# Building an Astronomical Calendar

Okay, I have this great idea: I want to build some kind of giant astronomical calendar (think Stonehenge) such that it would have synched up with a celestial event that would have taken place 10,000 years ago, but wouldn’t work in modern times (due to stars shifting, whathave you), and I want to make it out of something like stainless steel or other material which wouldn’t have been available to people 10,000 years ago. The reason for this (heh, heh, heh) is to confuse the bejeezus out of future archeologists who find the thing. (Rat Archeologist #1: “This doesn’t make any sense! It only lines up with stars that were in the correct position millenia before humans had discovered stainless steel!”
Rat Archeologist #2: “Well, those humans were a crazy lot! That’s part of the reason they went extinct.”)

So, what all do I need to know to be able to do this? I know that to get a sundial oriented in the correct way, you need to have some idea of your lattitude and longetude, and I assume that there’s something similar I’d need to know to figure this out. Also, what kinds of astronomical events would work? I want something that would have stayed the same for at least a decade or so, but would have shifted to a different position by the beginning of the 20th Century at the latest.

No idea as to the technical aspects of it, but maybe point it at a star that has since blown up, or track a comet that no longer exists.

And most importantly, remember to have it inscribed 8,000 BC

The axis of Earth’s rotation precesses on a 26,000 year cycle. So 10,000 years ago, the brighter stars such as Antares, Betelgeuse, and such would have been rising and setting at different locations on the horizon. It’s not too hard to calculate where these points would be, given the location of the celestial pole, the star in question, and your latitude; but you’d need some knowledge of spherical trigonometry to get the full answer.

If you want to pick a star (or two) and a time frame, I could probably run through the calculations for you; let me know.

The stars Thuban and Vega have been pole stars in historic times, because of the precession. So have other less-known stars:

[quote}Due to the precession of Earth’s rotational axis, Thuban was the naked-eye star closest to the north pole from 3942 BC, when it moved farther north than Theta Boötis, until 1793 BC, when it was superseded by Kappa Draconis. It was closest to the pole in 2787 BC, when it was less than two and a half arc-minutes away from the pole. It remained within one degree of true north for nearly 200 years afterwards, and even 900 years after its closest approach, was just five degrees off the pole. Thuban was considered the pole star until about 1900 BC, when the much brighter Kochab began to approach the pole as well.[/quote]

Using 2787 BC as a target with Thuban as the pole star would peg it closely with both a date and the ancient culture.

Nitpick: Vega wasn’t really a pole star in “historic times”, since the last time it was near the pole was about 14,500 years ago, long before the beginning of recorded history. But it would work fine for the purposes of the OP.

Another thing to worry about: the proper motions of the stars are going to be worth worrying about over the course of ten thousand years. [symbol]a[/symbol] Centauri, for example, would be one of the brightest stars in the northern sky if Vega was currently the pole star, but its motion through space relative to the Sun has moved it on the order of ten degrees in the sky, enough to throw off your faux observatory. You could play it safe and use stars that are fairly far away, and therefore haven’t moved much in the sky over ten thousand years, but then you’d have some notable omissions in your list (after all, all things being equal a bright star is likely to be closer.)