Thread by @culturaltutor

The way we organise time is strange.

365 days in a year, 12 months (of 31, 30, 28, or sometimes 29 days), 52 weeks, 7 days in a week, 24 hours in a day, 60 minutes in an hour, and 60 seconds in a minute.

How did this happen?! The story begins with an Ancient Egyptian sundial...

The Ancient Egyptians separated day and night into two blocks of time, each with 12 parts — as we can see on their sundials.

Daylight hours were measured with sundials and night hours by the movement of the stars.

Thus the length of those day and night hours were different.

Why did they choose 12 hours? Because the Egyptians used a duodecimal system, based on the number 12.

They seemingly developed this system because instead of counting with whole fingers they counted by the joints of each finger, of which there are 12 on a hand.

So that's the 24 hours, which came from the Egyptians, but what about 60 minutes?

For most of history minutes simply weren't a thing, and the minute hand wasn't added to clocks for a long time after their invention during the Middle Ages.

Only hours on this clock from 1510:

And yet the concept of minutes was actually invented by Ancient Greek mathematicians — while they were attempting to create a system of latitude and longitude.

To do this they used the sexagesimal system, based on the number 60 and inherited from the Ancient Mesopotamians.

Eratosthenes first divided the globe into 60 sections and Hipparchus made that into 360.

During the 2nd century AD an astronomer called Ptolemy, who lived in Alexandria in Egypt, subdivided each of those 360 sections into 60 smaller sections, and each of those into 60 again.

The divisions were called (when translated into Latin) "partes minutae primae" and "partes minutae secundae", which morphed into minutes and seconds respectively.

Over a millenium later these divisions were finally applied to timekeeping — they were perfect for circular clocks.

Why are there 365 days in a year?

The year is an astronomical event — the time it takes for the Earth to orbit the sun.

People have known this for thousands of years (even if they thought the Sun was orbiting the Earth) but they didn't always get the figure quite right...

Take the Julian Calendar, introduced by Julius Caesar in 45 BC after the old Roman calendar had fallen into chaos.

In order to calibrate his new calendar with the solar year, Caesar had to extend 46 BC.

It was 455 days long — officially the longest year in history.

The Julian Calendar worked on the basis that a year is 365.25 days long, hence the addition of a leap day every four years.

But the real figure is 365.2422, and so the Julian Calendar drifted one day every 130 years.

That sounds trivial, but it was used for over 1,500 years.

Thus, by the 16th century, it had drifted significantly.

In 1582, under Pope Gregory XIII, the Julian Calendar was modified and a more accurate version introduced: the Gregorian Calendar.

When it was adopted Thursday 4th October was followed immediately by Friday 15th October.

What about months?

Originally (as you can tell from the name) they were based on the 29.5-day cycles of the moon, of which there are 12 in a year.

But the trouble is that lunar cycles only add up to 355 days — and are therefore not aligned with the solar year.

Many ceremonial and religious calendars, because of their age, are either lunisolar (meaning they include some calibration to stay in line with the solar year) or purely lunar.

Hence the countless traditional and religious festivals that fall on different days each year.

And this also explains why the months we use have different numbers of days — another fixture of the Julian Calendar.

Before its introduction the Romans would occasionally add an extra month called Mercedonius to make sure their calendar stayed in alignment with the solar year.

Caesar instead established 12 regular months of days set at either 30 or 31, with Februrary being shorter and a leap day added to it every four years — hence, also, why month names have Roman origins.

If you want 12 months in a year there is no way to make them the same length.

Weeks, meanwhile, are the strangest of all, because unlike years, days, and months they aren't based on any astronomical event.

They are arbitrary subdivisions of the month, and seemingly originated because of the need for a shorter and more managable unit of time.

Thus, because they are arbitrary, weeks around the world have varied in the length — the Romans long used an eight day week.

The seven day week originated four thousand years ago with the Ancient Sumerians in modern-day Iraq, and thereafter spread abroad from Greece to China.

Why did they choose seven?

Perhaps because of the seven luminaries — the five planets visible to the naked eye, plus the Sun and Moon — after which their days were named.

A tradition that has lasted millennia, and is reflected in modern names for the days of the week.

This brief overview has missed plenty of important and fascinating details, but it shows just how strange and unshakeable the influence of history is.

The way we measure and date time, even in the 21st century, is built directly on foundations that are thousands of years old.

Of course, that hasn't stopped people trying to create a new system — and, with such an odd agglomeration of traditions and methods, why wouldn't they?

Think of something like Unix Time, which counts seconds passed since 1st January 1970 and is used in computing.

During the French Revolution a whole new calendar system was introduced, based on the number 10, with new names for months and days of the week.

12 months of 30 days, each 10 hours long, with 10 day weeks — extra days were added at the end of the year to align with the seasons.

But large-scale change is difficult, and creating an effective system is even harder.

Over the last five thousand years humanity has essentially been collectively developing and fine-tuning a way of organising time.

It may look messy... but it works.