Bad, bad, bad movie (I may disagree with many on this point, but it had me ready to scream on so many points). And Mythic Earth's "reality" is badly fractured. There's a concerted effort to try to integrate it, reconciling paradoxes, but there are too many inherent contradictions caused by RL dogma which is supposed to be "factual" in the setting. Mythic Europe is more of a sumover than a discrete, definite entity, with measurements and observations covering a very, very wide range of phenomena and outcomes unified by generally agreed-upon and tested "laws".
You´re not alone on that. Yet another bad rehash on the "is reality really real" theme. Not even close to the good ones in that genre... 
Which is why i like our version better, the world is like the real world with all the "extras"/"myth" added in, and different people just describe everything differently, and not always correctly so, but they dont know that, and it might still "stick together" and work so why should they care...
I would agree with Gustaf 
The only reason why today we think as the Earth orbiting around the Sun (or, rather, both orbiting around their common centre of gravity) is that it makes the whole theory simpler, and a simpler theory (that provides the same predictions) is considered "better". This very, very fundamental idea of modern scientific thought only starts with William of Occam though, a century after Ars Magica's setting, and one has to wait the 20th century to provide a more solid justification of it than William of Occam did.
Incidentally, would Foucalt's pendulum work in Mythic Europe? What about Galileo's experiment veryfing that different items of the same weight fall at the same speed? And Cavendish's experiment verifying that gravity is really between any pair of objects, not just between the Earth and something else? All these would have been extremely easy to set up in the Middle Ages without any specialized toolset. But I think that in Mythic Europe they would fail to provide the same results.
That's indeed the conclusion you get from reading Art & Academe. 
In canon Mythic Europe, they would indeed fail to provide the same results. Foucault's pendulum would not turn, heavier items fall faster than light ones, and there would be no gravity to detect in Cavendish's experiment.
The nightmare this causes in attempting to understand fundamental principles of "reality" is not to be underestimated. Aristotle was a long way from the "be all" Authority of the Classical world. Where, for example, does Archimedes (one of if not the greatest scientific minds of antiquity) fit into things? Or Eratosthenes? Or countless others? Ibn Sina (Avicenna)? He was one of the greatest commentators on Aristotle, yet he challenged many of Aristotle's assertions. Many of the great natural philosophers of the "Medieval era" didn't buy into Aristotalean infallibility.
For everyone here, I very, very strongly recommend reading "Lost Discoveries: the ancient roots of modern science, from the Babylonians to the Maya", by Dick Teresi. He delves into the many different types of scientific research--and the myths that presaged and/or confounded it--performed by many disparate cultures throughout history, including a number that fed into the thinking of Medieval Europe.
Within the context of the milieu, one should consider the fact that Hermetic Magi are, at heart, frequently empiricists. They conduct more experiments that delve into the fundamental nature of the world--and its supernatural aspects--than just about anyone else. They're the sort to pry into any kind of inconsistency, driven by curiosity and the desire to both improve their knowledge and make a reputation for themselves. The Order of Hermes may be the preeminent intellectual society of the age.[/i]
The real nightmare is that, as Galileo noticed, that heavier bodies fall faster than light ones would mean that no body can fall at all. 
But hey, it's magic 
Noted. I frankly don't think much of any pre-Greek thinkers, and don't think most discoveries leave a trace let alone become the roots of modern science - but I'd be gald to be surprised.
Yes, they are, and their experiments will confirm (within wiggle room) Aristotle as being correct because that's a core assumption of the setting. It's not realistic, per se, but neither is a secretive order of powerful wizards who're not trying to take over the world, honest. It makes for an interesting game world, as well as ensuring that there's a good reason for people to try to think differently in character without relying on modern reasoning.
That's exactly what Dick Teresi thought before he began the book. It was his intended treatise to demonstrate that almost all scientific thought descended from the Western Classical tradition. The amount of evidence refuting that contention was so overwhelming that it completely changed the nature of his work. It is exhaustive in its citations and references, opening up considerable bodies of further reading for anyone who is interested.
In some ways, the differences in the basic laws of physics are more appalling than the physical cosmology. One could otherwise treat Mythic Earth as an "artifact world" that is the product of sophisticated engineering and design within a self-contained "bubble" of spacetime. Altering something as fundamental as the laws of inertia make things turn very ugly, very quickly. Plus, it doesn't account for the development of the field of ballistic trajectories, which were well-established by the 13th Century, albeit more in the East.
About the only way I can handle the purported "canon" Mythic Earth posited in 5th Ed. (prior editions deliberately left open a range of potential structures, depending on the exact Saga parallel) is to treat Mythic Earth as a different type of holographic construct (in the sense that it is composed of information embedded in a field or brane{s}, depending upon what type of cosmology one favours) with the mediating information handled in different units. On some level, it is actually easier to treat it as a "virtual" construct with no actual mass as would exist in a "real" universe.
Although one has to stretch the bounds of causality in any version of Mythic Earth, it is simpler and, I believe, preferable to view it as a "real" world within a "real" universe that is affected by supernatural forces able to act on the material plane. Within the scope of their action, normal physical laws may be temporarily altered, but the BASE level of "reality" functions according to essentially the same laws we know.
Pantheistic Soliphism. Solves all possible conradictions.
I do think that those differences are part of what makes the game interesting. Forget about real-life modern science for a while.
Eh? Of course it is an artificial construct with no true physical reality: it is a game world. Part of the game is to cast your characters' actions within its framework.
Don't overthink it.
Fine. Just keep in mind we're probably not playing the same game.
After reading some reviews, I decided I can't afford to read this book. It seems the author, Mr. Teresi, is not an historian and makes awful gaffs like claiming the Greeks believed that the earth was flat (!). He seems to be a "skeptic" of the kind I don't like - the kind the embraces conspiracy theories, and some comments also say he has an anti-Western axe to grind. I am afraid reading this book will pollute my mind with mistaken facts and ideas.
I have instead added "The Beginnings of Western Science: The European Scientific Tradition in Philosophical, Religious, and Institutional Context, Prehistory to A.D. 1450", by David C. Lindberg, to my shopping cart. This seems like a serious historian looking at similar issues. Next time I make a book purchase, it'll probably be on it. (So I should read it in, oh... a few years )
I cast my vote for Jerusalem style.
a) Most churchmen at the time probably believed it (I am not talking about educated monks but about your unfriendly neighborhood priest who can barely read the Bible (captitalized for canonicity's sake).
Plus don't tell me the average illiterate knight or farmer didn't agree.
So in a world that depends on consensual reality (like an rpg world) consensual reality is reality.
Making the earth spherical is magic and scary and powerful magic too. It is also obvious nonsense: People on the lower side would fall down - and down means hell, doesn't it?
The review that you cite seems to have only looked at the introduction to the book instead of the body of the work, its 50 pages of endnotes, and 11 pages of densely-written bibliography. His background is science writing, including "The God Particle", perhaps the single best introduction to particle physics that I've ever seen, written with Dr. Leon Lederman, Nobel Prize Laureatte in Physics. The reference to the Greeks and the "flat Earth" applies not to the period of Hellenic natural philosophy and science, but the Bronze Age era, predating the development of many of the physical models with which we are familiar. As for the anti-Western "axe", allow me to supply you with a quote from its pages (given for purposes of critical review, and thus complying with its copyright):
This shall be a book of "unkempt historical details"--a tale of the non-Western roots of science. I began to write with the purpose of showing that the pursuit of evidence of nonwhite science is a fruitless endeavor. I felt that it was only responsible, however, to attempt to find what meager legitimate non-European science might exist. Six years later, I was still finding examples of ancient and medieval non-Western science that equaled and often surpassed ancient Greek learning.
My embarassment at having undertaken an assignment with the assumption that non-Europeans contributed little to science has been overtaken by the pleasure of discovering mountains of unappreciated human industry, four thousand years of scientific discoveries by peoples I had been taught to disregard.
Check it out at a library and skim through its pages to find out the details for yourself.
"Consensual reality" is closer to the original version of Mythic Earth in the earlier editions of the game. It is, however, fraught with its own faults--like the fact that if majority Belief rules, then the Earth would be under Chinese or Indian "reality" because there have almost always been far more people in Asia than in Europe. So, no "One True God" (of whatever aspect) for starters--unless you're looking at Brahman/Vishnu/Shiva, or maybe the Buddha (for Mahayanists; Therevadans would remind you that Siddharta was "just a man").
"Reality" is both objective and subjective. It's entirely true that we're dealing with a fictional world and universe, however, the more it purports to be based on the materials of previous eras, the more closely we should examine what those details might be. I love the strong, historical elements of the game, coming from a background of both science and history. At the same time, I feel I must object to the assertions that certain aspects of Medieval dogma prevailed over everything else, especially when people in that era challenged it. Contention, debate, discussion, and analysis are far more interesting.
They did, and the truth of the canon setting is that they were wrong. It's a weak argument to claim that the world doesn't make sense if, for instance, momentum doesn't behave as Newton claimed when a suitably good painting of a man can be used to aid his murder from a different continent. The potential for argument is still there, all that has changed is the underlying truth.
For myself, I find the shift in mindset to be great for helping me to seperate my characters knowledge and my players. It also allows for some wonderful things in setting. The ReCo(Pe) spell from Art and Academe which turns a man's arm to a mass of writhing maggots, for instance.