There is a section on the FAQ about this, though I think /u/jschooltiger's comment here does a better job of collating some of the more useful threads on the matter. (With answers mostly by jschooltiger, /u/restricteddata and myself.)

The short answer is, it's a fairly straightforward implication of a spherical earth. So this will have been understood, in at least the general sense that when it is 'night' for us it is 'day' somewhere else, pretty much since then. Just for example, Aristotle explains in his Meteorology (2.1) that while people once thought that the mountains in the north merely obscured the sun to cause night:

Some further evidence of the height of the northern regions of the earth is afforded by the view of many of the ancient meteorologists. They believed that the sun did not pass below the earth, but round its northern part, and that it was the height of this which obscured the sun and caused night.