Prehistoric Astronomical Observatories and Paleoclimatic Records in Bulgaria Estimate Astroclimate during 4000–4500 BCE: A Critical Assessment

Prehistoric astronomical observatories include a specific type of rock-cut monuments from the Mountainous Thrace in Bulgaria, with a specific shape and orientation in space, which are part of the characteristic representatives of the archeoastronomical sites on the Balkan Peninsula from the period of 4000–4500 BCE. Earlier societies focused on the triad “astronomical instrument”—celestial objects—trained observers. When choosing sites for the construction of oriented stone complexes for astronomical observations, prehistoric people were interested in the number of clear days and nights within the tropical year, which is connected with the paleoclimate of the region and to the astroclimate, which determines the possibility of observing heavenly bodies. Here we examine 13 prehistoric astronomical observatories using the methods of archaeoastronomy in order to determine the period of their operation. Since the existence of a large number of such objects is indirect evidence of a good astroclimate, we make an assessment of the paleoclimate in the relevant era in the Bulgarian lands in order to find out if it was suitable for astronomical observations. The estimations are made according to the geological data and solar insolation luminescence proxy records of the evolution of cave speleothems from Duhlata cave in the village of Bosnek, Pernik municipality, which is still the only available experimental record of past solar insolation in Europe covering the last 20,000 years. The number of clear days and nights are estimated, and a critical assessment of the possibility of successful observations of the Sun during equinoxes and solstices is made using the methods of “horizon” astronomy and meridional culminations. It is also shown that the climate at the end of the Ice Age was cooler than today. About 11,700 years ago (11,700 radiocarbon years before 1950 CE or 11,700 BP), the climate began to warm, and forest vegetation developed on the territory liberated from the glaciers. During the Upper Atlantic (6–8 thousand years BCE), the average annual temperature on the Balkan Peninsula and in particular in Bulgaria was about 2–2.5 °C higher than it is today. This climate allows some very good astroclimatic conditions for observations of the Sun near the horizon and increases the accuracy of the observational data in determining the time of occurrence in its extreme positions on the horizon. We show that changes in climate (and astroclimate accordingly) influence the type of prehistoric astronomical observatories.