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My question is pretty simple. How would the night sky change if you were in a different planet within the same solar system? Specifically the fixed stars. Would the constellations be essentially the same? Different? Of course, having more or less atmosphere will effect the night sky, but outside of that is the only real difference in the night sky between northern and southern ends or do different planets in the solar system have unique perspectives? Could it be assumed that every planet within an arbitrary solar system as isolated as ours would have roughly the same night sky throughout the system?

Note, I do not care about how planets would look different from the perspective of other planets, only the fixed stars.

Mark Morgan Lloyd
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skout
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3 Answers3

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Yes, the night sky formed by everything outside the solar system will look the same from every planet in our solar system. Similarly an observer in any other system will not see a difference between observing the sky from one or another planet in the very same system (yet of course different from what we see or from yet another stellar system).

For a comparison of scale: The orbit of Earth is 300 million kilometers in diameter. Neptune's orbit is about 9000 million kilometers in diameter.

Yet to the nearest star (Proxima Centauri), it is about 4.2 light years from here. That is at a distance of 39 million million (no typo!) kilometers from here, thus more than 10,000 times further than anything in our solar system from any other thing in our solar system.

Another way to look at is when using the distance unit parsec, short for parallax second. One parallax second is the distance of an object when it changes its apparent place in the sky by one arc second, that is 1/3600 of a degree when the Earth moves around its orbit. This is one of the primary ways to measure distances in our neighbourhood (e.g., with Gaia) up to ~10000 light years with a reasonable accuracy of about 10%.

The nearest star, Proxima Centauri, has a parallax of 0.77 arc second. Thus with the Earth's motion it changes its apparent place over the year by about 0.77/3600 of a degree. If we now change our orbit to Neptune's orbit, thus at 30 times Earth's distance from the Sun, then Proxima Centauri would change its apparent place in the sky during one revolution of Neptune around the Sun by 23/3600 of a degree or about 1/150 of a degree—but over a time span of 164 years. If you stretch out your arm and look at one of your fingers, it has approximately 2 degrees angular diameter; envision 1/300 of that: a difference would only be visible if you have immediate comparison when placing these two images above each other and closely looking for differences, but not looking at it one day and the other on another.

So to the unaided human eye, who travels from planet to planet, this difference would not be perceptible. But of course, if you are an astronomer and measure the positions of objects against the far far distant background, you would see tiny changes for the nearby stars.

planetmaker
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  • A huge improvement would be that probably we would have a really, really good sky from a solar system body without an athmosphere and humanity. – peterh Jul 19 '23 at 04:46
  • So succinctly, constellations change but far too subtly to be noticed by anything but precision equipment. This was not confirmed until the 1830s. – Mark Morgan Lloyd Jul 19 '23 at 07:42
  • Of course, the unit Parsec would have a different value on planets with a different orbital radius. – Paŭlo Ebermann Jul 19 '23 at 16:54
  • @PaŭloEbermann No. A unit is a unit and they are universally defined. Just because I fly to the Moon, the length of the meter doesn't change. So does the parsec - which is defined as the viewing angle with a base line of one astronomical unit, that is 149597870700m by definition. – planetmaker Jul 19 '23 at 17:20
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    @planetmaker: I think the point is that a civilization from a distance planet would define its "parsec" differently, basing it on the radius of their planet's orbit. (And also on how early civilizations on their planet decided how to divide the circle into angles.) – Michael Seifert Jul 19 '23 at 20:21
  • @peterh If you change that to large cities instead of humanity you would be accurate.. Though a similar effect can be achieved by going out to somewhere rural away from the big cities. – Questor Jul 19 '23 at 21:36
  • "(or any solar system we know)": This is ambiguous. I assume you mean "or from any two planets in the same solar system". But it could equally mean that the night sky looks the same from all solar systems, which of course is false. – TonyK Jul 20 '23 at 10:45
  • @Questor Yes, but even there is an athmosphere and, as far I know, one has to be really far from anything to get a really good sky. Just going to a local hilltop helps a lot, but not everything. – peterh Jul 20 '23 at 12:44
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    Individual constellations will be (nearly) unaffected, but the overall "night sky" will depend quite a lot on axial tilt. – Ben Voigt Jul 20 '23 at 20:24
  • The numerical results are very interesting, but it might be improved by adding an analogy. For example, if you're in a garden, looking at some mountains many tens of kilometres away, if you walk across the garden, does the view of the mountains change? – Oscar Bravo Jul 21 '23 at 06:39
  • Instead of saying million million kilometers, you could just use Pm. Simpler and shorter. There is a reason SI-units can have prefixes. – 12431234123412341234123 Jul 21 '23 at 14:00
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No, it will change! The amount it will change will depend on how far the star you are looking at is from Earth. Here is an image comparison of the same field taken from the New Horizons probe (picture taken 50% further from the Sun than Neptune, but still in the general "Solar system")[1]:

enter image description here

The star you see blinking back and forth is Proxima Cen, the closest star to Earth, and every other star in the night sky will move less than this. The field of view of this image is about 11 arcminutes on a side (1/3rd the moon diameter). The motion of the star is about 1/20th of the field of view, so the maximal change in star position will be about half the size of Tycho crater:

enter image description here

Look at the background stars, however. These are not moving around too much, so from a human perspective, there would be no change to the night sky (even Proxima Cen's motion would be unnoticeable from a human perspective).

However, in reality the position of every star in that image is changing slightly, and these changes would be readily detectable by a modern astronomical instrument. The precision of modern astrometric instruments (those meant to measure stellar positions) is about 30 microarcseconds [2], meaning that from Earth's perspective stars closer than about 1AU/(30 microarcseconds) = 33 kiloparsecs have detectable motions. If we expand to Neptune's orbit, this expands by about ~30 times, meaning ~1000 kiloparsecs, which safely encompasses the Galaxy's diameter of 30 kpc.

[1]https://www.nasa.gov/feature/nasa-s-new-horizons-conducts-the-first-interstellar-parallax-experiment [2]https://www.cosmos.esa.int/web/gaia/science-performance#astrometric%20performance

Mike
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    Interestingly, this question has an answer saying "yes" and another saying "no" and both are correct. That's the problem with asking "if" instead of "how much". – Pere Jul 19 '23 at 19:23
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    @Pere Kind of like asking if the view from your left eye is the same as from your right eye. They're essentially the same, but different enough to determine distance using parallax. – Barmar Jul 20 '23 at 12:36
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    @Pere and as answer to "how much" you get the two answers "not a lot" and "quite a bit" ;) It always depends on from where you're coming (or going), what is your reference or norm. In this particular case I interpret asking for 'constellations' the same is asking for the naked eye observer, not for an astronomer equipped with a precision-pointing telescope and plenty of time; yet I find Mike's answer very helpful, too. – planetmaker Jul 20 '23 at 18:37
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    This answer is not correct. The question isn't will they change when viewed with precision equipment. He asks, "will the constellations be essentially the same", (Yes) and would we have "roughly the same night sky", (also yes). – Oscar Bravo Jul 21 '23 at 06:36
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    @OscarBravo Eppure si muovono! – Mike Jul 21 '23 at 08:33
  • This answer is extremely misleading for the question asked. The example given (first image) is absolutely wonderful. However the bold headline at the start should be more like ... "Only by an extremely tiny amount, as you can see from this amazing example ..." – Fattie Jul 21 '23 at 11:20
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    One way to look at this. The question is about the night sky. Is the night sky the same throughout the system. The question is NOT about "is one incredibly tiny, massively magnified, microscopic close-up ... the same". – Fattie Jul 21 '23 at 11:38
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I'm adding a correct and clear answer, as,

  • the other two answers given here are very advanced and, thus, could be misleading.

How would the night sky change if you were in a different planet within the same solar system?

There is absolutely no change, whatsoever.

All of the constellations, etc, that you know, are absolutely identical with absolutely no change.

Would the constellations be essentially the same?

They would be exactly the same with absolutely no difference

Could it be assumed that every planet within an arbitrary solar system as isolated as ours would have roughly the same night sky throughout the system?

That's correct, but the word "roughly" is wholly wrong. The night sky is exactly the same from all planets in a solar system.

What if you have incredibly advanced, sci-fi like instruments?

Consider this question for a moment: can you see any difference in the night sky the same from Tokyo versus London? In fact yes there is an (incredibly tiny) difference in the position of stars, when you're a few thousand kilometers apart.

Thus obviously, sure, there is an incredibly tiny difference in the position of one or two stars, when you consider Earth versus another planet.

Sure, obviously, if you have mind-numbingly advanced alien-like equipment, and decades to study, hundreds of thousands of stars, yes, you could find one or two TINY differences.

So what differences could you see with the naked eye?

To be totally clear ...

There are no differences in any meaningful usage of the English word "no".

You can see about 6000 stars from our solar system.

If you move your position around our solar system:

Hold up three or four human hairs at arm's length. One or two stars might move by that much, that's it. This is absolutely non-discernible to the human eye.

Fattie
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    You jump from "naked eye" to "sci-fi technology". In the vast gap between those is real modern astronomy, itself spanning a couple of orders of magnitude in resolving power. There's no doubt that a real big observatory could see something - thus "exactly" is wrong. But could a portable telescope? – Chris H Jul 21 '23 at 12:44
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    "Absolutely no change, whatsoever" is not "one or two stars move a miniscule amount"; your answer is not "correct and clear", it is self-contradictory. – David McKee Jul 21 '23 at 16:01
  • Chris - real modern astronomy is sci-fi technology (I thought that was obvious and the point, but I'll edit) – Fattie Jul 21 '23 at 19:35
  • David - not at all. "Absolutely no change, whatsoever" to the "night sky". Say there are a few printed photos on a table. We ask, are two of them the same photo. Two are the same photo of the same cat - the answer is "yes". If we then ask, are (say) microscopic differences in the (say) pixels, ink blocks in the paper weave - that is a totally different question. – Fattie Jul 21 '23 at 19:39
  • Also, David, would you remove your comment, as you rather strangely quote something I didn't say?? (Was it another answer.) FYI that's not how minuscule is spelled, so whoever you are quoting should know that. I did say (cut and paste) (quoting) "What if you have incredibly advanced, sci-fi like instruments ... there is an incredibly tiny difference in the position of one or two stars" – Fattie Jul 21 '23 at 19:42
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    "Absolutely no change" is absolutely wrong. – Mark Morgan Lloyd Jul 23 '23 at 07:26
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    The "incredibly advanced, sci-fi like instruments" of the 1830s? That's when annual stellar parallax was first measured. – Sten Jul 23 '23 at 10:07