From “Early Celtic ‘Stonehenge’ Discovered in Germany’s Black Forest” (ScienceDaily, Oct. 11, 2011), we learn:
A huge early Celtic calendar construction has been discovered in the royal tomb of Magdalenenberg, nearby Villingen-Schwenningen in Germany’s Black Forest. … The order of the burials around the central royal tomb fits exactly with the sky constellations of the Northern hemisphere.
With the help of special computer programs, Dr. Allard Mees, researcher at the Römisch-Germanischen Zentralmuseum, could reconstruct the position of the sky constellations in the early Celtic period and following from that those which were visible at Midsummer. This archaeo-astronomic research resulted in a date of Midsummer 618 BC, which makes it the earliest and most complete example of a Celtic calendar focused on the moon.
Apparently, after the Romans took over, they replaced the moon-based calendar with their sun-based one, and these calendar stone circles fell into disuse.
See also: National Geographic: Site shows religion, not agriculture, prehistoric organizing force
and
Antikythera – an ancient mechanical instrument
It’s sobering to reflect on the many instances of progress of one kind or another just getting buried. Maybe there’s something in PayPal CEO Peter Thiel’s concern for today: “Is technological progress slowly winding down?” The main thing to see is that numerous times in history that did happen, and doubtless there were symptoms.