Merriam-Webster has https://www.iwuhren.de/produkt-Sammlungen/pilots/ a lovely short essay on https://www.cuhren.de/produkt-Sammlungen/tank/ the https://www.crwmontres.com/Collection/santos-de-cartier/ history of the concept of Civil Twilight, which mentions that as recently as the early 20th century, there were at least six different definitions of it. Practically speaking, the definition of Civil Twilight had less to do with astronomy, and more to do with defining how late in the day work could be done, indoors or out, without resorting to artificial light.