Name: Why Do We Call It AM and PM? The Surprisingly Ancient Reason
Uploaded: 2025-12-06T21:01:10.000Z
Duration: 8 min 25 s
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If you've spent your entire life checking the time,

Adverbs of degree

then you've probably come across something so normal that you never even questioned it.

They're printed on clocks, digital displays, airline tickets, calendars, and phones.

So familiar that we rarely stop to think about what they actually mean.

But behind these two simple abbreviations lies a long history of astronomy, ancient civilizations, Roman influence, and

the way humans have always tried to bring order to something as wild and unstoppable as time itself.

Today, we're diving deep into why we call the first half of the day, AM.

And why the world still uses this system, even when there's an alternative 24-hour clock available.

Let's explore right here on History of Simple Things.

To understand AM and PM, you need to go all the way back to the civilizations that lived and died under the movement of the Sun.

For thousands of years, humans used sunlight as their primary reference for time.

When the sun rose, when it was at its highest point, and when it set.

The midpoint of the day, when the sun is directly overhead, was incredibly important for navigation, rituals, farming, and even survival.

Ancient Egyptians used sundials thousands of years ago, dividing daylight into 12 hours based purely on the movement of shadows.

But it wasn't until the ancient Romans that the idea of naming the periods before and after the Sun's highest point became more formal and linguistic.

They used Latin not just for religion or literature, but for daily life.

So naturally, timekeeping terminology evolved in Latin as well.

AM stands for antemeridium, which literally means before midday or before the meridian,

with meridium referring to the Sun's position at its apex.

Meanwhile, PM stands for post meridium, meaning after midday.

The 12-hour cycle resets every time the Sun reaches that midpoint.

From midnight until just before noon, everything is considered antemeridium,

And from noon until just before the next midnight, everything is postmeridium.

What's interesting is that these terms refer specifically to the Sun's position,

relative to an imaginary line in the sky called the meridian.

Essentially a vertical arc running from the North Pole to the South Pole across your location.

At solar noon, the Sun crosses this meridian, marking that exact midpoint of the day.

So AM and PM are literally describing the Sun's cosmic travel schedule.

Now, here's something that confuses almost everyone.

At some point, why is 12 PM actually noon and 12 AM actually midnight?

Shouldn't noon be neither before nor after midday?

By strict definition, noon is neither AM nor PM.

But because the timekeeping system needed labels for schedules, clocks and communication, we applied the terms anyway.

Historically, there were attempts to fix this.

Some clocks used the word noon and midnight instead of AM, PM.

Some old railways labeled midnight as 00:00 and noon as 12 M.

But eventually, by widespread convention, not by logic,

12 PM became noon simply because it's easier to treat the numbers increasing after 12 o'clock as entering the post midday cycle.

Midnight got stuck with 12 AM because it begins the before midday hours.

It's not mathematically elegant, but it's how the world standardized it.

As you might expect, the 12-hour AM, PM format isn't universal.

A large portion of the world uses the 24-hour clock, also known as military time.

In that format, the day runs from 00:00 to 23:59 without resetting halfway.

Midnight is 00:00, noon is 12:00, and instead of saying 8 PM, you simply say 20:00.

It's logical, it reduces confusion, and it's used heavily in Europe, Asia, science, aviation and medicine.

So why does the United States, Canada, the Philippines and several other countries still popularly use AM and PM?

A lot of it comes down to cultural habit, historical practice, and

the fact that analog clocks, which dominated for centuries, were designed around 12-hour cycles.

The 12-hour format is also easy to read quickly and matches ancient human rhythms tied to daylight and night cycles.

Even though digital systems allow for 24-hour displays, many cultures simply felt no need to change.

However, the Latin term stuck because English, unlike some other languages, borrowed heavily from Latin.

In its early scientific, religious and academic development,

timekeeping was something the church and astronomers dealt with extensively and both depended on Latin terms.

So by the time English clocks and schedules became standardized,

AM and PM were already the accepted scholarly way of referring to the two halves of the day.

And because English speaking nations spread their influence globally through trade, colonization and technology, the system went with them.

The result is that even in countries that now use the 24-hour clock,

you can still find AM&PM in old documents, historic buildings and traditional literature.

In the big picture, AM and PM aren't just labels on your alarm clock.

Echoes of ancient astronomers, Roman scholars, sundial makers,

church timekeepers and early scientists who shaped how the modern world understands time.

The next time you set an early morning alarm or schedule a late night meeting,

remember, you're using a system older than most languages.

Older than modern nations and older than the technology that now depends on it.

And that system continues to work centuries later because of two simple Latin phrases: antemeridium and postmeridium.

If you have suggestions for our next video, feel free to share them in the comments below.

We'll be sure to give you an acknowledgment for your contribution.

If you enjoyed this video, please check out our other bingeable channels.

Thank you for joining us on this journey through the History of Simple Things.

Don't forget to like, subscribe and stay tuned for more stories woven through the smallest details.

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Why Do We Call It AM and PM? The Surprisingly Ancient Reason

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