Peter spoke up again:
"Is there no way we could prove it was the earth moving?"
"Optically that could be done, if at all, by measuring parallax from Mars or something."
"OK, how would that prove it?"
"If Earth and Mars both move in simple ellipses around the sun, then parallax from Mars will be as simple as parallax seen from Earth. It will however be slower and bigger, since Mars has a greater orbit that takes longer to complete."
"And if not?"
"If the Sun moves around Earth and Mars around the Sun, and alpha Centauri really waggles ... well depends on distance. If distance were the same, that would look the same from Mars as in Heliocentric case, but ..."
"Depends on whether alpha Centauri is further away or closer than our theory says. If it is closer, its waggling is smaller than Sun's orbit around us, and it will count for less neutralising than the complex movement of Solar orbit with Martian orbit."
"So there are basically three possible outcomes of measuring parallax from Mars:
- you prove Earth stands still and alpha Centauri closer than four light years
- you prove earth stands still and alpha Centauri further away than four light years
- you prove alpha Centauri four light years away, without proving whether earth or sun moves each year."
"Yes, that is it."
"For alpha Centauri, that is?"
"There are about ten thousand stars that show annual waggling known as parallax at all."
"So if all of them moved as the Heliocentric theory states, even when seen from Mars?"
"It would probably mean the Earth moves."
"If we accept the possibility that stars could be moved about by some spirits, by angels or something, then we could even so not exclude that the distances believed by astronomers were right for those ten thousand stars, but that they were moving in time with the sun around their places in a Heaven moving around us."
"And if one star of those, like alpha Centauri or another one ...?"
"Especially alpha Centauri! If anything with a very clear parallax shows something else from Mars than we would expect from Heliocentrism, there is less doubt about the measurement, and so a clearer case ..."
I shouted out: "... for Earth standing still, right?"
"Yes, but why do you shout?"
"Oh it is so totally exciting, Ed. And still just a bit confusing!"
"You agree, I assume, that when we are in a boat, the further away we are from something the less it seems to move? The trees on the river move very clearly, the mountains seem still to us, or if you like and say you know you are moving they seem to be following us, right?"
"On the other hand, if something far away moves in our sight, say the fifteen degrees that you measure with your outstretched hand, it moves more the further away it is?"
"So if we are four light years from alpha Centauri - the distance it takes light four years to run through ..."
"You mean light has a measured speed? It is not in one instant?"
"At least they say so, but even if they are wrong that only means 'four light years' is the wrong word for the distance, but it is still convenient."
"So if it is four light years away, either we move around Sun which stands still along with alpha Centauri, or alpha Centauri and Sun move same distance exactly parallel in two different places, and the Sun around us?"
"But if it is eight light years away, we do not move and it moves twice the distance of the Sun but not around us?"
"And if it is only one light year away, we do not move and it moves a quarter of the distance of the Sun but not around us and so on if it is even closer, like a light month, and so on?"
"Brilliant as a brick, Lu!"
"And observing alpha Centauri from both Earth and Mars is like observing some distant peak from two different boats to make sure if it is a Mountain or a Giant's Head?"
"Exactly, but in this case also to make sure the one observation point 'is really a boat' so to speak. If it is not, if it is in the unmoving centre of a daily rotation from moon up to stars, we cannot know the distance from itself - from ourselves that is - alone to alpha Centauri."
"Is it important to know it?"
"Not unless you plan going on a space voyage."
"And I suppose the waggling of this Centaur is well measured?"
"Well ... if some stars show 'backward parallax' as Father once told me, it is supposed to be due to mistakes in measuring."
"So we have not got any further than Galileo proving Robert Bellarmine wrong, then?"
"Not really, no."
"What if he was right?" I asked.
And Peter to answer:
"Then we cannot say the Popes were wrong to condemn him for heliocentrism."
"But wasn't there a cruel process with torture and all that?"
"Not for him, no. He was threatened with torture but he was too old according to the rules actually to be tortured: old men and cardiac patients were never tortured."
"But the burning him at the stake must have hurt pretty much, I bet."
"Sure, if he had been burnt, he was not. Not if our professor in history was right. He recanted and then spent the rest of his life in prison at home."
"Why is everyone saying he was then?"
"Because Giordano Bruno was."
"And he was a Heliocentric?"
"Who believed in many different solar systems and who believed each of them had its own maker or even just its own soul - and that God was just the soul of our solar system."
"Well, it does not sound as if he was a martyr for Christian truth then."