Sijin: The Traditional 2-Hour Clock
Think of it as slicing the 24-hour day into 12 big pizza slices, each named after a zodiac animal.
Definition Sijin (時辰) is the fundamental unit of the traditional East Asian timekeeping system that divided a 24-hour day into 12 equal periods. Each sijin corresponds to exactly two modern hours, named after the twelve zodiac animals (Earthly Branches)—such as the Rat, Ox, and Tiger—to organize daily life.
Dividing the Day into 12 Generous Slices
Long before smartphones and ticking wristwatches, people tracked time by reading the sun's position and the shift of shadows. They divided the 24-hour day into twelve generous blocks, and each block was called one sijin (a traditional double-hour).
One sijin equals exactly two modern hours. That meant a full 24-hour day consisted of 12 sijin. Each of these twelve blocks was named in order after the twelve zodiac animals—starting with the Rat (Ja), Ox (Chuk), Tiger (In), and Rabbit (Myo).
For example, late night from 11:00 PM to 1:00 AM was the Hour of the Rat (Jasi), while midday from 11:00 AM to 1:00 PM, when the sun reached its peak, was the Hour of the Horse (Osi). Without needing strict minutes or seconds, these two-hour windows were plenty precise for farming, eating, and sleeping.
To our ancestors, time was not a frantic race measured by ticking seconds, but a relaxed rhythm aligned with the natural cycle of the sun.
Hidden Traces in Everyday Words
This traditional time system is not just an ancient relic locked away in history museums. It remains deeply embedded in everyday East Asian vocabulary, especially in Korean words for time.
Take the Korean word for noon, jeong-o. It literally means the exact middle of the Hour of the Horse (Osi). Similarly, midnight is jajeong, meaning the exact middle of the Hour of the Rat (Jasi).
Even the everyday terms for AM and PM come from this system. In Korean, morning is ojeon (literally "before the Horse hour") and afternoon is ohu ("after the Horse hour")—dividing the day before and after the midday Horse hour. Every time someone checks the morning or afternoon schedule, they are tapping into this centuries-old tradition.
Other hours also reflect the natural habits of animals—such as the Hour of the Ox (1:00 AM–3:00 AM), when roosters first prepare to crow, or the Hour of the Rooster (5:00 PM–7:00 PM) at sunset—leaving rich metaphors across traditional literature and sayings.
Need More Precision? Dividing the Two Hours
Did people back then have no way to measure periods shorter than two hours? Not at all. Royal courts, astronomers, and military commanders needed far greater precision for rituals, astronomical charting, and security.
To achieve this, each sijin was further split into smaller units. The most common was one gak (刻), representing 15 minutes, derived from the notched marks carved on traditional water clocks. One two-hour sijin was divided into 8 gak. This is why traditional idioms speak of every single gak as precious.
At night, the darkness was organized into five night watches called gyeong (更) to coordinate military patrols and city gate curfews. The famous "third watch" (samgyeong, 11:00 PM to 1:00 AM) signaled the dead of night.
By layering refined secondary units like gak and gyeong beneath the two-hour sijin, our ancestors operated a remarkably practical and scientific timekeeping system.
🤔 Common misconceptions
A sijin was the same length as a modern one-hour unit.
One sijin corresponds to two modern hours. Because a 24-hour day was divided into 12 units, each traditional time block lasted twice as long as an hour today.
The Korean words for AM (ojeon) and PM (ohu) are direct modern translations of Western terms.
They are traditional East Asian terms based on Osi (the Hour of the Horse, 11:00 AM–1:00 PM), referring to the time before and after that midday mark.
🧺 Where you meet it
A sijin is a traditional East Asian unit of time that divided the 24-hour day into twelve 2-hour periods, each named after a zodiac animal.