Why don't stars move in the night-sky as the moon does?

  • I want to know why don't we see a change in the position of stars during the night while we do see a change in the position of the moon. I've checked other online sources and some answers say that it is because they are too far away, is this true?

    I mean for example if i'm laying down on the earth since sunset looking at the stars, and let's say I'm close to the equator, no matter the distance, I'll be looking at the stars and earth will turn about 180° until sunrise, so my angle of view will be about 180°.

    Why not just look up for more than twenty minutes. You will see the stars move. Or if you don't have the patience, use a long exposure on your camera and look at the lovely arcs, as @Tildal shows.

    This question appears to be off-topic because it is based on amazingly incorrect symptoms which the op could learn himself if he actually looked up.

    @Rory: i guess i'll try a long exposure pic using my nikon, thanks for the idea

    It's a valid question, the fact that he is wrong in what he thinks doesn't make it less so. It is because he doesn't know that stars do move that he asks, so the answer, as @TildalWave did, explains so.

    I didn't want to speculate for the reasons of OP's false assumptions in my answer, there's simply too many possibilities, but one that's fairly common is that we tend to be predictable and periodic at that too, so we'd look at the night sky at more or less same times each night. Meaning that stars would be, on a daily basis (but not year long), more or less at the same position. This reminds me of a similar reason why Mercury was long assumed to be tidally locked with the Sun, since it's in 3:2 spin-orbital resonance with Earth and we always observed it facing the same side relative to Earth.

    Stars move about a degree every four minutes. It takes the moon about 4.2 minutes to move that angle.

  • TildalWave

    TildalWave Correct answer

    8 years ago

    This is referred to as diurnal motion, due to Earth's rotation on its axis, and it affects apparent motion of stars differently depending on their position on the skies relative to the axis of Earth's rotation. For example, on northern hemisphere, the star that appears not to move at all is positioned so that the earth's axis of rotation points directly towards it, and is called a Polaris due to its role as a pole star. Stars further away from true north (not to be confused with magnetic north), will appear to prescribe a circle around it as the Earth completes one rotation on its axis:

                                 enter image description here

                                    A long exposure photograph of the night skies, showing their apparent motion

    So what you assert, that the stars don't seem to move isn't exactly true. Stars closer to celestial equator will move at equal apparent radial velocity as any object fixed on the night skies would (so at the speed of Earth's rotation), while the stars closer to celestial poles, while maintaining this radial velocity, will prescribe a smaller circle and appear to be more stationary. And the stars exactly aligned with the Earth's axis (like Polaris, but there isn't any such exact counterpart on the southern hemisphere), would appear not to move at all.

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Content dated before 7/24/2021 11:53 AM

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