Today, the fact that there are 24 hours in a day or 60 minutes in an hour is a universally accepted convention. Have you ever wondered why it is 24 and not 25 or 26? Why was the number 60 chosen instead of other round numbers like 80 or 100? In this article, we will explore the question: why is an hour divided into 60 minutes, a minute into 60 seconds, and a day into 24 hours?
How Did Ancient Peoples Measure Time? Based on the Sun
Currently, the most widely used numeral system is the decimal system (base 10), which researchers believe originated from the convenience of counting using fingers. However, many early civilizations used duodecimal (base 12) and sexagesimal (base 60) systems to divide a day into smaller parts.
Ancient Egyptian sundial discovered during excavations in the Valley of the Kings, dating back to around 1550 to 1070 BC
Based on documents and evidence found, researchers confirmed that ancient Egyptians used sundials. Historians conclude that most early civilizations relied on the Sun to divide time in a day into smaller parts. The remnants of the earliest sundials were simply a stick or a pole stuck in the ground, with time determined by the length and direction of the shadow cast by the stick in the sunlight.
As early as 1500 BC, ancient Egyptians developed relatively sophisticated sundials. These were T-shaped sticks placed on the ground, adjusted to divide the time between sunrise and sunset into 12 equal parts. This division reflects that ancient Egyptians began to use the duodecimal numeral system.
According to researchers, the number 12 was chosen as a significant number based on the number of lunar cycles in a year. Another theory explaining the number 12 is that it corresponds to the number of finger joints on each hand (excluding the thumb). Nevertheless, the sundial method had a disadvantage: the length of each day varied depending on the seasons, with sunny days in summer being longer than those in winter.
Without artificial light, people in this era viewed bright and dark times as two opposing realms rather than components of a complete day as we do today. Calculating time at night (the period between sunset and sunrise) was significantly more complex than during the day.
However, during the era of sundials, ancient Egyptian astronomers noticed a set of 36 stars dividing the celestial sphere into equal parts. Daytime was marked by the appearance of 18 stars, while 6 stars were used to mark dawn and dusk. The total night period was divided into 12 parts corresponding to the remaining 12 stars. As a result, the night was divided into 12 equal parts (a signal of the duodecimal system).
Roman water clock preserved at the Agora Museum, Athens
By the time of the New Kingdom in Egypt (1550 to 1070 BC), the aforementioned measurement system had been simplified into a set of 24 stars, 12 of which marked nighttime intervals. This was followed by the emergence of water clocks (clepsydra) used for nighttime timekeeping. Considered the most accurate timekeeping device of ancient history, water clocks were found at the temple of Amun in Karnak, Egypt, dating back to 1400 BC. This was a vessel with a sloped interior surface and a receptacle for catching water below. Nighttime would be divided into 12 parts corresponding to predetermined amounts of water. Similar water clocks were also found at the ruins of ancient Greeks and Babylonians.
Why is an hour 60 minutes? A minute 60 seconds? Why the number 60? The Origins of the Terms Minute and Second?
Since both day and night were divided into 12 parts, the model of 24 hours in a day gradually took shape. However, the fixed-length hour model was not established until the ancient Greek era, when Greek astronomers began to systematically use this model and apply it as a standard in their calculations. In studies from 147 to 127 BC, the astronomer, mathematician, and geographer Hipparchus proposed dividing a day into 24 hours based on the 12 hours of daylight and 12 hours of nighttime on the equinox. However, for many centuries afterward, varying hour lengths continued to be used according to the seasons, and the fixed hour model only became popular in Europe with the advent of mechanical clocks in the 14th century.
Hipparchus and other Greek astronomers applied astronomical techniques developed earlier by the Babylonians who settled in Mesopotamia. The Babylonians performed astronomical calculations based on the sexagesimal system (base 60), a calculation system inherited from the Sumerians around 2000 BC. To this day, there is still no precise explanation for the choice of the number 60. One hypothesis suggests it was chosen for convenience in representing divisions since 60 is the smallest number that can be divided by 10, 12, 15, 20, and 30.
Although the sexagesimal system is not widely used in everyday calculations, it remains in use for measuring angles, geographic coordinates, and time. In fact, the round clock face we see today originates from a system developed 4000 years ago by the Babylonians.
The famous Greek astronomer known by the nickname beta, Eratosthenes (circa 276 to 194 BC), used the sexagesimal system to divide a circle into 60 equal parts, forming the system of geographic latitude with horizontal lines running through famous locations on Earth at that time. A century later, Hipparchus standardized the latitude lines, representing parallel lines to fit the shape of the Earth as perceived in that era. At the same time, he envisioned a longitude system with lines covering 360 degrees running from pole to pole.
Mathematician, geographer, and astronomer Claudius Ptolemy, who subdivided hours and named minutes and seconds
Subsequently, in his astronomical work Almagest (written in 150 AD), the Roman philosopher Claudius Ptolemy explained and expanded upon Hipparchus’s earlier research by subdividing the 360-degree latitude and longitude system into smaller segments. Each degree was divided into 60 parts and named partes minutae primae (or first minute), which we simply refer to today as minute. Each part was further subdivided into 60 smaller parts, which he called partes minutae secundae (or in English, second minute), now known as second. This regulation marked the birth of the terms minute and second that we still use today.
However, the concepts of minutes and seconds were not widely used in everyday life for many centuries following Ptolemy’s time. During that period, timekeeping devices took on various forms but were divided into halves, thirds, quarters, and sometimes into 12 parts without being split into 60 parts. Moreover, contemporary people were still unaware that 1 hour consisted of 60 minutes as we know it today. It wasn’t until the late 16th century that the advent of mechanical clocks included the division of 1 hour into 60 minutes on the clock face.
Conclusion
Thanks to ancient civilizations that sought to define and track time, modern society has come to adopt the convention that 1 day consists of 24 hours, 1 hour has 60 minutes, and 1 minute contains 60 seconds. This brings us to understand why a second in English is called “Second” (meaning the second division of time). However, advancements in the science of time have changed the way we define time with increasing accuracy. In 1967, according to modern physical conventions, 1 second was redefined as the duration of 9,192,631,770 cycles of the electromagnetic radiation emitted by a cesium-133 atom when it transitions between two hyperfine energy levels. This definition ushered in a new era of atomic time measurement and the establishment of Coordinated Universal Time (UTC).
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