Is there a way to convey date information to a future civilisation who has no knowledge of our calendar system using astronomical details or something?
Thanks,
Rob
In your time capsule, include a sample of radioactive material along with a written record indicating the initial concentration and the half-life; the finders can do the math to determine the date the sample was fabricated. This is more or less how carbon dating works, although it has a limited range of measurement, since after an excessive length of time there’s not enough carbon-14 left to allow for an accurate measurement. For your time capsule, you can include a material with a longer half-life (to allow for measurement after a longer storage period) or a shorter half-life (to allow for more accurate determination of shorter storage periods), or both.
I think that’d depend how far in the future.
I suppose marking to as much accuracy as possible the point in the sky our axis of rotation is currently pointing to, along with key positions of the brightest background stars in that area; our axis slowly pivots like a gyroscope (precession), tracing out a 26,000 year long circle against the sky.
By luck, the North Star – Polaris – just happens to be close to this point. It won’t be in a few more thousand years.
Noting the positions of all of the planets, including Pluto, should do it. Exact repeated confugurations happen what, every several hundred years, tops?
You could give the frequencies of several close by pulsars. The frequencies decrease (I’m pretty sure at a determinable rate) as energy is radiated away.
An accurate star map would also serve as the proper motions of nearby stars is quite measurable. Barnard’s star has a proper motion of about 10 arc seconds per year. In 1992 Rho Aquilae moved out of the Aquila constellation and into Delphinus. This would only be know to someone who had our star charts with precise lines delineating the constellations drawn on it, but just observing the relative positions do not depend on this.
The Mayans seem to have managed it.
Do they give the date of any event that we can peg to the day?
Thanks,
Rob
You mean other than the end of the world?
Voyager Golden Record: *The record is constructed of gold-plated copper. The record’s cover is aluminum and electroplated upon it is an ultra-pure sample of the isotope uranium-238. Uranium-238 has a half-life of 4.51 billion years. It is possible that a civilization that encounters the record will be able to use the ratio of remaining uranium to daughter elements to determine the age of the record. *
It also contained a Pulsar Map as did the Pioneer Plaque. Supposedly a future civilization could fix the system of origin in space as well as time.
If I’m not mistaken, historians have been able to place past events to the exact day when there just happened to be an eclipse. So, we could perhaps do the same in the future. Make sure your event happens during an eclipse. Easy peasy. 
The Babylonians also made records of the observations of other planets, especially Venus, that have since enabled us to pinpoint some other events down to the day.
How long it takes for an “exact configuration” depends on how precise your measurements are, but even with fairly rough observations, you’re well into many thousands of years, at least. Neptune alone has a period of 164 years, so there’s no way you could possibly have a match after less time than that (Pluto and the other iceballs can be even longer, but there’s no guarantees that future civilizations would notice the same particular tiny iceballs that we’ve noticed).
Scientists have used the temperature of white dwarf stars to estimate the age of the universe, since they cool at relatively predictable rates. This might even be the dating mechanism with the longest useful period. Assuming that the star survives, doesn’t take on any mass in the future, and can still be located, it will be useful for hundreds of billions of years.