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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 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.

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  • $\begingroup$ forums.xkcd.com are down, but let me point you to 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. $\endgroup$ – Carl Witthoft Jan 28 at 19:42
  • $\begingroup$ 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. $\endgroup$ – Carl Witthoft Jan 28 at 19:45
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    $\begingroup$ See astronomy.stackexchange.com/q/33150/9527 $\endgroup$ – usernumber Feb 24 at 10:00
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The position of stars in the sky has been used by navigators for a long time. For instance, astrolabs 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 you are in, you can determine the latitude by looking at the position of Polaris above the horizon. But to know longitude, you must also know the date and time.

Because of precession and 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.

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  • $\begingroup$ I think there's an existing question about this (that I think I answered) and you might be able to pull it off using the positions of the planets, the moon (which moves fairly quickly) and the Sun. But, realistically, it's a long shot $\endgroup$ – user21 Jan 28 at 16:25
  • $\begingroup$ 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. $\endgroup$ – usernumber Jan 28 at 16:33
  • $\begingroup$ Thanks! Now I at least have a few starting points to geek out :) $\endgroup$ – Augunrik Jan 29 at 13:30
  • $\begingroup$ Fair enough, but I assumed 'time' just meant 'time in general', not 'time to the precise minute' $\endgroup$ – user21 Jan 29 at 15:53

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