If you're seeing this message, it means we're having trouble loading external resources for Khan Academy.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Giving up control

When observing a sunset it is natural to assume the sun is moving around us. Aristarchus of Samos (c. 310-230 BC), a Greek astronomer, was first to maintain that the earth rotates on its axis and revolves around the sun. This was a dangerous assertion at the time and could not win over supporters of the geocentric model, a model that persisted for over a thousand years.

Image Credit: Brit CruiseIn 1543, Nicolaus Copernicus (1473 – 1543) published “On the Revolutions of the Celestial Spheres”, in which he explained what many had suspected: that the sun is at the centre of the universe and we move around it along with all the other planets. This is called the Heliocentric Model.

Image of heliocentric model from Nicolaus Copernicus' "De revolutionibus orbium coelestium".This theory resolved the issue of retrograde motion by arguing that these pretzel orbits were an illusion due to our vantage point. It is a perfect example of Occam’s razor: if you have two competing hypotheses, the one with the fewest assumptions should be selected. The simplest explanation tends to be correct. If you look closely you'll see his model correctly assumed the moon is orbiting around us. Notice how this explains the strange wandering motion of planets when observed from telescopes based on Earth.

It's the motion of the Earth relative to Mars which causes this wandering effect. The next simulation will allow you to explore both the geocentric vs. heliocentric models of our solar system. Have fun!