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

Who invented dioptra?

In his Mathematical Syntax, or Almagest, Claudius Ptolemy (2nd C. C.E.) credits Hipparchus of Nicaea (2nd C. B.C.E.) with the invention of an instrument, called diopter, for measuring the apparent diameters of the Sun and Moon.

What is a dioptra used for?

The dioptra is the original survey instrument developed by Greek Astronomers for use in measuring angles.