Stumbling across news of the upcoming “Galileo Was Wrong” conference made me feel somewhat like Dr Watson in a Study in Scarlet who observes:

“My surprise reached a climax, however, when I found incidentally that he was ignorant of the Copernican Theory and of the composition of the Solar System. That any civilized human being in this nineteenth century should not be aware that the earth travelled round the sun appeared to be to me such an extraordinary fact that I could hardly realize it.”

The fact we are now in the 21st, rather than the 19th, century makes it even more extraordinary that there are people organising a conference dedicated to the idea that the Earth is at the centre of the universe.

Unlike Holmes though, these modern-day ignoramuses are aware of the scientific arguments against geocentrism but choose to ignore or twist them because of their Catholic faith and a rather literal reading of the bible.  Even before looking at their website I could guess that their main argument would involve abusing Relativity theory.

To understand their arguments one must know that there is no such thing as absolute rest in Relativity theory.  What this means is that saying that an object is travelling at 60 mph doesn’t mean anything.  You can only specify your speed relative to another object.  For example a probe might be travelling at 1000 mph relative to the Earth (which is where it was launched from) but at 200 mph relative to the Moon (which is its target).

One consequence of this is that if there are two rockets in space moving apart at 200 mph then an astronaut in rocket 1 may well think his rocket is perfectly still and that rocket 2 is moving away at  200 mph while an astronaut in rocket 2 will think the exact opposite and neither of them is “wrong”.

If both of them do experiments in their rockets to try to find out which one of them is “really” moving they will fail.  So long as neither rocket accelerates, any experiments they do will give identical results on both rockets.

You can probably see where our Catholic friends are going to go with this.  They argue that instead of seeing the Earth as orbiting the Sun, isn’t it equally correct to view the Earth as stationary and the Sun as going around it?

There are some problems with this argument though.  First, even if it were equally valid to look at the Earth as being stationary with the Sun orbiting it this wouldn’t show that Galileo and Copernicus were wrong and the Church and Bible right. All it would mean is that both viewpoints are equally valid.

That, however, is a fairly trivial objection.  A far more important point is that we know that the gravitational force of the Earth on the Sun is equal to the gravitational force of the Sun on the Earth.  However because the Sun is a few hundred thousand times more massive than the Earth, the Earth’s gravity is too puny to affect such a large lump of matter very much, while the Earth is affected to a far greater extent by the Sun’s gravity.  Therefore, from any reasonable viewpoint it is the Earth that is going around the Sun.

A final and more subtle point goes back to what I said earlier about our two astronauts in rockets.  So long as neither rocket accelerates, any experiments they do will give identical results on both rockets.

However, in one view (the  scientific one) the Earth is going around the Sun and in the other (the view of the religious nutters) the Sun goes around the Earth.  Objects moving in a circle are constantly changing direction – this means they must be accelerating.  Therefore someone doing experiments who is stationary relative to the Sun will get different results to someone  doing experiments who is stationary relative to the Earth.

Someone who carries out experiments in a spaceship that doesn’t accelerate will observe all the usual forces of nature: electromagnetism, gravity, the weak nuclear force and strong nuclear force.  Someone who carries out experiments in a spaceship going in a circle will observe these four forces as well as a few extra forces like the Coriolis force.  These extra forces are called fictitious forces because they do not arise from any physical property of an object (compare this to electromagnetism which arises from electric charge).  Fictitious forces only appear when we look at things that are not moving at constant velocity, in order to make the laws of physics work.

An example of a fictitious force on Earth is the Coriolis force.  If you were to fire a cannon due north from the equator the path of the cannonball would appear to curve slightly to the east.  One way of looking at this is to say the Coriolis force is pushing the cannonball eastwards.  However the real reason for the curvature is because the Earth spins on its axis.  It spins fastest at the equator and least at the poles.  A cannonball fired from the equator would be spinning eastwards around the Earth’s axis at the same rate as the equator but as it travels north (and closer to the north pole) the Earth directly underneath would be spinning east more slowly than the cannonball.  This would make the cannonball appear to curve in flight to the east from the viewpoint of someone on the ground.

This effect is something that has to be corrected for when artillery shells are fired long distances.  The fact that we can see such an effect at the Earth’s surface is a dead giveaway that the Earth is in fact rotating around its axis.  Similar but smaller forces would show that it is the Earth that goes around the Sun and not the other way around.

(It gets a bit more complicated when you realise that the Sun is actually affected slightly by the Planets’ gravity and that it is also orbiting around the centre of the Galaxy but we can ignore these minor complications for our purposes.)