Whilst I understand why stars move across our skies,(Earth rotation) I struggle to understand why they do not appear to move relative to each other.

To explain: Take the best know constellation the Plough. It is a familiar site to most people. But it has been there apparently un-changed for years. If we (I mean earth, moon, stars and everything else ) are hurtling through space, then surely changes should be apparent unless we are all travelling in the same direction at exactly the same speed.

  • $\begingroup$ You picked a bad example! Most of the stars in the Plough are in fact part of the co-moving UMa group and so are moving in the same direction at the same speed! But the answer is basically that they do, just at a rate which is imperceptible to the human eye (but quite measurable with telescopes). $\endgroup$ – ProfRob Jan 14 '16 at 12:26
  • $\begingroup$ cseligman.com/text/sky/starmotions.htm en.wikipedia.org/wiki/Proper_motion (and, technically, the Plough/Dipper is an asterism, not a constellation) $\endgroup$ – user21 Jan 14 '16 at 15:30

They do move - just far too slowly for you to detect by eye even over several human lifetimes.

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Even the closest stars are a very, very, very long way away so their apparent movement relative to each other is going to be very small.

You can see the same effect when looking out of a moving vehicle through the side windows. You see the objects closest to you rushing past, but objects on the horizon appear to move much more slowly. Now, scale that up by many orders of magnitude to interstellar distances and you'll see why the stars don't appear to move relative to each other.

There's software that simulates the night sky and you can run time backwards and forwards - if you wind it far enough you'll see the constellations change.


Here's a nice image of Barnard's star moving against the stellar background: enter image description here From this interesting website: AstroWright


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