Stonehenge. The most famous megalithic site in the world. Construction was begun in about 2800 BCE. and continued for over 1000 years. It was first noted in 1721 that the central Avenue was directed toward the summer solstice sunrise. More recently, many other astronomical relationships have been claimed, to such an extent that some consider Stonehenge a sophisticated prehistoric astronomical computer.

I don't believe that such a conclusion can be made. It is important to realize that with the erosion of the stones, and their settling and shifting over several thousand years, it is not really possible to say with a high degree of precision what their original positions were, nor where exactly on the stones measurements should be made. With resulting position errors on the order of a degree or more, no complex astronomical relationships can be ascertained (or rather, any number of fanciful ones may be derived.) Certainly, like other early henges, the stones were set up to either predict or indicate the summer solstice, and possibly a few other seasonal events. The Neolithic and Bronze Age farmers who built it must have long understood methods of predicting the solstices given the importance of such knowledge to their agriculture. It seems likely to me that Stonehenge was not an astronomical site, but rather a religious one that was built the way it was in order to make a spiritual connection with the forces of nature that so determined the fate of its builders.

Building Decoration, Royal Greenwich Observatory.