How many letters does the top answer for “an optical instrument, invented by hipparchus, for taking altitudes, leveling, etc.” have?

The most common answer is diopter (7 letters).

Crossword clue: an optical instrument, invented by hipparchus, for taking altitudes, leveling, etc.

2 answers found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

diopter 7 letters web-def

a unit of measurement of the refractive power of a lens which is equal to the reciprocal of the focal length measured in

dioptra 7 letters web-def

An optical instrument, invented by Hipparchus, for taking altitudes, leveling, etc.