Q: In a recent post, you say the noun “dial” evolved from the Latin word for “day.” So how did it become a circular item for measuring or adjusting? My guess is that the round clock face had something to do with it. Am I close?
A: Not all dials are circular, of course. The dial on a radio for example, may be a horizontal or vertical panel. But as you’ve observed, many dials are indeed round.
As for your guess, the round face of the traditional analog clock probably had something to do with the circular sense of the noun “dial.” But a much earlier influence may have been the belief in the ancient world that the Sun revolved around the Earth.
As we say in our “Dial A for anachronism” post, the word “dial” is ultimately derived from diēs, classical Latin for “day,” but the more immediate sources were in Anglo-Norman, Middle French, and post-classical Latin.
The Oxford English Dictionary cites two words in the medieval Latin that was used when “dial” showed up in English in the early 1400s: dialis (daily), and diale (clock face).
The English word may also have been influenced by the medieval Latin phrase rota dialis, or daily wheel, according to the Chambers Dictionary of Etymology.
The phrase rota dialis apparently referred to the rotating face of an early mechanical clock with a fixed hand, as in the following example.
In a 1368 poem, the medieval author Jean Froissart, writing in Middle French, compares the revolving dial on such a clock to what was believed in his geocentric era to be the Sun’s revolution around the Earth:
“And this dial is the daily wheel that in a natural day makes one precise turn, just as the sun makes its own turn and encircles the earth in a single day” (Et ce dyal est la roe journal / Qui, en un jour naturel seulement, / Se moet et fait un tour precisement, / Ensi que le soleil fait un seul tour / Entour la terre en un seul jour.)
As we know now in our heliocentric age, the Earth’s rotation on its axis and its elliptical orbit around the Sun create the impression that the Sun is moving across the sky. And the position of the Sun overhead produces the shadows that have revolved around sundials since ancient times.
In fact, the classical Latin term for a sundial was solarium, from sol, or “sun.” And the Latin term was used for a sundial in English until the end of the 16th century, according to our searches of the database Early English Books Online.
The earliest example for “sundial” in the Oxford English Dictionary is from the English lexicographer John Minsheu’s 1599 update of A Dictionarie in Spanish and English, by Richard Percyvall: “Relox del sol, a sunne diall.”
The OED’s first example for “dial” itself is from a 1410-12 nautical inventory in which the term “dyoll,” according to the dictionary, “is likely to refer to a sandglass.” On the other hand, the Chambers Dictionary as well as John Ayto’s Dictionary of Word Origins assert that the earliest use of “dial” in English referred to a sundial.
Whether “dial” originally referred to a sundial or to an hourglass, we suspect that the circular sense of the word was influenced by the circular shape and movement of medieval clock faces as well as the pre-Copernican belief that the Sun revolved around the Earth once a day.
from Grammarphobia http://www.grammarphobia.com/blog/2016/12/dial-2.html