Explanation:
there are many things. I am not sure what your teacher told you.
one of the first strange observations leading to this conclusion was the "loop-di-loop" in the track of Mars. because of the different orbits of Earth and Mars, sometimes one (particularly Earth as the faster one) swings around the sun in relation to the other, and with that first moves away and suddenly moves closer again, making the track of Mars appear on Earth as if it would make a loop in the sky.
