<p><a href="https://en.wikipedia.org/wiki/Celestial_navigation" rel="nofollow noreferrer">The position of stars in the sky has been used by navigators for a long time</a>. For instance, <a href="https://en.wikipedia.org/wiki/Astrolabe" rel="nofollow noreferrer">astrolabs</a> give you the time based on your position (or vice versa). And until GPS came along, airline pilots learned how to determine where they were using the stars. This comes with a few caveats however. </p>

<p>Determining the date and place from just looking at the sky is ambiguous. Since the Earth rotates, the position of stars and planets at a given place and time is indistinguishable from their position a bit later (say a couple hours) and a bit further West (how many miles further West depends on your latitude). 
If you know what <a href="https://en.wikipedia.org/wiki/Epoch_(astronomy)" rel="nofollow noreferrer">epoch</a> you are in, you can determine the latitude by looking at the position of <a href="https://en.wikipedia.org/wiki/Polaris" rel="nofollow noreferrer">Polaris</a> above the horizon. But to know longitude, you must also know the date and time. </p>

<p>Because of <a href="https://en.wikipedia.org/wiki/Axial_precession" rel="nofollow noreferrer">precession</a> and <a href="https://en.wikipedia.org/wiki/Astronomical_nutation" rel="nofollow noreferrer">nutation</a>, the star that is aligned with the axis of rotation of the Earth will change over long periods of time (roughly 25 000 years). So the position Polaris isn't even enough to know your latitude if you don't know what millennium you are in. </p>

CC BY-SA 4.0
2020-01-28T16:33:00.843Z
The Moon only moves about half a degree per hour, so you need an angular precision less than one arcminute if you want to give the time and date.
usernumber
CC BY-SA 4.0
2020-01-29T13:30:09.117Z
Thanks! Now I at least have a few starting points to geek out :)
Augunrik
CC BY-SA 4.0
2020-01-28T14:40:47.063Z
2020-02-20T10:05:37.427Z
2020-02-20T10:05:37.427Z
usernumber
<p>Disclaimer: I'm an astronomy noob</p>

<p>You sometimes see in (B?) movies, that they infer dates from sky patterns. 
I was asking myself if you can encode a place and date in a mostly meaningful pattern? How would you do it? Would it be possible?</p>

<p>I was looking at <a href="https://in-the-sky.org/skymap.php?no_cookie=1&amp;latitude=49.02&amp;longitude=12.10&amp;timezone=2.00&amp;year=2016&amp;month=10&amp;day=9&amp;hour=20&amp;min=0&amp;PLlimitmag=2&amp;zoom=173&amp;ra=19.29834&amp;dec=-14.44471" rel="nofollow noreferrer">1</a> and maybe the relation between the visible planets? 
(Moon, Mars, Saturn)</p>

<p>Why: I want to make something special for my girlfriend (who likes "stars"), the date is special to us.</p>

CC BY-SA 4.0
2020-01-28T19:42:52.280Z
forums.xkcd.com are down, but let me point you to http://1190.bicyclesonthemoon.info/ott/view  .  A few hundred pages in you'll find people who determined the date from the star positions embedded in the drawings.
Carl Witthoft
CC BY-SA 4.0
2020-01-28T19:45:23.327Z
TO clarify: my comment was to show that it's possible, not that it's explained there. But there are several software packages that will show you where the stars & planets are at any date/time.
Carl Witthoft
CC BY-SA 4.0
2020-02-24T10:00:03.480Z
See https://astronomy.stackexchange.com/q/33150/9527
usernumber
<amateur-observing><time><night-sky>
Encode place and date in star pattern
Augunrik
# Encode place and date in star pattern

(Asked by: username_0 on 2020-01-28)

Disclaimer: I'm an astronomy noob

You sometimes see in (B?) movies, that they infer dates from sky patterns.
I was asking myself if you can encode a place and date in a mostly meaningful pattern? How would you do it? Would it be possible?

I was looking at [1](https://in-the-sky.org/skymap.php?no_cookie=1&latitude=49.02&longitude=12.10&timezone=2.00&year=2016&month=10&day=9&hour=20&min=0&PLlimitmag=2&zoom=173&ra=19.29834&dec=-14.44471) and maybe the relation between the visible planets?
(Moon, Mars, Saturn)

Why: I want to make something special for my girlfriend (who likes "stars"), the date is special to us.

**Tags:** amateur-observing, time, night-sky

> **Comments:**
> **username_1**: forums.xkcd.com are down, but let me point you to http://1190.bicyclesonthemoon.info/ott/view  .  A few hundred pages in you'll find people who determined the date from the star positions embedded in the drawings.  
> **username_1**: TO clarify: my comment was to show that it's possible, not that it's explained there. But there are several software packages that will show you where the stars & planets are at any date/time.  
> **username_2**: See https://astronomy.stackexchange.com/q/33150/9527  

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## 1 answers

### Answer by username_2 on 2020-01-28. Score: 2 | Accepted answer

[The position of stars in the sky has been used by navigators for a long time](https://en.wikipedia.org/wiki/Celestial_navigation). For instance, [astrolabs](https://en.wikipedia.org/wiki/Astrolabe) give you the time based on your position (or vice versa). And until GPS came along, airline pilots learned how to determine where they were using the stars. This comes with a few caveats however.

Determining the date and place from just looking at the sky is ambiguous. Since the Earth rotates, the position of stars and planets at a given place and time is indistinguishable from their position a bit later (say a couple hours) and a bit further West (how many miles further West depends on your latitude).
If you know what [epoch](https://en.wikipedia.org/wiki/Epoch_(astronomy)) you are in, you can determine the latitude by looking at the position of [Polaris](https://en.wikipedia.org/wiki/Polaris) above the horizon. But to know longitude, you must also know the date and time.

Because of [precession](https://en.wikipedia.org/wiki/Axial_precession) and [nutation](https://en.wikipedia.org/wiki/Astronomical_nutation), the star that is aligned with the axis of rotation of the Earth will change over long periods of time (roughly 25 000 years). So the position Polaris isn't even enough to know your latitude if you don't know what millennium you are in.

> **Comments:**
> **username_2**: The Moon only moves about half a degree per hour, so you need an angular precision less than one arcminute if you want to give the time and date.  
> **username_0**: Thanks! Now I at least have a few starting points to geek out :)  

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