Ever look at an organization chart? It shows the reporting relationships among employees or members of some organization. Usually there’s a box at the top for the head honcho. There may be several boxes below that for people who report to the honcho, and more boxes below those for their employees.
We might look at this and nod, and think “Yes, this is my understanding of the organization.” It may even have some predictive power. It might indicate who to ask for approvals. A box labelled “T.B.D.” suggests that someone will be transferred or hired to fill that opening.
But it’s just a model! It’s a logical construct … a way to visualize relationships. People don’t actually work in boxes. (Well, they do in some organizations.) And there are no lines connecting them. The org chart is an abstraction of certain properties of how people relate to each other.
Likewise, our concept of how the planets orbit around the sun, and the moons around the planets, is just a model … an abstraction. We think of these huge physical bodies all travelling on neat ellipses year after year, but there are no neat ellipses. Actually, the moon orbits the earth, which is orbiting the sun, which is also travelling through the galaxy, which is moving away from the center of the universe. So instead of ellipses, the planets and moons follow intertwined helical paths, spiraling through space.
And even that is just a matter of point of view. In fact, the heliocentric model of planets orbiting the sun is no more real than the geocentric model of the sun and planets travelling around the earth. It’s just easier to visualize and to do the math. Really, heliocentrism is not so much a discovery as an invention … an invention of a model that’s easy to predict from.
These models need constant maintenance. New discoveries make us revise these models, or even throw them away altogether. Relativity may have to be refined or replaced as a result of the new evidence that neutrinos travel faster then light.