th century it was found that it is by far the most massive object in the solar system. Also, with the right equipment we can see the positions of the nearest stars change as the Earth moves in its orbit round the Sun. So the heliocentric theory rules.

Before that the geocentric theory was used, based on a literal interpretation of The Bible. Ptolemy's theory with the Earth at the centre used a system of circles and epicycles to predict the positions of planets, and it worked quite accurately.

The heliocentric theory was proposed by Nicolas Copernicus in the early 1500s. Later Galileo had a famous dispute with the Catholic Church over it. Galileo started by saying he had proofs of the heliocentric theory but the Cardinals were too stupid to understand them. Eventually he was forced to admit there was no proof based on the scientific evidence available at that time.

The proof came in the next century after Newton's laws of motion and law of gravity were used in detailed calculations that explained the planets' orbits and allowed the mass of the Sun and planets to be calculated.