Heliocentrism: A Thought Experiment

"There is talk of a new astrologer who wants to prove that the earth moves and goes around instead of the sky, the sun, the moon, just as if somebody were moving in a carriage or ship might hold that he was sitting still and at rest while the earth and the trees walked and moved. But that is how things are nowadays: when a man wishes to be clever he must . . . invent something special, and the way he does it must needs be the best! The fool wants to turn the whole art of astronomy upside-down. However, as Holy Scripture tells us, so did Joshua bid the sun to stand still and not the earth."
 - Martin Luther, 1539 

“eppur si muove” - Galileo?
Thought Experiment: Imagine going outside at sunrise on a clear day and sitting facing the south. Observe the Sun move across the sky until it sets. Also observe no apparent movement of Earth during this time. For example, you may observe the absence of a perpetual wind indicating that you are moving, and a ball thrown straight up into the air doesn’t land behind you. The most obvious conclusion: the Earth is stationary and the Sun (and other planets and stars) move around Earth! Right?

Every sunbather or person trying to stay in the shade has made this type of direct observation on their own, but three quarters of Americans think a heliocentric model of our solar system is true - with elliptical rather than circular orbits for the more savvy among us...acknowledging also, if necessary, that the focus of our orbit is not the Sun but the barycenter of the Solar System, which also orbits the Milky Way, yada yada. Or to put it more technically, most of us think that there is no inertial reference frame in which the Earth is stationary at the center and the Sun moves in a circle around it. But why is this counter-intuitive (and Biblically contradicted) fact about our world so widely accepted today? Do you just believe it because that's what you were told? What evidence confirms it?
"I want to argue that clear thinking, combined with a respect for evidence — especially inconvenient and unwanted evidence, evidence that challenges our preconceptions — are of the utmost importance to the survival of the human race in the twenty-first century, and especially so in any polity that professes to be a democracy."
 - Alan Sokal
Framing Questions:
  1. What evidence convinces you that Earth moves relative to the Sun?
  2. What evidence supports the idea that Earth is spherical v. flat?
  3. What evidence supports the idea that the stars are much further away than the Sun, Moon, and other planets?
Explore the (mathematical) models and observations that they predict related to the three main lines of evidence below. Note how no single line of evidence conclusively proves heliocentrism but can lead to non-parsimonious geocentric models with more epicycles (ex. to explain the retrograde motion of planets). After all, it wasn't until Kepler added elliptical orbits to the Copernican model that it better fit the best data we had at that time - Tycho Brahe's very detailed and comprehensive astronomical observations. Galileo's discovery of moons orbiting Jupiter and the phases of Venus got him into some trouble with the Church on this issue, but it was really Kepler's elliptical orbits and his laws of planetary motion, which better predicted Brahe's data, that led to a simpler heliocentric model being widely regarded as true. Not until 1838 did we have the first successful measurements of stellar parallax (Friedrich Bessel), and by then almost every educated person was already convinced that heliocentrism was true.

In hindsight, these are the main lines of evidence that are usually offered to directly support heliocentrism today:
But the point of this thought experiment is not to "prove" (or "disprove") heliocentrism, which many of us would be hard pressed to do, but to explore what and how we (provisionally) know things about the world and how we (iteratively) find more things out - using (mathematical) models that make predictions which we then subject to testing or observations to see if they are true (real).
"It is important to note that well-tested theories in the mature sciences are supported in general by a powerful web of interlocking evidence coming from a variety of sources."
 - Alan Sokal 
Takeaways: Geocentrism is one of many valid reference frames (sunrise and sunset, star charts), and unless we want to travel to another planet in our solar system most of us will never need a heliocentric model (much less a galactic coordinate system to talk about the motion of galaxies). And yet science shows us that Geocentrism is as wrong as a Flat Earth (also a valid reference frame - for building houses for example).

References: Heliocentrism & the Copernican Revolution

Primary Historical Sources:
Modern Physics & Astronomy:

Modern Philosophy & History:

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