I had a passive interest in archaeology and archaeoastronomy, but a strange stone at the Guayabo National Monument in Costa Rica sparked my interest. Photograph of the stone when it was used as a paving stone in the walkway that surrounds the central mound.
Green area indicates cloud cover over the mountains.
The stone displayed with the rectangular coordinate system showing cloud cover over the southern mountains during May to October. How cloud cover builds up during the rainy season obscuring the sky behind them. During the late 1970's my family and I lived in a second floor apartment in Moravia Costa Rica. Moravia is a small and quiet town northeast of San Jose. A nice thing about this apartment was the small balcony that we shared with my neighbor on the second floor.