The Lunar Calendar in Japan

By Steve Renshaw and Saori Ihara

Revised January, 2000

There are a number of good sources on means of reckoning a lunar calendar, and these should be consulted for more in-depth methods [see Dershowitz and Reingold (1997) for example; Helmar Aslaksen also has an excellent article on the Mathematics of the Chinese Calendar]. Basically, as you would expect, the primary unit of a lunar calendar is the lunar "month". This is supposed to be based on "true" periods of lunation and can range from 29 days 8 hours to 29 days 19 hours [average lunation period=29.53... days]. A "month" is determined by taking the closest number of actual days corresponding to the period of lunation. Thus, a month can be either "short" with 29 days or "long" with 30 days. For any particular region or culture, the "start" or first day of a lunar month would be the day that the new moon occurs.

Obviously, the first day of a lunar "month" could sometimes differ for cultures with different longitudes; thus a lunar calendar based on longitude in China would not necessarily be accurate for Japan. Even with an accurate reckoning of the start of the month, 12 such "months" do not add up to the requisite number of days (365.24...) of an actual solar year. Ancient calendar scholars in a number of cultures found that after three solar years, the lunar calendar would be behind the sun's actual progress by about 33 days. For cultures based on agriculture, this was an intolerable system. To correct for the discrepancy, a leap "month" was added in appropriate years. Finding common multiples in days, it was initially determined that a 19 year cycle (Metonic Cycle) with 7 appropriate intercalary leap months could keep both solar and lunar reckoning in relative congruence (Chinese scholars in later times used a cycle of 76 years, and some experimented with even larger cycles). To keep the solar calendar aligned with actual seasons, it was required that the winter solstice always occur in a non-intercalary 11th month. Thus, generally in early lunar calendars, the new year began on the second new moon following winter solstice. Normal years based on a lunar calendar may have 353-355 days. Leap years may have 383-385 days. [It should be noted that in more modern times, as Dershowitz and Reingold (1997) point out, "...true, not mean, behavior of the moon and sun is used in calculations, and as a consequence, months twelve and one are very rarely followed by a leap month - in particular, Chinese New Year is not always the second new moon after the winter solstice" (p. 185). Calculations are based on a pre-determined "reference" meridian for each country or territory.]

A lunar calendar can be quite precise and synchronized with the seasons so long as correct astronomical data are used. While both calendar and navigational needs were important in the advancement of astronomy in Europe, it was improvements in lunar calendar reckoning that drove most of Chinese and later Japanese advances in astronomy. For both China and Japan, early calendars served more of an astrological function than one in keeping with accurate observation of astronomical phenomena. Chinese practices in both observation and calculation did, however, steadily advance over the centuries. Before the 18th century, improvements and reforms of the lunar calendar seemed to occur in China long before they reached Japan. After Jesuit missionaries visited China in the 16th Century, reform there was quite rapid. Meanwhile, both Emperors and Shogunates in Japan continued to value knowing "good" days and "bad" days (with associated astrology) more than precise calendar reckoning.

Solar Terms

Names for Months

MONTHS IN THE JAPANESE CALENDAR

EXPLANATION: JAPANESE NAMES FOR MONTHS

ENGLISH	MODERN JAPANESE NAME	OLD NAME	OLD NAME'S MEANING
January	IchiGatsu	MuTsuki	Harmony, Happy Spring
February	NiGatsu	KisaRagi	Seasonal Change of Dress
March	SanGatsu	YaYohi	Grass Grows Dense
April	ShiGatsu	UTzuki	Summer, Plant Rice
May	GoGatsu	SaTsuki	Rice Sprouts
June	RokuGatsu	MiNaTzuki	Watering Month (put water in the field)
July	ShichiGatsu	FuTzuki	Month of Letters
August	HachiGatsu	HaTzuki	Month of Leaves
September	KuGatsu	NagaTsuki	Autumn Long Month
October	JuuGatsu	KaNaTzuki	Month of Gods*
November	JuuIchiGatsu	ShimoTsuki	Month of Falling Frost
December	JuuNiGatsu	ShiHasu	"Poor Looking" Winter

• People believed that in NanaTsuki (October), all the gods and deities gathered at Izumo Shrine (near modern Matsue in Shimane Prefecture). In a sense, for other Prefectures, it was a "Month of No Gods".

The Solar System and Names for Days

For some time following adoption of a lunar calendar, Japan used Chinese sexagenary cycles for naming days and years. However, there is evidence that from 807, a seven day week with names related to the "planets" had found its way to Japan. Before this year, as in China, Sunday had no special significance (workers did not get any days "off"), and the most important aspect of each day was determining whether it was "good" or "bad" (a practice that continues in "unofficial" lunar calendar use to this day).

In 806, the Buddhist monk and scholar Koubou Daishi wrote that Japanese calendar scholars had trouble determining good and bad days because they did not know "Mitsubi" (the secret day). Having just returned from China, he wrote of hearing of a day called "Mitsu" and apparently determined that this day must be the "secret day". It turned out to be the day "Sunday". Actually, though sources are somewhat obscure, he may have heard the Samarkand equivalent to Sunday pronounced "mee-ruu" and mistook this for "mitsu". At any rate, from 807, "Mitsubi" was written into calendars as "Sun" day or "NichiYoubi".

It seems evident that by 1007, seven day weeks with nomenclature similar to that in Western cultures were common in Japan. In the diary of Michinaga Fujiwara, September 23, 1007 is recorded as a "Kayoubi" (Tuesday); simulating in Gregorian form, this is indeed the case. The seven "sei" (solar system bodies including the Sun, Moon, Mars, Mercury, Jupiter, Venus, and Saturn) were as important to early calendar scholars in Japan as they were anywhere else, regardless of the inadequacy of accurate methods for determining their motion. As in the West, similarity of names for days and names for the then known solar system gained acceptance with the latter being "cemented" by the Meiji Restoration.

JAPANESE KANJI (CHINESE CHARACTERS) FOR "PLANETS" AND DAYS

EXPLANATION: "PLANETS" AND DAYS

JAPANESE ROOT	ENGLISH	JAPANESE "PLANET"	ENGLISH	JAPANESE DAY	ENGLISH
Taiyou, Hi Nichi	Sun	Taiyou	Sun	NichiYoubi	Sunday
Tsuki, Gatsu Getsu	Moon	Tsuki	Moon	GetsuYoubi	Monday
Honoo, Hi Ka	Fire	KaSei	Mars	KaYoubi	Tuesday
Mizu Sui	Water	SuiSei	Mercury	SuiYoubi	Wednesday
Ki Moku	Tree	MokuSei	Jupiter	MokuYoubi	Thursday
Kin	Gold	KinSei	Venus	KinYoubi	Friday
Tsuchi Dou	Land	DouSei	Saturn	DouYoubi	Saturday

Calendar Reform in Japan

In many ways, theirs is a sad story. Shibukawa and Yasutomi seem to have been more theoretical while Tani held sway in empirical investigation and calculation. Seeing the current calendar to be in error by up to two days, they had an uphill struggle against a general obsession with astrology, mystic assumptions of the emperor, disinterest by the Shogunate, and their own lack of information from the "outside" world. By this time, Jesuit missionaries, well aware of events in Europe, were helping Chinese calendar scholars to develop accurate ephemerides and thus more accurate calendars. Unaware of these developments, yet seeing "consistent" inconsistencies in their own observations and calculations, the three scholars of Japan, especially the empirically oriented Tani, worked hard to develop an accurate means of reckoning. While they were able to get a more accurate version of an older (1282) calendar from China adopted, they were nevertheless restricted by time and place from making advances that would have truly made their observations meaningful. While other reforms were introduced in the later Edo era, it was not until the Meiji Restoration that Japan joined the world stage in having a consistent and relatively accurate method for recording dates. This, of course, was the Gregorian calendar.

In modern times, many of the aspects of the "old" lunar system remain in Japanese culture including determination of some festivals and observances (such as "Tanabata" on the seventh day of the seventh month), names for years after the current Emperor (1996 is Heisei 8), and the old Chinese sexagesimal and astrological associations (familiar animals such as mouse, cow, cat, rabbit, etc.). While many Japanese faithfully continue to plan events with their "supplemental" lunar calendars, the Gregorian system is officially recognized for all legal transactions.

Matasaburou's Calendar

Discrepancy between solar terms in the calendar Matasaburou used and actual astronomical phenomena may be seen in another example. 1664 was indeed a leap year, and in Matasaburou's December 12th (Gregorian) entry, he mentions that "May" (an extra 5th month) was the leap month for 1664. However, if this were so, then the extra "May" would have included the principle term "Great Heat" (120 degrees in solar longitude); as we know, this would not be an accurate intercalary month. In fact, using truer astronomical observations, the extra month would have been inserted after "June" (an extra 6th month). It is possible that Matasaburou simply forgot which month was a leap month, but an equally probable "cause" for this inconsistency could be the lack of accurate reckoning mentioned above.

Finally, in Matasaburou's February 6, 1665 (Gregorian) entry, he mentions that day as the first day of Spring. Using actual astronomical measures, February 3, 1665 would have been the more accurate date for this sectional term. In our assessment of Matasaburou's diary, we show the lunar dates that he recorded. The Gregorian dates listed were obtained by matching known phenomena (such as the lunar eclipse on February 11, 1664 and the solar eclipse that occurred on January 16, 1665) and "interpolating" intervening dates.

Izuru, S. (Ed.) (1994) Koujien. (Japanese Etymological Dictionary). Iwanami Shoten, Tokyo.

Kasahara, K. (Ed.) (1995) Nippon no Rekishi (The History of Japan). Kodansha, Tokyo.

Masuda, K. (Ed.) (1995) Kenkyusha's New Japanese-English Dictionary. Kenkyusha, Tokyo.

North, J. (1995) The Norton History of Astronomy and Cosmology. W.W. Norton and Co., New York.

Pannekoek, A. (1961) A History of Astronomy. Interscience Publishers, Inc., New York.

Scheffler, A.O. and P.P. (1991) CalMaster2000: Dates, Holidays, and Astronomical Events. Zephyr Services, Pittsburgh, PA.

Shunichi, U. et al (Eds.) (1991) Nihon Zenshi; Japan Chronik. Kodansha, Tokyo.

Sugimoto, M. and D.L. Swain (1989) Science and Culture in Traditional Japan. Charles E. Tuttle Co. Inc., Tokyo.

Uchida, M. (1990) Koyomi to Tenmon: Ima Mukashi. (Calendars and Astronomy: Now and Then). Maruzen Co. Ltd., Tokyo.

Watanabe, T. (1984) Kinsei Nihon Tenmon Gakushi; Vol I. (A View of Japanese History of Astronomy in Modern Times; Vol I). Koseisha Kosaikaku, Tokyo.