# Can we determine the date just from sky observations?

Assuming everyone "freezes" for several thousand years. Then everyone unfreezes at the same time. Could we tell how much time passed based on sky observations without fancy telescopes?

(Just started watching Dr Stone and I had this on my mind)

I'm guessing we could tell something based on how less sun does the moon cover compared to now during an eclipse. Am I on the right direction? Can we tell that with basic tools that don't need very precise cutting?

• Sure, the ancients did not have fancy telescopes, and their observations are correct. – anna v Aug 4 at 3:34
• @annav I don't quite understand what the "fancy" means, ancients did have some fancy tools. I think the question just want us to look and tell. – ShoutOutAndCalculate Aug 4 at 3:43
• Would this be better on Astronomy? – Aaron Stevens Aug 4 at 3:44
• I mean modern telescopes, ancient telescopes might be out of the question as they would need to be very precise. – Goodwine Aug 4 at 4:05
• To close voters (and @AaronStevens) -- astronomy questions are explicitly on-topic here and so we should not be closing them based on that. – tpg2114 Aug 12 at 17:30

Easily observable features (i.e. without the need for any telescopes) are the north pole and south pole of the sky. These are the points in the sky around which all stars seem to rotate during their nightly course.

Currently the north pole of the sky is near the star Polaris ($$\alpha$$ Ursae Minoris). But due to the precession of the equinoxes this is changing slowly through the millennia.

The north pole of the sky circles around the ecliptic pole in 23.5° distance once in 25772 years. The timeline of the north pole from 10000 BC to 14000 AD looks like this (notice the "Polaris" star near the "+2000" mark):

Another long-term effect changing the apparent positions of the stars in the sky is their proper motion. The stars move slowly (mostly with speeds of a few milliarcsecond/year) across the sky relative to the background of the more distant stars.

Here is how the constellation Ursa Major (i.e. the Great Bear) looks now and 100000 years from now.

image from astropixie: ursa major

• Thanks, although it wouldn't be good enough if more than 25.7k years passed, I think it would be a good indicator where IIUC one can estimate coarsely to centuries and then decades as better tools are made – Goodwine Aug 14 at 6:42
• @Goodwine For times larger than 25.7k years you can use the proper motion of stars (see my edited answer) – Thomas Fritsch Aug 14 at 8:22

A simple protractor, a clear view of the night sky, many years (at least a few decsdes) of careful observation, and some scientific reasoning is enough to build formulas, charts, or models that can tell you approximate date and time anywhere on the earth.

A good resource for this would be links found by googling "sky navigation ship".

1. The earth's polar direction do change.

2. The solar sun is traveling in the space, so does the motion of the galaxy.

However, you got a measurement issue: you want a number without precision measurement. Yet, by your means of determining the date, it does require precise measurement.

But I guess, one may use relative position of the planet to determine the year.

However, if you can't even make the precise measurement to establish a first reference, it's highly impossible for you to measure the date in a digital form.

• How precise would we need to get? Probably just knowing how many years, decades? have passed, not the exact date – Goodwine Aug 4 at 4:07
• @Goodwine You probably need to watch some video as S. McGrew suggested. The seasoning shift and the extra minutes of the year could not be ignored in the length of millennium, thus it's kind of hard to get into a precise day, the best precision I could think of was still in the range of months. The ancients could do it with their tools, as anna v suggested. – ShoutOutAndCalculate Aug 4 at 17:13