Whenever he thought about it, he felt terrible. And so, at last, he came to a fateful decision. He decided not to think about it. ~John-Roger and Peter McWilliams
Q:

Dear 100 Hour Board,

This question comes from my roommate:

Why does "a.m." and "p.m." change at 12 o'clock? Wouldn't it make more sense to have it be 1-12 instead of 12, 1-11?

Also, why did they choose 12 to be "noon" and "midnight"?

- Dead Cat

A: Dear Dead Cat,

A.M. and P.M. mean, respectively, ante meridiem and post meridiem. Translated from Latin, that's "before mid-day" and "after mid-day." Since our clock defines 12 (noon) as the mid-day point, anything before noon is A.M., and anything after noon is P.M. There is, of course, the question of whether noon should be grouped with "before noon" or "after noon," and the decision was to group it with the rest of its hour. After all, having 12:00:00 AM be one second before 12:00:01 PM would just be odd.

Now, the real question is why noon isn't defined as 1:00. In ancient Rome (and even before), the day was divided into 12 hours, and the night into 4 watches. Thus, the day went from 6 "hours" before noon to 6 "hours" after noon. (Note that these "hours" were actually simply 1/12 of the daylight that day, which meant that an hour was longer in the summer than in the winter.) In the ancient Roman system, time was more directly measured from the meridian; 3:00 P.M. meant "3 hours after the meridian" and 3:00 A.M. meant "3 hours before the meridian." (Yes, that's right; in Ancient Rome, 3:00 A.M was a perfectly reasonable time to be at work.)

When mechanical clocks came into existence in the 1300s, they kept the standard 12 hours of daylight and added 12 hours during the night, which meant a full 24 hours. However, since it's easier to tell time on a 12-hour analog clock than a 24-hour analog clock, they kept the 1-12 numbering already used for post meridiem, and redefined the before-noon hours to be as we know them today.

Now please, don't go quoting me in a research paper; I've skimmed over a lot of details and made some overgeneralizations. But hopefully you've got a better idea of why our clocks are the way they are.

-Yellow