# Is it (practically) possible to determine the stellar coordinates of a planet from a photograph of the night sky?

For my story, I need to know the following:

I have a (long exposure) analog (as in "not digital") picture of the night sky taken on a random planet somewhere in the Milky Way galaxy. I know it is theoretically possible, but can the coordinates of the planet (or at least the solar system) practically be determined with present astronomical knowledge and present or foreseeable near-future technology?

Criteria:

• The picture is taken with a quality camera with long (>minute) exposure on the most sensitive commercially available (as: it can be bought in stores) film.
• The camera is available and is in the same condition as when the picture was taken. (So lenses, aperture, etc. can be examined in a laboratory.) Also all its settings are untouched.
• I know a very good estimate (minutes resolution) of when the picture was taken. (In "real" time. Either no time dilation is in effect or the time frame is corrected with the time dilation.)
• I have detailed information of the atmospheric compounds of the planet. (In sake of simplicity lets assume it is Earth-like.)
• The picture could have been taken anywhere in the galaxy. (No light-cones/no information principle is in effect.)
• Only present astronomical knowledge can be used. (No pictures from outside the galaxy or from other stellar points.)
• Only present or possible near-future technology can be used. (Limited processing power, no sci-fi technology, only Earth-mounted/space telescopes etc.)
• The result is extremely important, so for a limited time (Hours, maybe days... You can't use all computers on Earth for decades.) Every available resource on Earth can be used.

Edit:

The story will take place on an (late XX.- early XXI. century) Earth-like planet (not Earth) but I skipped that part for simplicity's sake.

Edit2: (Thanks to Scott Downey)

We have a nice approximation of the rotating speed and no real info of the latitude. The photographer appeared by irregular means (like teleportation) and have only spent a few minutes on the planet surface.

• You'd probably get better answers to this question over at Astronomy. Jan 5, 2016 at 15:20
• Now I'm getting nightmare-flashbacks to having played Starship Titanic. Finding your home solar system from a photograph of your house's night sky was the last puzzle. I still don't know how the developers intended for that puzzle to be solved... Jan 5, 2016 at 21:24
• If your picture could include the Andromeda galaxy, you would have a much better chance. Jan 6, 2016 at 15:54
• @RBarryYoung I doubt it. For one, the galaxy would appear as a point-object same as any other star. For two, it's only a single point. You need a minimum of 3 to identify a view plane. Jan 6, 2016 at 17:16
• @mg30rg I apologize for the capitalization error; the many parentheses evidently confused me. I changed the "a" to "an" because it referred to Earth, hence it would be "an Earth", not "a Earth", independent of the parenthetical phrase. I may have been wrong. At any rate, I assure you that my intent was not too insult. Feb 20, 2016 at 16:02

The problem can be solved relatively quickly only if the photo happens to have an identifiable astronomical feature, such as a globular cluster, cepheid variable or something like one of the Magellanic Clouds visible. The more identifiable features that are in the picture the easier it will be to identify the location in space.

This means that the photo will only be useful in a relatively small volume of space inside the Milky Way galaxy. As you can see in the picture, only a small portion of the galaxy is visible to us. A photo that somehow was delivered from one of the other arms would have virtually no features that are visible to us except exogalactic features like the globular clusters and Magellanic Clouds, and the probability of enough of these being visible in a single photograph is extremely low.

You also mentioned that light cones and other issues are being ignored, so there will be an additional problem in locating your planet in the picture: you might have a general idea "where" the picture is taken from, if you can see the locations of two or more widely separated features like globular clusters and Magellanic Clouds, but you will still have the issue of "when" the picture was taken. If the picture comes from 30,000 LY away, then the stars are in the positions they were 30,000 years ago. Even in a galactic scale, there will be displacement due to the rotation of the galaxy, so the farther away the picture is from, the greater the amount of uncertainty due to "drift".

Thinking on a somewhat bigger scale, you could resolve some of these difficulties not with digital cameras, but with more pictures covering a wider area of the sky. A panoramic shot would be nice, and as mentioned in the comments, using an equatorial mount to counter the rotation of the planet would make this process far easier.

## Almost certainly "No"

Without a spectroscopic analysis or other measurement to tell us the distance of the objects present in the picture, it's generally not possible to determine the object's absolute magnitude. If you can't tell how bright it is or what its color is, then you can't tell which star you're seeing in the picture.

So the answer to the general case of "if you're plopped down on a random planet somewhere in the galaxy and want to know where you are from a photograph," the answer is "no."

## Under exceptional circumstances, "Possibly yes"

Let's say that your circumstances are not the general case. Let's say you do have some background knowledge of locations (e.g. you know you're on Earth at some point in the timeline and you want to know when - or you know you're near the Orion nebula), then your specific knowledge of the stars likely to be present and their probable brightness might make it so that you can figure out your location.

The answer to the special case of "you're stranded in a known general region of the galaxy and you want to know the specific star you're circling," is "possibly."

## Change the scenario, "Probably yes"

Let's alter the scenario a bit. Let's say your spaceship crashed with some basic astronomy tools present (such as a telescope, spectrometer, & database). Then you may be able to determine the spectrum of different stars, compare them with your database, and could reconstruct your location.

Assuming you have a telescope, spectrometer, and astronomical database that includes the stars in your current night sky; the answer to the question of "if you and your spaceship are stranded on an unknown planet and you want to know its location," is probably "yes."

Let me re-emphasize that if you crashed in a location that can't see the stars in your astronomical database, then you're back to "No."

### Terminology

Star Spectra (what you'd see coming out of a spectroscope)

The different spectra above show the different metallicities of stars. The more spectral lines the more metal (anything other than hydrogen or helium) in the star (and the later generation, evolutionarily speaking, the star is from). Each star's spectral line has slightly different strength depending upon the abundance of that element in the star's photosphere. If your measurements are precise enough the spectral lines can act like a finger print uniquely identifying a star. I don't expect our stranded voyagers to have the equipment or expertise necessary to be able to do that though.

Absolute Magnitude $$M_{abs} = m - 5 \cdot \left(\left(\text{log}_{10}D_L\right) - 1 \right)$$

The problem is we don't know the distance ($D_L$) or the absolute brightness ($M_{abs}$). Using the star spectra and checking the Hertzprung-Russell diagram we can make a highly educated guess as to the approximate absolute magnitude - which means we can get a ballpark guess as to the distance to the given celestial object.

• Would down voters please explain? Jan 5, 2016 at 21:27
• I hit +1, but I don't see any downvotes. I think this is an exceptional answer. Jan 5, 2016 at 21:49
• I think one of the reasons they downvote is the same of my reason to not upvote: your mention of stereoscopic equipment. What good would it do for you? If you had two pictures taken from astronomical distances then maybe it had some uses, but regular stereoscopic equipment has no use in case of stars. They are too distant for that. Also I mentioned in my question, that while we have no clue of the location we have exactly pinpointed the timeframe, and the usage of analog film provides some limited capability to determine the colors too (no spectrum-analysis is possible however). Jan 6, 2016 at 8:35
• @mg30rg, I didn't mention stereoscopic equipment. I said spectroscopic equipment. Equipment designed to split the light into its spectrum. Different stars have different spectral signatures. Although we might not have the definition to use this as an exact fingerprint of each star, it does give us lots of identifying information about the stars (it's type of the Hertzprung-Russell diagram for instance. Jan 6, 2016 at 15:03
• @mg30rg In addition, you can take useful stereoscopic pictures if you just wait half a year. And while an analog film might not have much spectral resolution, you can jury-rig a simple spectroscope from an ordinary camera with things you could find in home depot for under \$100. Jan 6, 2016 at 15:37