Dioptra

A dioptra (sometimes also named dioptre or diopter, from Greek: διόπτρα) is a classical astronomical and surveying instrument, dating from the 3rd century BC.

Called "one of the greatest engineering achievements of ancient times," it is a tunnel 1,036 metres (3,399 ft) long, excavated through a mountain on the Greek island of Samos during the reign of Polycrates in the sixth century BC.

When used for astronomical purposes, the user would align the sights with a specific star or celestial object, and then measure the angle using protractors attached to the instrument.

Its ability to measure both vertical and horizontal angles with high precision made it a versatile tool for both astronomy and surveying.

The dioptra's origins trace back to the Hellenistic period when Greek scientists and engineers sought to improve observational accuracy in astronomy and surveying.

Notably, Hero of Alexandria's detailed work on the dioptra exemplifies the pinnacle of Hellenistic engineering prowess, showcasing the instrument's versatility and precision.

The instrument's ability to measure angles with high precision allowed engineers to plan and execute large-scale infrastructure projects with greater accuracy and efficiency.

The theodolite, which emerged in the sixteenth century, eventually surpassed the dioptra in accuracy and ease of use due to technological advancements and refinements in optical and mechanical components.