.The preparation of calendars was a foremost responsibility of the Bureau of Astronomy (Gwansanggam), the agency that presided oveThe preparation of calendars was a foremost r matters related to astronomy, calendars, fortune telling, and water clocks. The head of the Bureau of Astronomy was the equivalent of today’s Prime Minister. A calendar was a fundamental resource for calculating the positions of stars, and was used to produce an almanac for the forthcoming year. An almanac contained information on various dates and times of vital importance for everyday life, in particular agricultural activities. On each winter solstice (the day with the longest nighttime hours, December 22 by the solar calendar), the Bureau of Astronomy distributed the almanac for the new year to government officials as well as commoners.
.Seven Stars Constellations Calendar
.From the Three Kingdoms period (1st century B.C.-7th century A.D.), Koreans adopted Chinese calendars because they were unable to develop a calendar of their own. King Chungseon (r. 1308-1313) of the Goryeo Dynasty (918-1392) adopted the Shoushi calendar developed by Guo Shoujing (1231-1316) of China’s Yuan Dynasty. This was the most advanced calendar of its time, founded on precise calculations of the time and position from the starting point, the winter solstice, which specified the length of one year as 365.2425 days and one month as 29.530593 days.
.However, the Shoushi calendar was prepared for Yuan China, and as such was not properly oriented for the location of Joseon Korea. Recognizing this, King Sejong (r. 1418-1450) built a royal observatory on the palace grounds so that astronomical observations could be conducted for the purpose of developing a calendar that reflected the actual latitude of the capital Hanyang (Seoul). He instructed calendrical specialists Yi Sun-ji (?-1465) and Kim Dam (1416-1464) to develop a calendar that adjusted the Shoushi calendar so that Hanyang was the central reference point. Sejong used this calendar, called Chiljeongsan Naepyeon (Seven Stars Constellations Calendar), to establish a new time system. Chiljeong, or Chiryo, refers to seven heavenly bodies: the sun, moon, and five stars.
.King Sejong also ordered clockmaker Jang Yeong-sil to create Jagyeongnu, an automated water clock that automatically announced the time at specific intervals. After this clock was installed at Borugak Pavilion of Gyeongbokgung Palace, from the first day of the seventh lunar month of 1434 it became the standard for telling time in Korea. The time-interval signals of Jagyeongnu were relayed to Gwanghwamun Gate in the daytime to announce high noon, and to the Jongnu bell pavilion on Jongno Street at dawn and in the early evening to announce the opening and closing of the city gates. To announce the curfew at dusk, the bell was rung 28 times, while 33 rings were sounded at dawn to indicate the end of curfew and opening of the city gates.
.Integrated Time System
.The Seven Stars Constellations Calendar defined the length of one day as the period from midnight to midnight of the next day. This length of time was divided into 12 double-hours, which were divided into cho “beginning” and jeong “middle.” A double-hour was equal to two of today’s 60-minute hours. The 12 double-hours were named after the 12 animals of the Oriental zodiac: rat, ox, tiger, rabbit, dragon, snake, horse, sheep, monkey, chicken, dog, and boar. In conjunction with the 12 double-hour system, the calendar utilized a 5 night-watch system, under which the time between sunset and the next day’s sunrise was defined as the time of night, which was divided into 5 night-watches (ogyeong), that were further divided into 5 night-watch points (ojeom). The night-watches were numbered 1 (first) through 5, while the night-watch points were referred to as “first night-watch, initial point,” then “first night-watch, second point,” and so on up to “fifth night-watch, fifth point.” Because the length of the night changed throughout the year, it had to be calculated according to the season. Therefore, an operating manual for the water clock, entitled Nujutongui, (Operating Manual of the Clepsydra), was developed on the basis of the Seven Stars Constellations Calendar, applying both the system of “night-watches and night-watch points” and the system of “12 double-hours and 100 equal intervals.” Meanwhile, at military camps in outlying areas and in rural areas, where the water clock could not be used to tell the time of night, a meridian star observation method was used to calculate the time from the passing of the 28 solar stages along the zodiac, according to season.
.Clock for Commoners
.The background of the creation of the angbuilgwi scaphe sundial is recorded in the Annals of King Sejong on the 19th day of the fourth lunar month of 1437: “From concern that the ignorant common people are slow to know the time, two sundials have been made with images of the 12 double-hour figures drawn on the inside so that the common people can easily know the time. One sundial has been placed alongside Hyejeonggyo Bridge, the other by the street on the south side of Jongmyo, the royal ancestral shrine.” These sundials were the first clocks made and displayed for public use so that people could tell the time of day.
.Angbuilgwi is also known as omok or concave sundial. It is a universal sundial made by cutting off the upper half of a celestial globe at the equator, to form a bowl, and then drawing length and width lines on the inner surface. The hour-circle line is engraved horizontally, and beneath it the 12 zodiac animals are drawn. Six vertical lines are delineated from top to bottom across the equator, along with 24 lines marking seasons in fortnight periods by solar declination. At the South Pole, a pole-pointing gnomon is set up to cast a shadow, so that by observing the position of the shadow cast over the 24 lines, the time can be determined, according to the season.
.From the 18th century, numerous sundials were produced according to the 24 integral quarter-hour system, based on Western time-reckoning, with Hanyang’s latitude of 37 degrees 39 minutes 15 seconds north serving as the standard. As such, in the late 19th century, the family of Gang Yun became known as far away as China for producing a variety of accurate sundials, from portable scaphe sundials to planar versions. Planar sundials were popular among the common people, together with seonchu pendants, which could tell the time and indicate the geographical direction at the same time.
.Guardian of Standard Time
.The most renowned Korean water clock, the Jagyeongnu, was created in 1434 by the master clockmaker, Jang Yeong-sil. This mechanized clock used an analog-type mechanism, a float-rod clepsydra, to measure the time, and at the same time used digital-type time-annunciating mechanisms to announce time intervals according to both the system of 12 double-hours and the system of 5 night-watches and night-watch points.
.The double-hours, night-watches, and night-watch points were announced by audible time-signal mechanisms that used time-annunciating jack work with wooden figurines to strike a bell on each double-hour, a drum on each night-watch, and a gong on each night-watch point. The time-indicators for double-hours included both an audible time-indicator, which sounded a bell 12 times each day, and a visible time-indicator that used 12 time-period bearing double-hour jacks, inscribed with the names of the 12 double-hours, to position 12 wooden figurines (symbolizing the 12 zodiac animals) into a window space when the bell sounded.
.The five night-watches and the night-watch points were announced with digital-type audible time-indicators in which figurines that announced the watches and points struck a drum or a gong based on the respective watch and point. All the audible time-indicators relied on the force of a ball that fell in accordance with the time-telling device of the water clock. This time-telling device could be considered a forerunner of the modern digital counter. During the 15th century, Jagyeongnu was the only clock in East Asia with a digital-type time-telling feature.
.Because the water clock developed by Jang Yeong-sil was thought to be similar to a palace clock produced by Emperor Shundi of China’s Yuan Dynasty, King Sejong gave it the title of Jagyeonggungnu or “Automatic Striking Palace Clock,” which is the origin of the Jagyeongnu name. The distinguished British science historian Joseph Needham (1900-1995) believed that the idea for using a falling ball for the time-telling device of Jagyeongnu came from the Elephant Clock, the fourth water clock described in the Book of Knowledge of Ingenious Mechanical Devices that was written by the Arab scholar al-Jazari in 1206, which had been introduced to Korea by way of Yuan Dynasty China.
.In 1536, an improved version of Jagyeongnu at Borugak Pavilion was revealed, known as the New Borugak Water Clock, which automatically announced the beginning and end of the night curfew period. Borugak Jagyeongnu was destroyed during the Japanese invasions of 1592-1598, while the surviving relic, the Automatic Striking Water Clock of Borugak (National Treasure No. 229), is a remnant of the original New Borugak Water Clock, consisting only of the clepsydra vessels, with the time-telling device having been lost. Today, this artifact is featured on Korea’s 10,000-won bill to commemorate its heritage of world-class scientific technology, which remains a source of pride for the Korean people.
.In 1438, the 20th year of the reign of King Sejong, Jang Yeong-sil developed another clock, the Automatic Striking Water Clock of Heumgyeonggak, known as a “heavenly clock,” which adopted principles of Jagyeongnu to display astronomical phenomena. Thereafter, the tradition of Jagyeongnu was carried on by Song I-yeong, an astronomy scholar during the reign of King Hyeonjong (r. 1659-1674), who in 1669 created an armillary sphere, the remains of which are now designated National Treasure No. 230, Armillary Clock.
.Jagyeongnu represented an epochal innovation in the history of East Asian clock-making technology that highlighted the cumulative brilliance of the Joseon Dynasty’s politics, philosophy, science, technology, religion and art. Joseph Needham called it the “Striking Clepsydra.” As such, this time-keeper was a truly extraordinary creation that served as the guardian of the Joseon Dynasty’s standard time and demonstrated the ingenuity of the Korean people, who can be said to have opened a new chapter in the field of automation and robotics. Today, Jang Yeong-sil’s Jagyeongnu, which was developed at the time of King Sejong, is being re-created. Soon enough, Jagyeongnu will be restored to its former glory.