Sijin: The Traditional Korean Two-Hour Time Unit

People long ago had no smartphones or wristwatches ticking off the seconds. They read the time from how high the sun stood and how long shadows stretched. They cut the 24-hour day into twelve big chunks, and each chunk was called a sijin (時辰).

One sijin is equal to 2 hours in modern time, so a 24-hour day holds 12 sijin. Each of the twelve was named after one of the Earthly Branches in order: ja, chuk, in, myo, jin, sa, and so on. Each branch comes with a zodiac animal you probably know: rat (ja), ox (chuk), tiger (in), rabbit (myo), dragon (jin), and snake (sa).

The middle of the night, from 11 p.m. to 1 a.m., was jasi, the Rat hour. When the sun sat right overhead, from 11 a.m. to 1 p.m., it was osi, the Horse hour. One catch: the old sijin followed where the sun actually stood in that region. Converted to Korea Standard Time, jasi runs about 30 minutes later, from roughly 11:30 p.m. to 1:30 a.m. Nobody needed minutes or seconds to farm, eat dinner, or go to bed. Two-hour blocks were plenty.

For people then, time wasn't something that chased you down to the second. It was a roomy stretch that moved along with nature's rhythm and the turning of the seasons.

The time system of the Joseon dynasty isn't a dusty story locked in a museum. It's alive in plenty of words Koreans use every day without a second thought.

Take jeongo, the Korean word for noon. It comes from the idea of the middle of the osi (午時), the Horse hour. In the same way, jajeong, the word for midnight, means the middle of jasi (子時), the Rat hour.

It goes further. Ojeon (午前, before noon) and ohu (午後, after noon) are Korean words for a.m. and p.m. They simply split the day into before and after the Horse hour. So every time we separate morning from afternoon, we're unknowingly using our ancestors' time unit.

Each sijin also had nicknames drawn from nature. Chuksi (1 a.m. to 3 a.m.) was called gyemyeong (鷄鳴), "the rooster crows," and yusi (5 p.m. to 7 p.m.) was irip (日入), "the sun goes down."

So could people back then not measure anything shorter than 2 hours? Of course they could. The royal court and government offices needed far more precise timekeeping for astronomical observation, state ceremonies, and military operations.

So they also used a smaller unit called the gak (刻). The word comes from the notches (carved marks) on a water clock. For a long time the day was divided into 100 gak, so one gak was about 14.4 minutes. After the Siheon calendar was adopted in late Joseon, the day was divided into 96 gak, making one gak exactly 15 minutes. It's the same gak you find in Korean sayings like "fighting over a chokgak" (every minute counts) and "one ilgak feels like three autumns" (a short wait that drags on like three years). Under the 96-gak system, one sijin (2 hours) held exactly 8 gak.

The dark hours after sunset had their own five-part system called the gyeong (更). Soldiers made their patrols and opened and closed the city gates by it. The length of a gyeong changed with the season, since nights are longer in some seasons than others. In Korean historical dramas, you often hear samgyeong (roughly 11 p.m. to 1 a.m). It was the signal that the night had grown deep.

So beneath the big frame of the sijin, our ancestors wove in fine helper units like gak (刻), gyeong (更), and jeom (點). That let them keep time in a flexible and scientific way to suit the situation.