Eli5 How do we still see the same constellations after thousands of years?


We are a ball of rock spinning around the sun in a galaxy that is also rotating and moving through space. How is it that we still see the same constellations as seafarers thousands of years ago considering all of this movement?

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Space is big. You just won’t believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.
*- Douglas Adams, The Hitchhiker’s Guide to the Galaxy*

In short, space is so large, that even the speeds and times we have been experiencing, we don’t observe hardly any differences. Things are just that far away.

Because space is massive. Like, however big you think it is it’s bigger than that.

When we’re talking about cosmic distances and timelines it takes *a lot* before you see any change.

The galaxy would take hundreds of thousands of years to make a full rotation, and figure that even then the farther out stars are not going to change a whole lot relative to each other. The <10k years of written history we have would only count for a few degrees of rotation, and even less of a shift between individual stars.

The short answer is that space is *really really* big and even stars moving very quickly relative to each other have a *very* long way to go before their movement is noticeable to us. A few thousand years is nothing to deep time. There *have* been changes over human history, like some stars going supernova and the resulting nebulas forming. The constellations have changed just a little bit. But it takes tens of thousands of years for significant changes.

The constellations do drift, but extremely slowly relative to everything within our solar system. The galaxy is so much bigger than the solar system, so relative star motions are extremely slow on a human scale. It will take tens of thousands of years for stars to move modest amounts relative to each other.

One thing that does change a bit faster is the axis on which the earth rotates. It’s a bit of a coincidence that we have a star nearly aligned with the north axis, resulting in a northern star. About five thousand years ago a different star, Thuban, was close to where our axis pointed, and so it always pointed north and didn’t move in the sky. Slowly as our axis wobbles, the location that the sky appears to rotate around moves, from Thuban and now to Polaris, the current north star. But most of the time there isn’t any notable star particularly close to that spot in the sky.