In my experience as a science editor, scholarly and scientific publishers I've worked with restrict the use of "circa" to years or dates: circa 1940, circa 3500 BCE, circa 250 MYA. In other contexts "approximately" or "about" or the approximate symbol (tilde) "~" is preferred: approximately 170 hectares, about two weeks ago, ~50 ft.

Last edited by M. McQuoid; 11/06/2008 5:04 PM.