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Our Earth is spinning and also orbiting the Sun, so how do the constellations appear fixed and constant? Almost all galaxies and stars are said to be fixed relative to constellations.

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    $\begingroup$ This question should really be in the astronomy stack exchange. Also it would benefit a lot from spellchecking. $\endgroup$ Mar 11, 2018 at 18:22
  • $\begingroup$ @SteveLinton that's probably true. Consider holding off answering it until it gets there? I'm not sure (someone correct me if so), but I believe you might make it more difficult to migrate by posting an answer here. $\endgroup$
    – uhoh
    Mar 11, 2018 at 18:24
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    $\begingroup$ I answered first, then realised which site I was looking at. Sorry. $\endgroup$ Mar 11, 2018 at 18:25
  • $\begingroup$ Yeah - me too. Nevermind. Can it be moved? $\endgroup$
    – rghome
    Mar 11, 2018 at 19:50
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    $\begingroup$ Short answer: they are not constant. The constellations actually do distort a little as the Earth goes around the Sun. It is just that this slight wiggle is so small that it is not at all perceptible to the human eye, and this is why people first thought that the stars were fixed on a sphere. Even measuring the looping apparent motion of the planets was not achieved until the 16th century. By the end of the 19th century, stellar parallax had not been measured on more than 60 objects. $\endgroup$
    – MichaelK
    Mar 12, 2018 at 15:34

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The short answer is that the stars are very far away. The rotation of the Earth causes all the stars to appear to rotate at the same speed (very nearly) so that their positions relative to one another do not change. The movement of the Earth about the Sun causes the position of the Sun and the other planets to appear to change compared to the stars, but its effect on how the stars appear is too small to measure without sophisticated instruments. In fact the nearer stars do appear to move back and forth slightly each year compared to the more distant ones, although the change is too small to see without a telescope. We use that change to measure their distances.

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The difference between the distance from the earth to the sun and the distance from the earth to the stars is so huge that any movement of the earth around the sun is not detectable by the human eye.

This question is often asked in the context of the pole-star and why it stays fixed. The pole-star is 430 light years away. The sun is 8 light minutes away. The difference is 430 x 365 x 24 x 60 ÷ 8 = approximately 28 million.

In other words, the effect is the same as looking at a car 2.8 kilometers away and noticing that it moved 1 millimeter. In imperial units, looking at a truck 4.4 miles away and noticing it moved a tenth of an inch. This is way beyond the ability of the human eye to see.

There are closer stars, the closest being about 4 light years, so the apparent movement would be 100 times larger. But an object moving 10cm at 2.8km (or 10 inches at 4.4 miles) is still not noticeable, especially since there is no background to act as a reference.

That said, these movements can be detected by astronomers, and measuring the distance to a star by its apparent movement (parallax) during the year is a commonly used technique. You can see this post Furthest object determined by parallax, which discusses the furthest object measured by parallax (at least thousands of light years away).

As for the rotation of the earth on its axis: that does cause the stars to move. You can easily find time-lapse photos online of the night sky, or you can sit out yourself on a clear night and watch, but since the movement is slow (the same rate as the sun moves) you will have to be patient.

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  • $\begingroup$ Your link gives distances of around 10,000 kpc. or 32,000 light years - quite a bit less than "millions" $\endgroup$
    – hdhondt
    Mar 11, 2018 at 23:46
  • $\begingroup$ Though not in the main answers, hidden in the comments of the last answer is a reference that indicates millions. $\endgroup$
    – rghome
    Mar 12, 2018 at 13:25
  • $\begingroup$ And the article linked to only mentions parallax measurements in the kpc range, not Mpc. $\endgroup$
    – hdhondt
    Mar 12, 2018 at 22:22
  • $\begingroup$ Ok - thanks. I don't know enough about it, so I will side with caution and update my answer. It is only a side-note to the answer. $\endgroup$
    – rghome
    Mar 12, 2018 at 23:11
  • $\begingroup$ Why does the metric system use cars, and the imperial use trucks.... Just curious. $\endgroup$ Mar 13, 2018 at 7:42

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