eli5 If we are a spinning ball – spinning in our own solar system – which is spinning within an arm of a galaxy – why are the constellations in the same place?

Question

eli5 If we are a spinning ball – spinning in our own solar system – which is spinning within an arm of a galaxy – why are the constellations in the same place?

In: Physics

Answer 1

As others have said, because they’re so far away that the differences are imperceptible to the naked eye. They actually do move around a little, and we use that as one of the methods for measuring astronomical distances. Basically, we measure how far we see a star move over the course of half a year (so halfway through one orbit around the sun), and then use math (trigonometry) to figure out how far away it is.

You can see a version of this effect yourself in everyday life. Close one eye, and focus the other on a thing that’s fairly close by (like a meter or so). Then switch which eye you have closed, and it will seem like the object moved slightly. This is because your eyes look at things from slightly different angles, and your brain then assembles the two images into one (which is one of the ways we perceive depth).