Earliest quantum mechanics experiment?

I have just watched a BBC programme investigates the latest hypotheses on the meaning of Stonehenge. Was it a beaker temple for the winter solstice? Very interesting how technology and science are used to provide seemingly convincing explanations to these things. The programme was interesting, but for me, triggered a number of thoughts which lead to, I think, some unanswered questions...

Firstly; if Stonehenge was a monument designed and built to be aligned with the sunrise on the exact day of the Winter solstice - how did they know the exact day? They didn't have a calendar to go by in the year 2500 BC did they? Was it someone's responsibility to measure the time between sunrise and sunset (when they knew they were getting close to the solstice) so they could put the stones in exactly the right place? If this was the case, what was used for accurate timing? Or did they use another approach - for example, the maximum height in the sky that the sun rose to? That would give you the answer but you would still need some measuring device. I assume that neither sextants nor clocks were available. The only other way that I think they could possibly have measured when the solstice occurs was by using the length of the shadows when the sun was at its highest in the sky, or more simply, where the sun rises from. This seems the most likely explanation to me of why Stonehenge was built in several phases. The markers were the first thing that was built (creating a circle). These markers are the 56 holes. If these were placed where the sun rose (easiest to measure) throughout the year then coupled with a record of the length of shadow (best way to measure the maximum height of the sun in the sky) they had (inadvertently?) created a calendar. If the importance to them was to know when the days would start lengthening again, then they are on their way to finding out exactly where to place the large stones where the sun shines through on the winter solstice - slap bang between the two markers that indicate the shortest shadows. So, imagine, placing a stone every, say, "week", within a year you have something that can allow you to predict where the sun rises.

So, I know - 56 doesn't go exactly into 365, but interestingly it does go about 6.5 times - does this mean they had a similar concept of a week as us? Even stranger, and I don't know where I am heading with this, but 56 isn't really that far away from 60. 60 is a nice fraction of 360, which is the number of degrees in a circle. This still doesn't explain how they managed to get it exactly right (and I am not even considering leap-years 'coz they probably weren't invented 4500 years ago).

OK, that's rubbish... Purely because I have skipped over the fact they don't have anything to measure the length of shadows with... That is, unless in each hole they place a pole. Each pole was cut to be the same length... Then they would have something.

I'll come back to thinking about this later. Finally though, if the largest stones created a slit by which the sun would appear on the summer and winter solstice, is this the earliest example of Young's slit experiments?! Were they creating wonderful patterns of light on walls?

It's an interesting subject.