# How Did We End Up With 24 Hours In A Day?

Steve Mansfield

Not 26 or 30, but exactly 24 hours in a day, and why is it divided into two 12 hour periods with indications of am and pm? Then we have 60 minutes in each hour, which is then subdivided into 60 seconds. And if you are looking for sports accuracy, this second is then split into thousandths of a second. Who created this system? Curious, isn’t it!? The reason behind time being measured like this takes us on an amazing journey through history.

This movement of the earth in the solar system was the basis for developing the calendar year adopted in 1582 and is still in use today. So it would be natural to think that the hours minutes, and seconds that are the primary focus of clocks and watches came from the movement of the planets too, and it does, but as with the development of the day and the year, it is just not that simple.

#### Early Days

It was the ancient Egyptians who first documented the measurement of the solar day. They initially divided the daylight period into 10 periods using a shadow clock. This is basically a stake in the ground (the “gnomon”) that is inclined to the horizon at an angle that equals the angle of latitude of the location.  The length of the shadow indicated the time that the gnomon throws.

Before starting the 10 daylight periods, a single period was added to cover the twilight, and one was added to cover dusk.

There were 10 periods for daylight, one for twilight, and one for dusk. Thus, there were now 12 equal periods that covered all of the daylight. Records indicate that from 1500 B.C., these 12 periods were being regulated using a T-dial.

#### What’s a T-dial?

It is a T-shaped bar placed in the ground and calibrated such that there are 12 equal intervals between sunrise and sunset. It is speculated that the ancient Egyptians selected a 12-period system because the duodecimal system (counting in the base 12) was used in all aspects of life at that time.

It was possible to measure the passage of daytime, but without artificial light or any modern aids measuring the night-time passage, the period posed a few additional challenges. Yet, the ancient Egyptians managed to find a remarkable solution.

#### And How?

Egyptian astronomers observed 36-star groups they called ‘decans’ that divided the night sky into equal parts. These decans were chosen so that one decan rose on the horizon in consecutive equal intervals as the earth rotated on any night. This system was so well developed that tables were widely available to aid in observing the time. These tables have even been discovered on the inside of coffin lids – we can only assume this was to help the dead read the time?

From sunset to sunrise, a total of 18 of the 36 decans were visible! Each twilight period had 3 decans assigned, which left 12 decans to measure the total period of darkness – again, we are back to the duodecimal system. Thus, the rise of each decan marked a period that represented one-twelfth of the night-time period.

Over time the system was simplified so that gauging the night’s passage was made easier to observe for more people – but all the simplifications still relied on observing the stars. It was not until the advent of water clocks (clepsydra) that there was a significant advancement in time measurement. A magnificent specimen of a clepsydra was found at the Temple of Ammon in Karnak dated back to 1400 B.C. This specimen was a vessel with slanted interior surfaces to decrease water pressure and inscribed with calibrations that divided the night into 12 equal periods during various months.

#### Did You Know That Equal Periods do not Equal Hours?

It is important to note that at this stage, all these systems allowed for the daytime and the night-time to be divided into 12 equal periods but, as the seasons’ pass, the daytime and night-time periods are of different lengths in a single day and the night-time period will change in length day to day as will the daytime period.

For instance, In summer, the day-time periods were longer than night-time periods, while in winter, the period lengths were the other way around – except at the equinoxes when daytime and night-time periods were equal.

Even at this point, and we are only at around 1,000 B.C., it is interesting to consider that the day has already been split into 24 periods, with 12 for the night-time and 12 for the daytime as we have already alluded to, several theories of how it ended up with 12 periods for daytime and night-time!

#### Want to Know One of the Most Compelling Ones?

The number of finger joints on each hand (excluding the thumb) makes it possible to count to 12 using the thumb.  Try it!

This is also consistent with the importance the Egyptians placed on the number 12. This is attributed to the fact there are 12 lunar cycles in a year and their general adoption of the duodecimal system.

#### The Greeks: An Hour Becomes an Hour

Clearly, having the day and night periods varying in length, even though each day was equal in length to the preceding, was not ideal. The leap to a consistent period for an hour originated during the Hellenistic period (323B.C. ~ 31B.C.), when Greek astronomers began using a fixed-length hour system for their theoretical calculations. Hipparchus (190 B.C. – 120 B.C.), a Greek astronomer, geographer, and mathematician, whose work primarily took place between 147 and 127 B.C., proposed dividing the day into 24 equinoctial hours, based on the 12 hours of daylight and 12 hours of darkness observed on equinox days.

Despite this suggestion, laypeople continued to use seasonally varying hours for many centuries. Hours of fixed length became commonplace only after mechanical clocks first appeared in Europe during the 14th century.  This is most likely explained as everyone could observe the sun movement accurately. Still, until mechanical devices to observe the time were commonly available, the most accessible timekeeping method maintained its popularity.

#### Hours and Minutes, Who Introduced These?

It was the ancient Babylonians who introduced the subdivision of hours and minutes into 60. They had a predilection for using numbers to the base 60 (sexagesimal) and are credited with introducing hours and minutes divided into 60 and dividing a circle into 360 degrees. The Babylonians divided their days into 360 parts called ‘ush .’ Each ush equaled four minutes in our modern time system. One hypothesis is that the Babylonians were interested in 360 because they estimated that this was the number of days in a year. The adoption of a base 60 system was most likely driven by the fact that 60 is easily divisible by the first 5 whole integers (1,2,3,4 and 5) and 10, 12, and 15 – this made complex calculations using fractions remarkably simple.

Although it is no longer used for general computation, the sexagesimal system is still used to measure angles, geographic coordinates, and time. In fact, both the circular face of a clock and the sphere of a globe owe their divisions to this 4,000-year-old numeric system initiated by the Babylonians. The Greek astronomer Eratosthenes (who lived circa 276 to 194 B.C.) used a sexagesimal system to divide a circle into 60 parts to devise an early geographic system of latitude horizontal lines running through well-known places on the earth at the time.

A century later, Hipparchus normalized latitude lines, making them parallel and obedient to the earth’s geometry. He also devised a system of longitude lines that encompassed 360 degrees and that ran north to south, from pole to pole. In his treatise Almagest (circa A.D. 150), Claudius Ptolemy explained and expanded on Hipparchus’ work by subdividing each 360 degrees of latitude and longitude into smaller segments. Next, each degree was divided into 60 parts, each subdivided into 60 smaller parts. The first division, partes minutae primae, or first minute, became known simply as the “minute.” The second segmentation, partes minutae secundae, or “second minute,” became the second.

#### Minutes & Seconds but No-one Can Use Them!

Minutes and seconds were defined, but they were not practical for everyday timekeeping until many centuries after Hipparchus and Ptolemy’s work. Clock displays divided the hour into halves, thirds, quarters, and sometimes even 12 parts, but never by 60. In fact, until relatively recently, the hour was not commonly understood to be of 60 minutes duration – hence “quarter to ten” for reading time. It was not practical for the general public to consider minutes until the first mechanical clocks were sufficiently accurate to display minutes that appeared towards the end of the 16th century.

#### Modern Day Timekeeping

Thanks to the ancient civilizations that defined and preserved the divisions of time, modern society still conceives a day of 24 hours, an hour of 60 minutes, and a minute of 60 seconds. Advances in the science of timekeeping, however, have changed how these units are defined.

Seconds were once derived by dividing astronomical events into smaller parts, with the International System of Units (SI) at one time defining the second as a fraction of the mean solar day and later relating it to the tropical year. This changed in 1967 when the second was redefined as the base unit for timekeeping and referenced to atomic events. Thus, a second is now officially defined as the duration of 9,192,631,770 energy transitions of the cesium atom. This recharacterization ushered in the era of atomic timekeeping and Coordinated Universal Time (UTC).

#### But Could it Be That Simple?

Apparently not, To keep atomic time in agreement with astronomical time, leap seconds occasionally must be added to UTC. Thus, not all minutes contain 60 seconds. A few rare minutes, occurring at a rate of about eight per decade, actually contain 61.

Finally, the milliseconds’ concept was not terribly relevant to the general population until there was an accessible and accurate way to measure these tiny increments, which was impossible until the advent of the quartz movement in 1969. Before this, mechanical movements could not accurately measure such small increments of time (a mechanical movement with a beat rate of 28,800 BPH or 4Hz can have a maximum resolution of one-quarter of one second).  With the availability of quartz movements and digital displays, the second subdivisions were made to base 10 in line with the SI units, almost universally observed globally.

#### Peoples’ Time

Given the millennia over which our time measurement system has been developed, it is only fitting that we have recently added a new modern twist to timekeeping with the millisecond – for future generations to wonder over, no doubt!  But what is most striking is that more accurate timekeeping was generally available before such accuracy was used in daily life.  The availability of measurement instruments available to the masses has driven the adoption of accurate timekeeping throughout the world.  This was even true through the 18th and 19th centuries as more accuracy was driven and standard time zones were introduced.

Do you want to understand how these adjustments came to be and the advancements in mechanical watchmaking during its heydays?

Lucky for you, we are giving away a FREE the seminal work, “A Rudimentary Treatise on Clocks, Watches & Bells,” which was written in 1883 by Edmund Beckett, Lord Grimthorpe, who was the president of the British Horological Institute – it is a real masterpiece that answers questions about time, timekeeping and the development of mechanical timepieces of all size.

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## Comparison Ohi2 vs. Ohi4

The OHI movements, or One Hand Indication, are unique to SNGLRTY. During the development phase of the watches Daniel and Steve used this moniker before naming it SNGLRTY. There are two distinctive movements to choose from, OHI-2 and OHI-4.

The OHI-4 movement is built on the Decorated and Fully Adjusted SW-300 tractor movement from Sellita. On top of the tractor movement the SNGLRTY complication plate is assembled and incorporates the “reverse minute gearbox” that is available exclusively from SNGLRTY. Depending on your selection, the complication plate will also relocate the date wheel from the top of the tractor movement to the top of the complication plate. Relocating it in this manner increases the size of the date disc and moves it closer to the top of the watch face improving its readability considerably.
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Finally, depending on the movement you select the watch case will have a different profile as the OHI-4 movement is thinner than the OHI-2 movement. The key differences are that the case for the OHI-4 movement has a double domed crystal and a flat caseback. The OHI-2 case has a flat crystal and a curved caseback. All the details are in the product page.