Japan's first calendar came from China via Korea. In the middle of the 6th century, the Yamato Imperial Court, which ruled Japan at the time, invited a priest from a country called Paekche (Kudara in Japanese), in what is now Korea, to learn from him how to draw up a calendar, as well as astronomy and geography. Reportedly, Japan organized its first calendar in the 12th year of Suiko (604).
The calendar used then was called "Tai-in-taiyo-reki," a lunisolar calendar, or "Onmyo-reki."
Each month was adjusted to the cycle of moon's waxing and waning. Since the moon orbits the earth in about 29.5 days, adjustment was required and this was done by making months with either 30 days or 29 days, the former, "Dai-no-tsuki (long month)," the latter, "Sho-no-tsuki (short month)." Aside from the moon's orbit round the earth, the earth orbits the sun in 365.25 days, which, as we all know, causes the seasonal changes. Thus, merely repeating long and short months gradually produced a discrepancy between the actual season and the calendar. To compensate for this, a month called "Uru-zuki," or intercalary month, was inserted every few years to produce a year with 13 months, with the order of longer and shorter months changing year by year.
Unlike our contemporary calendar in which there is no change in the order of months, back then the fixing of a calendar was deemed so important that it was placed under the control of the imperial court and, in the later Edo period, under the superimposed military shogunate regime.
Spread of the Guchu-reki and Kana-goyomi calendars
The calendar established by Onmyo-ryo was called "Guchu-reki," one in which various words indicating seasons, annual events and daily good omens were written in Chinese characters and called "Reki-chu (calendar notes)." The Guchu-reki derives its name from the fact that the notes were written in detail.
This Guchu-reki, which was in service until the Edo period, was used particularly by noblemen in ancient and medieval times, individuals wont to base their everyday activities on the calendar. They often wrote a personal diary in the blank spaces or on the back of their personal calendar. These entries remain left valuable historical records of the era.
20th year of O-ei (1413) Guchu-reki (Jugo Mansai nikki)
3rd year of Koei (1344) Kana-goyomi (Moromori-ki. Vol. 16)
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