Derivation of the Word "Theodolite"

Abstract

ALTHOUGH the etymology of the word theodolite has been discussed from time to time¹, no satisfactory solution has hitherto been established. It was first used in England, and the earliest reference to it is contained in a book by Leonard Digges (completed and published by his son, Thomas) called “Geometrical practical treatize, named Pantometria, diuided into three bookes, longimetria, planimetria, and stereometria, &c.,” first published in 1571, with a second edition in 1591, wherein the “composition of the instrument called Theodelitus” is represented as a “circle diuided in 360 grades or degrees, or a semi-circle parted in 180 portions”; or again, “the composition also of the Square and Planisphere or Circle named Theodelitus for measuring lengthes, breadths, and distances”. It had a “double scale”, an “index”, “the sightes”, and the circle was 2 feet in diameter, and “fastened in the top of some staffe”. He does not state how the name was derived, and spells it “theodelitus” and “theodolitus” alternately. William Bourne (“Treasure for Travailers”, 1578) named the same instrument “horizontall or flatte sphere”, and not theodelitus; but when he speaks of the alidade he calls it only once alideday, but otherwise always athelida. After this de Morgan, who first discussed the derivation in the Philosophical Magazine, concluded that the “theodelited circle” of Digges, who, however, does not use that adjective, and “athelidated circle” of Bourne, were various corruptions of the Arabic word al-idhâda (a sort of rule), from which the word alidade, which carries the sights or telescope of a theodolite, is derived.