The Rhythmic Sky Page 2

1. Rotation of the Earth

Let us first talk about motion in general.

Who is moving?If I would say: “object A moves” than this would be meaningless, because how do I know A is moving? The concept of motion is a relative concept: we can only say that object A is moving with respect to another object B. There is nothing like “absolute” motion, all motions are relative, one object moves with respect to something else.

Think about you sitting on your bicycle and cycling along the road. You would probably say that you and your bike are moving with respect to the road, the houses, trees and people around you, because that is what you see. But it would be equally OK to say that everything around you, essentially the whole Earth, is moving in opposite direction with respect to you. It really is a matter of perspective, from which position you are looking.

clip art: www.webweaver.nu

Giordano Bruno


What has that got to do with the night sky or basic astronomy?

Well ask that question to Giordano Bruno, who just over 400 years ago was burned alive at the stake in Rome, for his opinion that the Earth and not the Sun was moving. With this basic knowledge we discuss here, he might have been able to save his life. Well probably not, because his defence against the Catholic Inquisition was about religious concepts and ultimately about the conflict between religion and science.

But the point here is that motion is relative and that how you experience motion, really depends on your perspective. You could equally well say that the Earth moves around the Sun, as you can say the opposite. When we now, 400 years later, study the motions in the Solar System, the easiest perspective as seen from space, is to put the Sun in the centre and let planets and everything else orbit around it.

But is it so strange that for millennia people were convinced that the Earth was stationary? Not really when you stand in your backyard and see the Sun move through the sky during the day. Clearly the Sun is the only thing that is moving!

Giordano Bruno, credit: Wikipedia.