Jantar Mantar Observatory
This instrument measures the altitude of the sun, according to the height of the shadow (center of photo) cast by the gnomon (the dark upright pole to the center right). In this photo, the reflected glare from the sun has washed out the scale grid in the center section. It is more visible on the upright sections to the left and right.
As the sun rises and falls in the heavens, the shadow falls and rises correspondingly as it moves around the instrument. The sun is highest in the sky when the shadow is lowest on the scale. Wedges are cut out of the instrument to enable observers to move freely inside. A second instrument (not shown) has the wedges cut out in complementary positions to the first, so that an observation can be taken at any given time on one of the two instruments.