What ways are there to represent dates that don’t depend on any language, like something we’d use on a Voyager-style message? I’m thinking more of writing/drawing on a 2D surface, so no audio or 3D. The first thing I thought of was of course celestial positions, but I couldn’t find any discussions. Are there any well-known celestial bodies that can be used to represent a date as simply as possible? Is it precise enough to represent individual days, or just the year?
How about using the positions of the four Galilean moons of Jupiter, as seen from Earth? Throw in the phase of our moon at the same time and you’d have five points of reference, all of which can be seen with a decent telescope. You could call it the moon calendar. You could easily see the differences from one day to the next. What worries me is whether you could tell the difference between their positions on, say, June 27th of last year compared to Thursday of next week.
If you had accurate enough instruments, you could measure the distances of the four outer planets: Jupiter, Saturn, Uranus, and Neptune. Their distances from the sun and distances from each other would probably give you a unique set of data to determine any given date within several thousand years. But you’d need way more than just a telescope to make such measurements accurately.
I think that you would have to start by defining ‘date’. After all our calendar is just an arbitrary number and can vary according to which system you subscribe to.
If we imagine a universe where humans have settled on far flung stars; my guess would be that they would use some local system for time and date, based on their home world’s rotations. Since it is all relative, there would be no use for some standard ***universal ***time and date.
Duplicate post
Once you fling the human race out to interstellar distances the concept of simultaneity breaks down for dates. But the OP seems to be worried about solar system levels of distance, or just earthbound, but arbitrary times.
Planet coordinates are a good one, and will work for a reasonable delta in time. You could imagine simply extending the Voyager style diagram to be a plan view of the solar system, with the orbits engraved, and the positions of the planets inscribed. That will get you a system that could reasonably expected to be accurate to a single day.
However, as the time delta increases the accuracy will drop off. The planet’s orbits are not stable, and eventually the instabilities will mount up, and the accuracy will drop and drop, until eventually a point will be reached where you won’t be able to derive the correct solution. Part of the problem is that there are a gazzilion candidate solutions, and you need to find the one that is the best fit. Eventually over time, the simple model of orbits will fit any configuration you might draw, and there will be an infinitude of solutions that are very close. You will probably want to pick the solution that is the smallest time away from now out of these, but as the instabilities and errors mount, you will find yourself, first getting slight inaccuracies in the date, and then suddenly getting wildly wrong dates as you pick the wrong candidate as the best fit.
The better you model the real orbital mechanics the better things will be, but then your universal calendar will need to convey the parameters of the model you chose, as well as just the positions. Which renders it it somewhat useless for purpose.
Eventually the system will so sufficiently unstable that the solar system will eject one of the minor planets entirely, and then your date system won’t even match the planets that are left. But so long as you are not trying to designate dates a few billions of years hence this is not likely to be an issue. The general instability of the orbits will limit the utility to a few million years (as a wild guess.) It would not be too hard to simulate to get a better feel for the range.
Hmm ok maybe the Voyager comparison wasn’t very good. I was thinking more for future humans to find, on Earth, in the next 1000 years or so, if they didn’t speak any current language. So maybe more like signage for nuclear waste dumps. Even then, I’m surprised there isn’t an easy, standard method. I always thought people could figure out the date from a picture of the night sky.