How far would we have to travel to make all of our familiar constellations unrecognisable? (in terms of human recognition)**

We know that groupings of stars in constellations is illusory. Two apparently close stars may be vastly distant from one another but appear to be close because they look that way in the sky.

Consider FTL (faster than light travel).

Suppose travellers pop out of FTL into an unknown space (perhaps because of a fault in the onboard AI). How far from Sol would they have to be in order to no longer to recognise any of the constellations?

No doubt a sufficiently developed AI in our ship could orient us in relation to a suitable 3D star map but what if we were out of range of the maps? More to the point, since the AI has proved faulty, what can we recognise as humans?

How far must we be away from Earth in order to recognise none of the familiar constellations?


I am aware that an exact answer may be difficult but I don't think the question is subjective. For example, if we found ourselves in orbit about Alnitak (which Earth dwellers say is in Orion's belt), we clearly would not see Orion's belt in our sky. However it is still possible that some other constellations would look the same at that distance. It's easy to come up with some distance because we could theoretically be in a different galaxy. However I'm looking for a rough closest distance where none of our familiar constellations would be recognisable. How can that be calculated?

To be specific

The constellations to be considered as recognisable by humans (using a telescope if necessary) is in the following list.

Below is the list of all the 88 modern constellations recognized by the International Astronomical Union https://www.constellation-guide.com/constellation-list/

  • $\begingroup$ "No doubt a sufficiently developed AI in our ship could orient us in relation to a suitable 3D star map but what if we were out of range of the maps?": if you were out of range of starmaps, how would you even see the stars the constellations are composed of? $\endgroup$ – Christopher James Huff Jan 10 at 15:33
  • $\begingroup$ @Christopher James Huff - Sorry I don't understand your question. You would just look out of the windows. You would either recognise some of the constellations or you wouldn't. Note the presumption is that the AI is faulty and that is why the travellers are in this predicament. Also it is quite possible that the maps used by the AI are relatively local to Earth. If I go for a drive in the UK, I don't usually carry a map of New Guinea. However, if I do suddenly get transported to New Guinea, I will recognise the Sun and Moon and will therefore know I'm still on Earth. $\endgroup$ – chasly - supports Monica Jan 10 at 15:35
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    $\begingroup$ The entire Tycho2 catalogue containing the 2.5 million brightest stars as seen from Earth is 160 MB of compressed text and is freely available. That'll get you far beyond the volume of space where the constellation stars are visible. Also, localization based on stars is computationally something you could handle with a smartphone app, just need to get telescope imagery to it. $\endgroup$ – Christopher James Huff Jan 10 at 16:17
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    $\begingroup$ If everybody on the ship is from roughly the same area of earth, then "to the other side of the earth" would do it. I've done a fair bit of stellar navigation, and I'd recognise exactly no constellations if you dropped me in the southern hemisphere. $\endgroup$ – user3482749 Jan 11 at 0:47
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    $\begingroup$ The average ship plying Earth's oceans has an emergency navigation kit in the Captain's cabin, plus one in the chief navigator's, plus another one in each lifeboat along with the emergency rations and radio. Such a kit consists of a sextant, extracts from the Bowditch manual, an almanac (possibly out of date, but still usable for star and sun sights), a solar-powered scientific calculator, a good quartz-crystal timepiece, and a large-scale chart of whichever oceans the ship expects to find itself in. It's enough to find a friendly port. Expect spaceships to carry something analogous. $\endgroup$ – Chromatix Jan 11 at 9:18

Last summer, for another answer on Worldbuilding, I wrote a script in an attempt to show how the positions of stars in the sky would change from the perspective of an observer outside the Solar System, using data from three catalogs: the Hipparcos, Yale Bright Star and Gliese databases. (The site which generated the CSV file I used is currently down, unfortunately.) As an example, I showed what the well-known constellation Orion looks like here on Earth and what it would look like if I traveled 10 (33 light-years) parsecs in the direction of $\alpha=0$ and $\delta=0$, where $\alpha$ is right ascension and $\delta$ is declination.

(I strongly suspect I made some numerical errors with the precise locations of the stars in the sky, but it seems that relative to one another, they're fairly accurate - and relative position is what's important here. The axes' scales are a bit off, so ignore the precise values for now.)

Here's Orion as it looks from Earth:

Orion as seen from Earth

Here's Orion as it looks from 10 parsecs away, in the specified direction:

Orion as seen from 10 pc away

The constellation as a whole seems to have shifted slightly, but some stars in particular have moved more than others relative to the new constellation. For example, on Orion's belt the middle star, Alnilam, has moved away from its companions because it's further away. Mintaka and Alnitak, being closedrto Earth, have shifted more and therefore remain together. Bellatrix, too, has moved significantly because it lies only 250 light-years from Earth, much closer than any of the other stars - now it appears to be where Orion's other shoulder was, whereas that star, Betelgeuse, is off the screen.

It makes sense that we'd seen changes in Orion. 10 parsecs is a few percent of the distance to some of these stars (around at least 4% for some), and given the variations in how far away they are, that does make a difference. Added to that is the fact that Orion is close to 90 degrees in the sky away from the direction we're traveling - had we moved in the direction of Orion, I'd guess there would be less distortion.

To maybe quantify this a bit: Say we have a star a distance $d$ away, and we move $x$ distance in a direction perpendicular to it. For $x\ll d$, we see that it should appear to shift by an angle $$\Delta\theta\approx\frac{x}{d}$$ Therefore, the angular shift of a star twice as far away than another will be half the shift of the closer star. Therefore, we'd expect Bellatrix to move five times more than Alnilam, which is roughly five times as far away.

Now let's say we're moving directly towards or away from the constellation. If the stars were all the same distance $d$ away and were separated by no more than a spatial distance $D$, the constellation would appear to have an angular size $$\alpha\approx\frac{D}{d}$$ At our new distance, $d+x$, the constellation would have a new angular size $$\alpha'\approx\frac{D}{d+x}$$ If the stars are at different distances, then their angular distances from the axis of travel will change individually by the above formula, with closer stars moving more and farther stars moving less.

I think that Orion is somewhat representative of the changes we'd see. The constellations in the sky don't involve stars which lie terribly far away because then individual stars would be too dim! Therefore, I'll handwave a little and say that traveling, say, 50 parsecs would be enough to render many of the constellations in the sky unrecognizable. Constellations perpendicular to the direction of travel would be completely unrecognizable; constellations along the line of sight might still be recognizable.

  • $\begingroup$ Thanks. That's better than I'd hoped for. I'll wait before accepting. $\endgroup$ – chasly - supports Monica Jan 10 at 15:22
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    $\begingroup$ Coloring the stars in Orion might help figuring who shifted where $\endgroup$ – L.Dutch - Reinstate Monica Jan 10 at 15:24
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    $\begingroup$ @chasly-supportsMonica: Moving in any direction will bring many new stars in the field of view, to the point where the "old" constellations will become drowned in the noise. We recognize constellations because they consist of bright(-ish) stars on a dark background; as the ship moves away, stars between the ship and the Earth appear brighter, and the "old" constellations fainter. $\endgroup$ – AlexP Jan 11 at 0:37
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    $\begingroup$ @AlexP, as an example, when viewed from Alpha Centauri, the "W" of Cassiopeia gains a sixth prominent star. $\endgroup$ – Mark Jan 11 at 2:12
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    $\begingroup$ We know the distances from earth for the stars, so we can build a 3D model and see which position for ourselves best fits the outside view. That requires some computing power if it's to be done quickly from a camera picture that only captures brightness, but you could likely get a good estimate by hand if you can measure spectra. $\endgroup$ – Simon Richter Jan 11 at 13:38

Hdes answer is very thorough and essentially correct, and I am not going to duplicate their mathematics, but the flaw in their answer is the corner case of moving directly away from a constellation.

Taking the southern cross as an example, the closest star is 88 light years. If I moved 88 light years away from it, the crosses angular size would halve, and it would slightly distort (as it's not a uniform plane),it would still be recognisable as a cross, just it would be half the angular size and have lost about three quarter of its brightness.

The human mind would still pick it as a cross.

The solution? Pick your new position carefully so that a star you've past is added to the constellation, giving it new meaning. The far side of proxima centuri is the closest place this will occur. About 4.5 light years away. The one surviving constellation will have proxima centuri added to it.

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    $\begingroup$ Of course, if we travel directly away from a constellation, we will then see our own Sun as part of that constellation...! $\endgroup$ – Greg Martin Jan 10 at 23:03
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    $\begingroup$ I've been beaten to it, but the closest start with which to do this is indeed The Sun. $\endgroup$ – OrangeDog Jan 11 at 13:08

It is lucky for space travelers that they have much easier ways to find their posiitons in space than by recognizing constellations. So if you want to reduce your fictional characters to recognizing constellations, you will have to disable those superior methods, perhaps by having the ship's AI control them and thus the AI's malfunction will interfere with the automatic use of those position finders.

So the human navigators will have to do everything by themselves. Posssibly that will include recognziing constellatiosn and comparing images of how they appear at the new positions with images of how they appear from Earth, and from other planets. If those images are available to them in print out form despite the AI's malfunction.

If the ship's navaigational AI is on the fritze, the crew would be lucky to have a hard copy book like Interstellar Navigation for Dummies or How to find Earth if Your Ship's AI is Defective, especially if it has lots of tables and diagrams and they don't have to relyon their memories.

There are a number of answers to various questions about interstellar navigation.

My answer to this question: https://worldbuilding.stackexchange.com/questions/123371/how-can-i-locate-myself-in-a-random-point-of-space/123429#123429[1]

Sugests a four step method to find one's way back to Earth if one is lost in space less than about one hundred million light years from Earth. That is a very vast distance, though only a tiny fraction of hte diameter of the observable universe.

Other questions with answers about interstellar navigation include:






And many others.

I note that when it comes to recognizing constellations, one of the most recognizable from Earth is Orion. The bright stars in Orion happen to be in about the opposite direction from the galactic core as seen from Earth. So if someone is lost in space and trying to find constellations, the first thing they might do is look in the difection opposite the galactic center to see if they can recognize Orion. If they can recognzie Orion they will know that they are close to Earth. And undoubtably someone more familiar with astronomy could point out the asterism or constellation which would be recognizable fromt eh greatest distance.

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    $\begingroup$ A problem with technology is that we become too reliant on it. If I drive through London, I no longer take a tattered copy of the A-Z with me. I simply rely on the satnav to get me to my destination. People nowadays are lost without their phones. In the old days, one could find a nearby telephone booth and call a friend. That option doesn't exist any more (at least not where I live). Someone on a tramp spacecraft operating on a low budget may be completely scuppered if the navigation equipment fails. They almost certainly won't know much more than what a few constellations ... $\endgroup$ – chasly - supports Monica Jan 10 at 22:52
  • $\begingroup$ ... look like from within their normal sphere of travel. Those will be very familiar just from looking out of the window. I want them to look around and recognise absolutely nothing. Nevertheless, your answer is very useful, especially about having the galactic centre as a point of reference. Thanks. $\endgroup$ – chasly - supports Monica Jan 10 at 22:54

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