This is a little niggle of mine. I’m sure someone will want to tell me that I’m wrong, but I’m feeling argumentative…
I believe (not 100% sure) that the day was divided into 24 hours by the Babylonians, who naturally faced the simple problem of deciding when to start counting. The sun rises a little earlier or later every day depending on the time of year and how far from the equator you are.
The only event that happens at exactly the same time of the day, every day, whatever the latitude, is high noon - the time at which the sun is at its highest point in the sky. The apparent elevation of the sun will vary, but the time that it happens is constant. Noon in important. Noon is the beginning and the end.
It’s so important it even has its own name. It’s called the meridian.
You may have noticed that before the meridian we can be happily counting up to 12, and after the meridian we start again. I guess it would make more sense to start counting at lunch time and keep going for 24 hours, but we don’t. And that creates a problem for pedants like me.
In order to know whether someone is referring to 10 before noon, or 10 after we need to label our time with ‘before noon’, or ‘after noon’. Fine so far. For convenience we use abbreviations of those labels, and traditionally we use abbreviations of the latin phrases with the same meanings:
a.m. - ante meridian, before noon
p.m. - post meridian, after noon
I learned all this when I was about seven or eight years old and am frequently amazed by the number of people who don’t know it. So this has been a public service announcement, so far. Now here’s my gripe.
12:00 is not a.m. or p.m.! How can noon be before noon? How can noon be after noon? Noon is the standard by which time is measured, everything else is before or after noon. There can be no such thing as 12pm - noon after noon. If 12 noon was pm then 12:01 would be 12ppm, and 24 hours later it would be 12pppm. This is clearly bollocks.
But I keep seeing it. Will you please ask everyone to stop if you see them doing it? It really bugs me.