When did midnight come to be seen as the division point between two days? I know that in many ancient cultures, the day began at sunset, which is why still today we speak of the night before a certain day as that day’s “eve” (i.e., evening). This seems to have been carried forward well into Europe’s Middle Ages and early Renaissance. Any details about when and how midnight came to be the diving point?
We start at midnight because the Romans started at midnight. Why they did so is a mystery to me. E. G. Richards (Mapping Time) lists the starting points of a few ancient cultures, but the only ones beginning the day at midnight are the Romans and ancient Chinese.
Thanks for the reference to Richards. In that book, he writes,
The British Navy, until the early ninteenth century at least, measured the day from noon to noon. Navigation required reading the angle of the sun at it’s highest position during the day. That reading was also taken to “Strike the bell, and turn the glass” and begin the new day.
Been re-reading Patrick O’Brian’s sea stories. 
Tris
No cite, but Tris’s point has always seemed to me the answer. IOW, midnight is the tail to the dog. Sunrise/sunset fluctuates, but noon is easily measured and consistent. Separating days by noon makes no sense, so we went to the opposite juncture, midnight, and declared by fiat that this (when most of us are asleep) is the admittedly-artificial dividing line between days.