What Do A.M. and P.M. Stand For?

One of the inconvenient things about breaking your day into two 12-hour chunks numbered 1-12, which turn over at midday and midnight, is that it becomes necessary to tell somebody which chunk you're talking about — are we meeting at eight in the morning or eight at night? For this reason the two periods are called a.m., short for the Latin term ante meridiem, or "before midday," and p.m., which is short for post meridiem, or "after midday."

But why should we have to complicate things with a 12-hour clock when the 24-hour clock seems so much more straightforward and convenient? The answer to this has a lot to do with early clocks.

The concept of a.m. and p.m. comes from the fact that ancient people used the convenient dark-light contrast of each 24-hour cycle to talk about what time it was: The sun told them what time it was in the day and the moon and stars did the same at night. The ancient Egyptians assumed day and night were of equal length.

As early as 3500 B.C.E. the ancient Egyptians built shadow-casting obelisks that shaded sections of the ground throughout the day — these sections were not as precise as modern "hours," but the concept was the same. By 1500 B.C.E. the more precise sundial had been invented, and with it, the "hours" we still know and love: 10 daytime hours with a sunrise and sunset hour tacked on to either end. The Egyptians also created a ​​merkhet, the first known astronomical device, invented around 600 B.C.E., which lined up with the Pole Star to calculate the hours of darkness based on the movement of select stars along the north-south meridian.

Later clockmakers used the circular model of the sundial to base the layout of their new technology. Although the first mechanical clocks in Europe, built for medieval clock towers, were 24-hour devices, by the 14th century, mechanical clocks began to be built with 12-hour faces, perhaps to save space and the eyes of pocket watch carriers.

Eventually clockmakers just threw in the towel and embraced a.m. and p.m., even though these designations didn't have much to do with the astronomical bodies anymore. However, by the late 19th century, starting with the Canadian Pacific Railway in 1886, more countries and military entities started telling time based on the 24-hour clock. With the exception of countries like the U.S., Australia, Canada and the Philippines, where the 12-hour system is officially used, most other countries officially use a 24-hour clock, although ancient habits die hard: If you ask someone in Ireland, China or France the time, they're equally likely to use a.m. and p.m.