Sundial gnomon (at top of south-facing circular opening).
In this simple sundial, the sun's image is projected through a south-facing pinhole (gnomon) creating a bright spot on the structure's floor or sill. As the days and seasons pass, the spot changes position as the sun rises higher or lower in the sky. The figure-eight pattern (analemma) on the floor marks the spot's position at noon at different times of the year.
Photos by I. Peterson