# How to describe the Sun's location to an alien from our Galaxy?

Let's say you got to the planet orbiting other star in our Galaxy through the wormhole. You think that the travel was instant but you're not 100% sure and it is not exactly known. The planet is inhabited by aliens who agree to send you back but they would need to know coordinates of the star where you are from. Assume that finding a planet would not be a problem then.

Over time you manage to understand their equivalent of SI, i.e. their equivalent of kg/m/s and you can at least roughly map their units to ours.

How would you uniquely describe where the Sun is to those aliens with as much precision as possible?

Trying to answer this question myself (and I'm not a scientist and hardly remember anything advanced from my college days) - I was thinking that one distinct factor could be the size of the Sun (d=1.4 million km), temperature of the Sun's surface (~5700K), the Sun's rotation period (27 days), perhaps also describe similar parameters of the closest stars e.g. Alpha Centauri and the distance from the Sun to those stars. Then knowing these numbers a star could probably be uniquely identified, assuming that alien astronomers managed to collect searchable star parameters in their database. The only flaw I can see is that those numbers are probably not very reliable given that distances from the Sun to the neighbor stars may not be the same if travel actually did took some time. And the only "fix" for this I can think of is also memorizing rough age of the Sun as of this moment - perhaps as well as neighbor stars to narrow down the search.

Is there a simpler / perhaps more comprehensive way to uniquely describe where the Sun is without memorizing too many numbers?

• Sep 28 at 7:14
• This image of the Voyager disk explains everything on it, including the position of the Sun. images.app.goo.gl/RGWb3kJpfkeFvng47 Sep 28 at 16:12
• Note that if anything has changed enough to be astronomically relevant, then enough time has passed that humanity will be as alien to you as your current location. Might as well stay there :D Sep 28 at 19:07
• How to describe the Sun's location to ourselves if black holes don't have a magnetic north? Finding a 'map' of ours is cheating, but not as much as knowing its trajectory after coming across the probe; it'd be back thataway, obviously. Sep 29 at 1:58
• @Brondahl plot twist - it was Earth all along! Sep 29 at 8:46

This has already been done. The pioneer 10 and 11 probes have a description of the solar system's location and Earth engraved for aliens to understand (or so one hopes). The physical parameters of our Sun itself though, are indeed not unique enough - our Sun is a very average star, and there are millions alike or similar.

The basic idea is to use triangulation of Earth's location by using pulsars (which act as cosmic beacons or light houses as they each have a unique timing each which only changes very slowly over cosmic timescales). Using enough of these, and their relative direction towards Earth can help derive Earth's coordinates. As fundamental unit the wavelength of the hyperfine transition of hydrogen is used and described.

Whether the depiction is accurate enough and actually can be understood and by another sentient species... is up to debate, e.g.:

• Thank you! I'm not sure though it is something that can translate to a few numbers that someone can easily/realistically memorize and carry in their head. I take it that the numbers must be memorized with very good precision + location of those pulsars should also be captured somewhere? Also, I guess another thing would be to better memorize numbers using Planck units or knowing how to translate to/from (or as you suggested "hyperfine transition of hydrogen")? Sep 28 at 5:40
• astronomy.stackexchange.com/a/39514/2531 Sep 28 at 7:13
• This doesn't work. Pulsars are ephemeral and their light is beamed. The same set of pulsars cannot be seen elsewhere or elsewhen in the Galaxy. Sep 28 at 7:19
• To amplify; despite what it says on the wiki page about that plaque; pulsars typically "pulse" for a few million years before switching off, may be unidentifiable from their periods long before this and often have velocities so high that they will have moved hundreds of light years in a million years. If we can assume no significant time-lag and we are still reasonably close to the Sun it could work, but the pulsed radiation is typically confined to a cone with an opening angle of a few degrees. This means that in any random Galatic position, the chance of seeing a pulsar is very small. Sep 28 at 11:39
• @Hobbamok I agree you might be able to pick out the Sun from $\sim 100$ similar stars. There are about 100 sun-like (0.9-1.1 solar mass - note that mass is not directly observable) within about 50 light years of the Sun. The main point is that you won't be able to see those pulsars from another position in the galaxy so you will have no accuracy whatsoever. It is not just me that says so - read the cited link to forbes.com. Sep 28 at 14:48

If the travel was instantaneous (or say, less than a million years) it should be relatively easy to relocate the Sun from triangulation using well-known objects visible from anywhere in the Galaxy. If you appeared at a different (future) epoch then you have the problem of determining the "when" as well as the "where". This should be a solvable problem so long as you are less than a few tens of millions of years into the future and you had access to some of our astronomical catalogues. Further ahead than that - very unlikely you could find the Sun again (because it has moved a long way!) wthout being able to get detailed data (by that I mean data that could uniquely identify the Sun, like its planetary system) for a large number of sun-like stars in the Galaxy.

Details

Let's imagine the aliens have got pretty good telescopes and measurement techniques.

First you need to pin down the time. If your travel was close to light speed,then the size of the Galaxy (100,000 light years) means everything is pretty much where you left it since stars don't move too far (more than a few light years) on 100,000 year timescales.

If you can then remember the positions (RA and Dec) of say M31, the Magellanic clouds, Omega Cen, the Galactic centre, or some other famous, unique Galactic objects (e.g. globular clusters) then it would be easy enough to triangulate the solar location.

If we can't assume a short time has passed then we need a clock. One possibility is the temperature of the cosmic microwave background. This is universally measurable to high precision and it cools at a rate that is very predictable at any time in the past. If you can remember what that temperature is now ($$T= 2.72548\pm 0.00057$$), then that places you in cosmic time - with a precision of about 3 million years (see https://physics.stackexchange.com/a/596895/43351 ).

If you are in this latter situation and more than a few tens of millions of years had passed, then it gets difficult. Everything is in (relative) motion, the Sun has travelled a significant portion of its orbit, which isn't well enough known to pin it down (and you certainly can't recall the details).

If you had taken a USB stick with the Gaia astrometry catalogue on, and the jump in times was less than a few tens of millions of years, then you would quite easily (in principle) be able to reconstruct and demonstrate where the Sun was. That is because the positions and space motions are determined with respect to an essentially fixed background of extremely distant quasars, and with enough precision that the evolution of stellar positions, for stars within about a thousand light years of the Sun, could be determined for the next 10-20 million years with considerable accuracy.

The Sun is an ordinary, middle-aged main sequence star. It has no big chemical peculiarities, beyond an unusually (bottom 10%) low Li abundance for its age. I'd say unless you had narrowed down the location to a box containing something like $$\sim 100$$ sun-like stars, then chemistry, rotation and age are unlikely to uniquely identify the Sun. Given the space-density of sun-like G-stars in the Galactic disk this means you would need to start with an error box in position of side $$\sim 50-70$$ light years.

A final thought - it is always possible of course that the aliens have a much better catalogue of stellar motions and positions than the Gaia catalogue. If you can estimate how far in the past you left Earth to 3 million years from CMB observations then it could be that the alien catalogue might allow you to trace back stellar positions quite accurately over a time period longer than a few tens of million years. In which case, your estimate of the past epoch combined with a reasonable knowledge of the sky positions of several well-known objects could be enough to narrow the Sun's present location down to a box with few enough candidates to pick the right one.

• Maybe instead of writing my own answer I should have just written this comment but: for the final step, what do you think of the idea of finding stars that are unique in the region, like Betelgeuse or ζ Ophiuchi, and figuring the way to the Sun from those? Basically a second triangulation once you know roughly where you are in the galaxy. I think this lets you work from a much bigger box than, "100 Sun-like stars". Sep 28 at 18:50
• @SteveJessop there is no problem in finding things to triangulate with, if the translation/teleportation is rapid. That's my answer. Luminous stars would do. Once you get to a few million years, the stars you refer to will be gone. Sep 28 at 19:35
• Thank you for very detailed answer! Seeing other answers I think this is the closest to what I'm looking for. It is a bit diappointing that most commonly known (in layman terms at least) physical traits of the Sun as of now won't be enough. I also came across this page: skyserver.sdss.org/dr1/en/proj/basic/spectraltypes "The best tool we have for studying a star's light is the star's spectrum. A spectrum (the plural is "spectra") of a star is like the star's fingerprint. Just like each person has unique fingerprints, each star has a unique spectrum." Sep 28 at 23:31
• @J.A. If you dig, you may find lots of work trying to establish "chemical tagging" as a technique. Unfortunately, it doesn't work and the spectra of stars are not sufficiently unique - the variation in chemical abundances is not large enough compared with observational uncertainties to make the statement you have read demonstrable. Sep 29 at 5:19
• A nice answer, but there's one thing I don't understand: if you triangulate with respect to "extremely distant quarars", then the results of this triangulation will be more or less the same throughout the Milky Way, won't they? So how will that help you locate our solar system? Oct 1 at 0:15

Since you're talking about aliens in our galaxy, I think the question comes down to whether we would expect the aliens to have a map of our shared galaxy which is sufficiently similar to our map that we can pick out shared landmarks.

I am not an astronomer, but if I were, and assuming the aliens have astronomy at least as good as ours (they have an interstellar teleporter!) I think the answer to this would probably be yes:

• We know quite accurately our distance to Sagittarius A*, so that narrows it down to (the surface of) a sphere. The aliens can reasonably be assumed to know about Sagittarius A*.
• We know roughly the orientation of the galactic plane and our distance from it. Together with the sphere above, that narrows it down to two huge strips circling the galaxy above and below the plane. The aliens can reasonably be assumed to know about the galactic plane and have their own rough idea of its location.
• We know which side of the galactic plane we're on compared with external objects such as the Magellanic clouds, so that establishes which strip.
• To complete the location and narrow us down to a single point (plus or minus all the inaccuracies in the above numbers), we need to know any angle between us, Sagittarius A*, and any one other object of which the aliens are aware and which we can mutually identify. Another galaxy would work, like Andromeda or a recognisable quasar.
• Equivalently, if you have a map of the galaxy "from above" then a lot of astronomers could just point to Earth's location on it fairly accurately. The spiral arms are quite fuzzy, but have characteristic shapes. If anything the danger there is the alien's map would be too good and not recognisable without serious processing. But you're looking for the Orion arm, which has characteristic size, shape, and orientation.
• Additional objects could reduce the region of uncertainty: I started with the galactic plane because I was thinking of how to relate to a 3-D star map, but actually it might be better to just ignore that and triangulate more.

This gives you a search region: I'm not sure exactly how big it is because I don't know the inaccuracies in the above figures. I don't know how clearly the aliens can "see" the stars in the region I give them, so I don't know how many stars there are in that region that "look like" the Sun. We need it to be one and we're done. If they can count the large planets and see any of their details (distance, mass) then the Sun has a characteristic signature. And if they're going to deliver me to Earth then they must be able to see the planets once they know where to look. So, look at all the stars in the region until you have the right one.

But perhaps they're just going to drop me in the Solar system in a lifeboat ship, 8 light-minutes from the Sun, and let me drive it home from there. They can't actually see the planets. Temperature and size of the Sun are not very unique, and in a small enough region they'd do, but I think for that we'd definitely need better than our approximation to the galactic plane.

We do know our location relative to groups of stars such as Alpha Centauri A, B, and Proxima Centauri. That's three stars of known temperatures and distances, which is enough of a characterisation to be unique in our neighbourhood. If, once they have the general neighbourhood, they are capable of "looking" in enough detail to pick that out, then we're the boring star 4ly away from it. A similar technique works if the area is small enough that Betelgeuse is unique within it. That's starting to be a decent region: it's 500-600 ly away and there's nothing like it closer.

Bad news for me is, I'm not an astronomer, and actually I don't know any of those things that I say "we" know. I had to look up the distance to Betelgeuse. I could not point even to the general location of the Sun on a map of the galaxy. I sort of know roughly how far out we are, and that's it. Not every astronomer knows either, especially from memory, when their life depends on it. But at least some astronomers will have a lot of details from memory, enough to do this. If they have an astronomical database on their laptop when they fall in the wormhole then even better.

As a caveat, though: if you're going to fall through a wormhole then you can travel through time as well as space. Just because minimal time passed as you passed through the wormhole doesn't really tell you anything about how far apart the ends are in time, just as it doesn't really tell you how far apart they are in space. Then unless the aliens can map through time as well as space, none of this information is useful because they don't know when to send you (and of course with relativity you have to be very careful talking about "elapsed time" anyway). If they're time-wise close enough to us, then time since known visible supernovae might be useful for this, since we know the ages of supernovae reasonably accurately (some of them literally to the day, since we saw them happen). The supernova doesn't have to be in this galaxy, but the aliens do need to have a record of it. So try to fall forwards in time through the wormhole, not backwards, so that your reference supernovae aren't in the aliens' future ;-)

If you travel a great distance in time, like a billion years or something, then I think you're in trouble. There's no common map of the galaxy.

• the orientation of the galactic plane and our distance from it is (all we can give them, +1) like giving them a zip code to find a single room in a high rise apparent, but it's better than, first you have to go find a golden disk floating in space, then... Sep 29 at 2:12
• @Mazura: sure, our deductions about the galactic plane are very inaccurate compared with our direct measurements of the angles in the sky between the objects we can see. I think in practice you tell the aliens everything you can remember about astronomy, and let them figure out what to use ;-) Sep 29 at 8:29

The simplest is to treat the galaxy as a plane and use polar coordinates. The center can Sagittarius A* (which is a massive black hole at the center). For angle, you can exploit the asymmetric, elongated shape of the galactic center. Simply pick a wavelength in which the asymmetry is most pronounced, memorize it, and get the angle of Sol from that axis.

These two coordinates will narrow down the location of the Sun by a lot. Technically, they will give you a bent tube going roughly perpendicular to the galactic disk, if you have trouble with the angle you may have to deal with two tubes (on opposite sides). There won't be too many stars in that tube, so it will probably be easier to find the Sun from there.

Better would be to find 4 or more objects and know the distance of the Sun from them. You can use this to triangulate the Sun precisely. Mathematically, 4 is enough, but if you know more, you will have redundancy in case your distances are off or the aliens can't see one of them for some reason. Bright objects outside the galaxy would be best, since their appearance would vary less with where in the galaxy you are looking from.

It might be useful to know some basic facts about the Sun, like peak wavelengths, mass, size and so on. That way you can confirm that you got the right place with your coordinates.

Depending on how far you end up, the aliens may be looking at where the Sun was up to 50k years in the past. However, they should be able to help you account for that easily.

As far as memorizing numbers, you could simply tattoo them on yourself instead.

Some very interesting answers have already been given, but a specific sentence in your proposition makes me want to try a different approach.

If your consider this segment :

alien astronomers managed to collect searchable star parameters in their database.

One might assume that the 'searchable parameters' include, as suggested, the star's surface temperature and rotation period. But additionally, others descriptive data like number of planets in the star's orbit as well as their composition (solid, gaseous), size etc... Moreover, as it would be the case in a relational database, information about the given data helps reducing the number of results drastically (ex : ordering)

Assuming you have average knowledge of the solar system, a small set of information should be enough to uniquely identify the Sun. Said set could be something like this:

• 4.6B Years old
• 8 planets (or 9 if you consider pluto) 4 solid, 4 gaseous in ordered sequence -> S(olid),S,S,S,G(aseous),G,G,G(,S)
• Asteroid ring between 4th and 5th planets
• 1 moon on the 3rd planet
• Obviously if the aliens know everything about every star system in the Galaxy then it isn't really a problem. We know almost none of this information for almost all of the other stars in our Galaxy and we know all of that information for no other star. Sep 29 at 15:34

If the space traveler has enough scientific knowledge they will have a much easier time describing the location of the Sun to the aliens.

If they have a watch or other timepiece and it is working they can use it to show the aliens how long Earth time units are.

For example, the aliens could measure how long 10 Earth seconds are compared to their drens. But a more accurate ratio of seconds to drens would result from measuring 100 Earth seconds, or 1,000 Earth seconds, or 10,000, etc.

A second is now defined by a scientific fact:

The second [...] is defined by taking the fixed numerical value of the caesium frequency, ΔνCs, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s−1.1

https://en.wikipedia.org/wiki/Second

Scientifically advanced aliens should have their equivalent of the periodic table of the elements. So if the space traveler has a copy of our table of elements and can show cesium on it to the aliens, they will known which of their elements it is.

There are 60 seconds in a minute, 60 minutes in an hour, and 24 hours in a day, or 86,400 seconds in a day.

I once calculated the number of seconds in a year in my head while walking to school though I don't remember what answer I got. There are 31,536,000 seconds in a calendar year of 365 days, and 31,622,400 seconds in a leap year of 366 days, and 31,557,600 seconds in an average Julian calendar year of 365.25 days, and so on.

A light year is defined as the distance in a vacuum travelled by light during one Julian calendar year 365.000 Earth days long, or 31,536,000 seconds. So if the traveler knows that and can show the aliens the lengths of Earth time unit, he an call them how long a light year is.

The Sun is near the inner rim of the Orion Arm, within the Local Fluff of the Local Bubble, between the Radcliffe wave and Split linear structures (formerly Gould Belt).[95] Based upon studies of stellar orbits around Sgr A* by Gillessen et al. (2016), the Sun lies at an estimated distance of 27.14 ± 0.46 kly (8.32 ± 0.14 kpc)[38] from the Galactic Center. Boehle et al. (2016) found a smaller value of 25.64 ± 0.46 kly (7.86 ± 0.14 kpc), also using a star orbit analysis.[96] The Sun is currently 5–30 parsecs (16–98 ly) above, or north of, the central plane of the Galactic disk.[97] The distance between the local arm and the next arm out, the Perseus Arm, is about 2,000 parsecs (6,500 ly).[98] The Sun, and thus the Solar System, is located in the Milky Way's galactic habitable zone.[99][100]

https://en.wikipedia.org/wiki/Milky_Way#Sun's_location_and_neighborhood

So if the space traveler knows the latest (2023) estimates of the Sun's distance from the Galactic center, he can tell the aliens that the Sun is about 25,180 to 27,600 light years from the Galactic center. And the aliens can search their their data base for stars in that distance range.

And if the aliens know the distance of a light year, they know the duration of a year 365 Earth days.

And maybe the space traveler knowns the length of a sidereal year.

A sidereal year (/saɪˈdɪəri.əl/, US also /sɪ-/; from Latin sidus 'asterism, star'), also called a sidereal orbital period, is the time that Earth or another planetary body takes to orbit the Sun once with respect to the fixed stars.

It equals 365.256 363 004 ephemeris days for the J2000.0 epoch.1

https://en.wikipedia.org/wiki/Sidereal_year

So the aliens may also search their database for the length of Earth's orbital period.

And maybe the space traveler also knows about the Earth's orbit and has managed to teach the aliens the length of a kilometer or a mile in their units.

The semi-major axis of the orbit of Earth is 149,598,023 kilometers or 92,955,902 miles.

So the aliens may be able to search their database for planets with orbits with semi-major axis of that length in their units.

Maybe he can't teach the aliens the length of Earth kilometers and miles and thus the size of Earth's orbit.

But if he can teach the aliens the length of a light year, he might be able to remember that a light year is 63,241.077 Astronomical Units or AU, and thus that an AU is 1/63,241.077 or 0.000015812 light years. And an AU is the average distance of Earth from the Sun.

So possibly the aliens may be able to search their database for a star within the right distance range which has a planet orbiting with the right orbital period and also has the same planet orbiting at the right orbital distance.

That might narrow down the number of candidate stars significantly.

Here is a link to a similar question in the Worldbuilding Stack Exchange.

https://worldbuilding.stackexchange.com/questions/193694/how-far-would-we-have-to-travel-to-make-all-of-our-familiar-constellations-unrec/193721#193721

And my answer has links to other questions about finding the way back to Earth and their answers.

To be continued.