Does the earth orbit the sun? Or does the sun orbit the earth? How do we know?

We know because of all the data collected by scientist and satellites. But there are some observations you can do to determine for yourself what orbits what.

1 / Let's take the example of t he phases of Venus. The next time Venus is visible in the sky (the brighter star in the sky), take a look at it with some binoculars.

You would be able to see that Venus shows the same kind of phase that we see with the moon. What does this mean? It means two things. First, We can see Venus because it reflects light from the Sun. Second, as the phases change, Venus is sometimes closer to us that the Sun and sometimes farther away. You would see a “full phase” Venus when it is on the other side of the Sun. How can both Venus and the Sun orbit the Earth but also have Venus move farther away? Oh, this is something that Galileo saw with his telescope.

2/ Moons of Jupiter. This is something else that Galileo did that you can repeat: see the moons of Jupiter. Again, you just need binoculars. You will be able to see the 4 big moons of Jupiter. So what? The idea that all the planets (and the Sun) orbit the Earth isn’t as strong once you show that there are objects that orbit another planet. These moons of Jupiter clearly orbit Jupiter and not the Earth.

There are more with observations of the sun, of Mars, ... all these observations prove the fact Earth orbits the sun !