Q From Alasdair Downes: We had a discussion recently about a theodolite and came to wonder where the word originated since theo- means God. The Chambers 20th Century dictionary simply said the etymology was unknown.
A There’s an intriguing story behind the word, but in essence it boils down to what Chambers says.
The portable surveying instrument that we call a theodolite was invented in the middle of the sixteenth century by Leonard Digges of Kent, who gave it a name that was expressed in the common Latinate form of the time: theodelitus. (The name changed to an Anglicised form later, and at that time the e in the middle shifted to o for no very good reason anybody can discover.) His theodolite, by the way, was not quite the same as the modern device, since it consisted of a circle for measuring horizontal angles only. It was described in a book that was published posthumously by his son Thomas in 1571: A Geometricall Practise, named Pantometria, divided into Three Bookes, Longimetria, Planimetria, and Stereometria, containing Rules manifolde for Mensuration of all Lines, Superficies, and Solides (but then you knew that).
The problem for those seeking the true origin of the word is that Mr Digges never recorded how he invented it. The suspicion is that he was a better surveyor and inventor than he was a scholar. The word looks Greek, as is theos, the word meaning “god” that we have in English words such as theology. It may be — this is only an educated guess, mind you — that he derived it from the Greek stem thea-, sight or view, which is also, through Greek compounds, the root of theorem (from theorein, to look at or be a spectator), and theatre (from theasthai, to see or to look at). If he did, we shall never know.