True, but there are frames of reference that make understanding (and the math) vastly simpler. I can calculate the orbits of the Saturn's moons using my location on earth as the origin. It will take me a lot of work, but I can do it.
It's a fun exercise to reframe the laws of physics in terms of your "stationary" frame on Earth and I recommend it to everybody. Consider the speed of light in the Andromeda galaxy... except under these physics we can't speak of the "speed" of light, but the permissible velocities as a function of the location in question.
Running through this exercise with some honesty can give one a greater understanding of why our physics is framed the way it is, and why it is that while all sorts of reference frames are valid, "inertial" reference frames are still important on their own merits.
The Earth is demonstrably not a "stationary" (inertial) frame, in the sense that it's constantly accelerating.
Is there a deeper meaning to "Consider the speed of light in the Andromeda galaxy" that I missed? The speed of light is known to be constant in every reference frame.
Your two paragraphs are connected to each other. The speed of light is known to be constant in every inertial reference frame. But reference frames are not required to be inertial, which is precisely why we call them inertial reference frames; the word "inertial" is not redundant.
You can reformulate all of physics into your Earthly non-inertial reference frame. You can formulate all of physics into a reference frame in which you personally are always stationary! Nothing stops you from doing it, and the physics will work, as much as they ever do (i.e., we know something's wrong with our theories). To the extent that the result is a hideous monstrosity, well, such is my point. Pondering the nature of that hideous monstrosity is something I think worth doing, at least for a bit. Not to the extent of actually writing the equations, though. It brings clarity to why inertial reference frames are so important that we almost consider "inertial reference frame" to be a single atomic word, because non-inertial reference frames are in general not very useful. (In specific they can be.)
The earth is still stationary with respect to itself. The Universe is accelerating around it. In the case of Saturn's moons the Sun and Earth are not significant factors, if I use the Earth as my origin I have to account for those anyway, but if I were to use Saturn as my origin I could safely ignore them (probably - I could come up with sci-fi reasons that they matter).
