# Geocentric vs Heliocentric Model of the Universe

If you look at the sky at night, it appears that the stars are moving. This is called apparent daily motion.

From this perspective, it is very easy to assume the earth is the center of the universe. This was the accepted theory called the  Geocentric model for many years. Great minds like Aristotle and Ptolemy proposed and supported this view of the universe. Geocentric means “earth-centered”  and it just seemed right.

However, there were some problems. For example, sometimes the planets appeared to go backward or have retrograde motion. In order to explain this small orbits called epicycles which are orbits inside an orbit was proposed to explain this backward motion.

This model did not sit well with Copernicus and In 1543, he published “On the Revolutions of the Celestial Spheres”, in which he explained what many had suspected: that the sun is at the center of the universe and we move around it along with all the other planets. This is called the Heliocentric Model.

He proposed that each orbit was a circle and that the earth like other planets revolved around the sun. This solved several problems including the retrograde motion of the planets arguing that such motion was only perceived and apparent, rather than real.

With the help of his telescope, Galileo Galilei discovered the moons of Jupiter and observed that smaller objects revolve around larger ones.
Johannes Kepler proposed that the orbits of the planets were not perfect circles but ellipses Our planets travel in an ellipse and actually speed up when closer to the sun and slow down when furthest away from the sun.

The retrograde motion of the planets was explained because it is caused by planets that move more quickly pass other planets and it appears the planet is going backward.
Sir Isaac Newton in his book Principia Mathematica proposed laws of motion in or to explain how objects interact with the world and with each other. His law of gravity identified the responsible force behind the celestial motion.

We now know the sun is one of many stars and is not the center of the universe but the planets do revolve around the sun and not the other way around.