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?



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

The distances involved when talking about space are insanely huge. There’s a reason that when we talk about big numbers we call them “astronomical”. Two stars that seem to be inches apart in the night sky are actually billions of miles apart. So if they move another 20 billion miles over a century, they’ll have moved an imperceptible 1/10th of inch by your view.

They are moving though. If you could role the clock back 35,000 years to the time when “we” (as in the first people who genetically resemble modern humans) first appeared, night sky would look quite a bit different.

The simple fact is that space is so vast, that the entirety of written history has happened in the blink of a cosmic eye. We human beings only have a direct account of what the night sky has looked like for a few thousand years, which just isn’t long enough on the a cosmic time line for things to have noticeably change.

They aren’t, if you mean the same place of the sky. The constellations move throughout the night/year. If you take a picture straight up every night at the same time, you’ll see the constellations move over the course of a year.

They do move… its just incredibly slow when compared to the objects moving around you.

Next time you’re in a car or bus driving… look out into a field. The closer an object is… the quicker it speeds by, the further out it is… the slower it moves.

However that being said… its moving slow to where we are in our own galaxy. If you were to go there and stand on a planet and look back at us… you could say the same thing.

In addition to the other comments, we can only see stars within our own galaxy. Even if we could see further, the rotational period of the galaxy is stupid long. A quick google shows it as around 240 million years. One of the earliest records of human civilization goes back around 5500 years, and the earliest Homo sapien skeleton was dated back to around 200,000 years.

If we translate those into into % of a rotation or degrees, we get like, .002% or .007 degrees of rotation for almost all of recorded history, or .08% or .288 degrees in all of known human existence. The amount the constellations would vary from other galaxies would be really small.

Disclaimer: I am really bad at math.

A decent analogy would be to think about living in Manhattan – you might move around the city all over but whenever someone asks you what direction San Francisco is, it’s always realllllly realllly close to the exact same direction because it’s “so far” away. But stars are a lot further.

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).