How the Earth moves, and how do we know?

More than 400 years after Galileo’s first telescopic observations, we’re more certain than ever that the Earth is moving through space. How do we know?

“Nature is relentless and unchangeable, and it is indifferent as to whether its hidden reasons and actions are understandable to man or not.” -Galileo Galilei

All of science is rooted in the idea that natural phenomena can be explained naturally, and that if we want to know how anything in the Universe works, all we need to do is ask the Universe the right questions, and the answers will appear.

So what about the question of the night sky, and why it appears to rotate the way it does?

Image credit: Peter Michaud (Gemini Observatory), AURA, NSF.

There are two straightforward explanations for this, and from observing the apparent motion of the night sky alone, they’re indistinguishable from one another.

1. The entire sky — and all the stars in it — spins around the Earth with a period of 24 hours, causing the stars to change position as we observe it from Earth.
2. The entire sky is — to the best of our observations — stationary, and appears to spin because the Earth is rotating beneath it.

These two scenarios, although they would both adequately explain this phenomenon, are vastly different from one another.

Images credit: Retrieved from the One Minute Astronomer, http://www.oneminuteastronomer.com/.

But the stars appearing to rotate about the celestial pole is not the only observation we have. By making other observations and interpreting them in the context of these two very different models, we can help determine whether one is superior to the other.

The first clue to which one is correct came back in 1610, when Galileo discovered that the planet Jupiter had satellites of its own orbiting it, and today happens to be the 404th anniversary of the discovery of the fourth (and final) Galilean satellite of Jupiter: Ganymede, the…