Dioptra

A dioptra is an ancient Greek astronomical instrument used for measuring the altitude of celestial bodies. Its name derives from the Greek word "dioptra," meaning "sight-tube" or "tracer." The dioptra typically consisted of a metal rule, often marked with a scale, to which a sight-tube or alidade was attached. This sight-tube could be rotated to aim at a star or other celestial object.

The user would then read the angle of elevation from the scale on the rule. While its exact construction varied, the fundamental principle involved aligning a sighting mechanism with a celestial object and then measuring the angle relative to a horizontal reference. This allowed for the determination of a celestial body's altitude above the horizon.

The dioptra was a significant tool in early astronomy and surveying. It enabled astronomers to make observations that contributed to understanding celestial movements and to calculate positions on Earth. While not as precise as later instruments, it represented a crucial step in the development of observational astronomy and its applications. The instrument's use is documented in the works of ancient Greek scholars, highlighting its importance in their scientific endeavors.