The Japanese Lunar-Solar Calendar

Introduction

For almost thirteen centuries Japan used a lunar-solar calendar, created in 602 (by the Korean priest Kanroku) and formally adopted in 604, replacing a less accurate system which days and months were 'calculated' based on the season and the moon. The lunar-solar calendar was only replaced at the time of the Meiji Restoration by a purely solar calendar, with a year of 365 days, on January 1, 1873.

Years

The Japanese used a number of different systems to designate the year.

One widely-used one, Chinese in origin, was based on imperial reign names, called nengō, in which years were identified by the name of the nengō period, and the number of years since the period had started. Use of this system commenced in 645, and it has been in constant use since 672; in fact, it is still used today.

Under the lunar-solar calendar, there was also a sixty-year cycle, the kanshi or eto, based on the combination of a 12-year zodiacal cycle (jūnshi), and a system of ten 'celestial stems' (jikkan). The zodiacal cycle is the same as the Chinese zodiacal calendar, with each year having an associated animal, e.g. rat, ox, etc. (Anyone who has ever eaten in a Chinese restarant will no doubt recall this from the placemats.) The stems were based on the five elements (earth, water, fire, metal, and wood), and were duplicated as 'younger brother' and 'older brother' elements.

Months

The lunar-solar year is divided into twelve months, some of twenty-nine days, the others of thirty; this is because a lunar month is twenty-nine and a half days. The short months are called shō no tsuki (literally, "month of short", with tsuki meaning both "month" and "moon cycle"), and the long ones called dai no tsuki ("month of long").

The pattern of long and short months changes from year to year, for religious reasons, and in fact there was a bias toward using a pattern which had not previously been used (although some patterns were re-used). Thus, it is necessary to consult a table (a partial one is given at the end of this page) to discover the pattern of long and short months for any given year.

Individual months start on the astronomical new moon, i.e. when the sun, moon and earth are in line (and thus the days on which solar eclipses can occur). The months do each have several names, but are usually known simply as "first month", etc.

The yearly cycle started with the moon in Pisces, in February and March; New Year's Day is nominally the new moon nearest to the first day of spring (as reckoned in the Asian system, where seasons begin half-way between the equinoxes and solstices), i.e. between January 21st and February 19th at this point in time.

Note that this offset for the start of the year means that dates towards the very end of a Japanese lunar-solar calendar year are actually in the next year, as reckoned by the Western calendar. So even though the year "Genroku 15" is normally converted to 1702, the 15th day of the 12th month of that year (the date of the attack of the Forty-Seven Ronin) was actually in 1703 in the Western calendar (January 31st, to be exact).

Intercalary Months

One additional complexity is that the lunar-solar calendar was not well aligned to the solar year, and tended to quickly drift out of synchronization with it. With 12 lunar months of twenty-nine and a half days, for a lunar year of 354 days, each lunar year starts about 11 days earlier, with respect to the solar calendar (which controls the seasons), than the previous one. To fix this, periodically (approximately every three years or so) an extra intercalary month was interpolated to bring it back into phase, so in those years there are a total of thirteen lunar months.

The choice of where to interpolate the extra month was determined in a somewhat complex way. The solar year was divided into twelve solar months by twelve 'solar points': the solstices and equinoxes, and two points equidistant between each of those major points. Because the lunar month of twenty-nine and a half days is shorter than the solar month of thirty and a half days, periodically a lunar month will fall entirely between a pair of solar points.

Such months are designated as intercalary months (uruu), and are given the same number as the previous lunar month. So, when there are two 'fifth months', the first one is just 'fifth month', but the following intercalary month is called 'uruu fifth month'.

Although it is theoretically possibly to calculate which years have an additional intercalary month, and when it falls, the calculation is very complex: the usual method is to just refer to a table, such as the one at the end of this page. Note that intercalary months also come in short and long forms, and this too is indicated in the table.

Table of Short and Long and Intercalary Months

Year Zodiac Sign Long Months Short Months Long Intercalary Short Intercalary
1688dragon2 5 7 9 10 111 3 4 6 8 12
1689snake1 2 5 7 9 10 113 4 6 8 121
1690horse1 3 6 9 10 122 4 5 7 8 11
1691goat1 2 4 7 9 10 123 5 6 8 118
1692monkey1 3 4 7 10 122 5 6 8 9 11
1693cock1 2 4 6 8 113 5 7 9 10 12
1694dog1 2 4 5 6 8 103 7 9 11 125
1695boar1 3 4 6 8 9 112 5 7 10 12
1696rat1 4 6 7 9 10 122 3 5 8 11
1697ox2 4 6 8 9 11 121 3 5 7 102
1698tiger2 5 8 9 11 121 3 4 6 7 10
1699hare1 3 6 9 10 11 122 4 5 7 89
1700dragon1 3 6 9 11 122 4 5 7 8 10
1701snake1 3 5 7 10 122 4 6 8 9 11
1702horse1 3 4 6 8 10 122 5 7 9 118
1703goat2 3 5 6 8 101 4 7 9 11 12
1704monkey1 3 5 6 8 9 112 4 7 10 12
1705cock
1706dog1 4 7 8 10 11 122 3 5 6 9
1707boar2 5 8 10 11 121 3 4 6 7 9
1708rat1 2 5 8 10 11 123 4 6 7 91
1709ox2 3 6 9 11 121 4 5 7 8 10
1710tiger2 3 5 7 9 111 4 6 8 10 128
1711hare1 2 3 5 7 10 124 6 8 9 11
1712dragon2 3 5 7 8 101 4 6 9 11 12
1713snake1 3 5 6 7 9 10 122 4 8 115
1714horse3 5 7 8 10 111 2 4 6 9 12
1715goat1 4 7 8 10 11 122 3 5 6 9
1716monkey2 4 7 9 10 111 3 5 6 8 122
1717cock1 2 5 8 10 113 4 6 7 9 12
1718dog1 2 4 6 9 123 5 7 8 10 1110
1719boar1 2 4 6 9 113 5 7 8 10 12
1720rat1 2 4 5 7 10 123 6 8 9 11
1721ox2 4 5 7 8 10 121 3 6 9 117
1722tiger2 4 6 7 9 111 3 5 8 10 12
1723hare1 3 6 7 9 10 122 4 5 8 11
1724dragon1 4 6 8 9 10 122 3 5 7 114
1725snake1 4 7 9 10 122 3 5 6 8 11
1726horse1 3 5 8 10 122 4 6 7 9 11
1727goat1 3 5 8 10 122 4 6 7 9 111
1728monkey1 3 4 6 9 112 5 7 8 10 12
1729cock1 3 4 6 8 112 5 7 9 10 129
1730dog1 3 5 6 8 10 122 4 7 9 11
1731boar2 4 6 8 9 111 3 5 7 10 12
1732rat1 3 7 8 9 112 4 5 6 10 125
1733ox1 3 6 8 9 11 122 4 5 7 10
1734tiger2 4 7 9 11 121 3 5 6 8 10
1735hare1 3 4 7 9 11 122 5 6 8 103
1736dragon2 3 5 8 11 121 4 6 7 9 10
1737snake2 3 4 6 9 11 121 5 7 8 1011
1738horse2 3 5 7 9 111 4 6 8 10 12
1739goat1 3 5 6 8 10 122 4 7 9 11
1740monkey2 5 6 8 10 121 3 4 7 9 117
1741cock2 5 7 8 10 111 3 4 6 9 12
1742dog1 3 6 8 10 11 122 4 5 7 9
1743boar2 4 6 9 10 11 121 3 5 7 84
1744rat2 4 7 10 111 3 5 6 8 9 12
1745ox1 2 3 5 8 11 124 6 7 9 1012
1746tiger1 2 4 6 8 11 123 5 7 9 10
1747hare2 4 5 7 9 111 3 6 8 10 12
1748dragon2 3 5 7 8 10 111 4 6 9 1210
1749snake1 4 6 7 9 10 122 3 5 8 11
1750horse
1751goat1 3 6 7 9 10 122 4 5 8 116
1752monkey1 3 6 9 10 122 4 5 7 8 11
1753cock1 2 4 7 10 113 5 6 8 9 12
1754dog1 2 3 4 7 10 125 6 8 9 112
1755boar1 2 4 6 8 113 5 7 9 10 12
1756rat1 2 4 6 7 9 113 5 8 10 1211
1757ox1 3 4 6 8 9 112 5 7 10 12
1758tiger1 4 6 8 9 10 122 3 5 7 11
1759hare2 5 7 8 9 11 121 3 4 6 107
1760dragon2 5 8 9 11 121 3 4 6 7 10
1761snake1 3 6 9 11 122 4 5 7 8 10
1762horse1 2 4 6 9 11 123 5 7 8 104
1763goat1 3 5 7 10 122 4 6 8 9 11
1764monkey1 3 4 6 8 112 5 7 9 10 1212
1765cock2 3 5 6 8 101 4 7 9 11 12
1766dog1 3 5 6 8 9 112 4 7 10 12
1767boar2 4 6 8 9 10 111 3 5 7 129
1768rat1 4 7 8 10 11 122 3 5 6 9
1769ox2 5 8 10 11 121 3 4 6 7 9
1770tiger1 3 6 8 10 11 122 4 5 7 9
1771hare2 4 6 9 11 121 3 5 7 8 10
1772dragon2 3 5 7 10 121 4 6 8 9 11
1773snake2 3 4 5 7 10 121 6 8 9 113
1774horse2 3 5 7 9 111 4 6 8 10 12
1775goat1 3 5 7 8 10 112 4 6 9 1212
1776monkey1 3 5 7 9 10 112 4 6 8 12
1777cock1 4 7 9 10 112 3 5 6 8 12
1778dog1 2 5 9 10 113 4 6 7 8 127
1779boar1 2 5 8 10 113 4 6 7 9 12
1780rat1 2 4 6 9 113 5 7 8 10 12
1781ox1 2 3 5 6 9 114 7 8 10 125
1782tiger1 2 4 6 8 10 123 5 7 9 11
1783hare2 4 5 7 9 111 3 6 8 10 12
1784dragon1 2 4 6 7 9 113 5 8 10 121
1785snake1 3 6 7 9 10 122 4 5 8 11
1786horse2 4 7 9 10 121 3 5 6 8 1110
1787goat1 4 7 9 10 122 3 5 6 8 11
1788monkey1 3 5 8 10 122 4 6 7 9 11
1789cock1 2 4 6 8 10 123 5 7 9 116
1790dog1 3 4 6 9 112 5 7 8 10 12
1791boar1 3 4 6 8 10 122 5 7 9 11
1792rat2 3 5 6 8 10 121 4 7 9 112
1793ox2 4 6 8 9 111 3 5 7 10 12
1794tiger1 3 6 8 9 102 4 5 7 11 1211
1795hare1 3 6 8 9 11 122 4 5 7 10
1796dragon2 4 7 9 11 121 3 5 6 8 10
1797snake1 3 5 8 10 11 122 4 6 7 97
1798horse2 3 5 8 11 121 4 6 7 9 10
1799goat2 3 4 7 9 121 5 6 8 10 11
1800monkey1 3 4 5 7 9 112 6 8 10 124
1801cock1 3 5 7 8 10 122 4 6 9 11
1802dog2 5 7 8 9 111 3 4 6 10 12
1803boar1 2 5 7 8 10 113 4 6 9 121
1804rat1 3 6 8 10 11 122 4 5 7 9
1805ox2 4 8 9 10 111 3 5 6 7 128
1806tiger1 2 4 8 10 113 5 6 7 9 12
1807hare1 2 3 5 8 11 124 6 7 9 10
1808dragon2 3 5 8 111 4 6 7 9 10 126
1809snake1 2 4 5 7 9 113 6 8 10 12
1810horse2 4 5 7 8 10 121 3 6 9 11
1811goat4 6 7 9 10 121 2 3 5 8 112
1812monkey2 5 7 9 10 111 3 4 6 8 12
1813cock1 3 7 9 10 11 122 4 5 6 811
1814dog1 3 7 9 10 122 4 5 6 8 11
1815boar1 2 4 8 10 123 5 6 7 9 11
1816rat1 2 4 6 8 10 123 5 7 9 118
1817ox1 3 4 6 8 112 5 7 9 10 12
1818tiger1 2 4 6 7 9 123 5 8 10 11
1819hare2 4 5 6 8 9 111 3 7 10 124
1820dragon2 4 6 8 9 10 121 3 5 7 11
1821snake2 5 7 9 10 121 3 4 6 8 11
1822horse1 2 6 8 9 11 123 4 5 7 101
1823goat1 3 7 9 11 122 4 5 6 8 10
1824monkey1 2 5 8 9 11 123 4 6 7 108
1825cock1 3 5 7 10 122 4 6 8 9 11
1826dog1 3 4 6 8 112 5 7 9 10 12
1827boar1 3 4 6 7 8 112 5 9 10 126
1828rat1 3 5 6 8 10 112 4 7 9 12
1829ox2 4 6 8 9 11 121 3 5 7 10
1830tiger3 4 7 8 10 11 121 2 5 6 93
1831hare2 6 8 10 11 121 3 4 5 7 9
1832dragon1 3 7 9 11 122 4 5 6 8 1011
1833snake2 4 7 9 11 121 3 5 6 8 10
1834horse2 3 5 8 10 121 4 6 7 9 11
1835goat2 3 5 6 8 10 121 4 7 9 117
1836monkey2 4 5 7 9 111 3 6 8 10 12
1837cock1 3 5 7 8 10 122 4 6 9 11
1838dog2 4 6 7 9 10 121 3 5 8 114
1839boar2 5 7 9 10 111 3 4 6 8 12
1840rat1 2 6 8 10 11 123 4 5 7 9
1841ox3 6 8 10 111 2 4 5 7 9 121
1842tiger1 2 4 7 9 113 5 6 8 10 12
1843hare1 2 3 5 8 114 6 7 9 10 129
1844dragon1 2 4 6 8 10 123 5 7 9 11
1845snake2 4 5 7 9 111 3 6 8 10 12
1846horse1 3 5 6 7 9 112 4 8 10 125
1847goat1 3 6 7 9 10 122 4 5 8 11
1848monkey2 5 7 9 10 111 3 4 6 8 12
1849cock
1850dog1 3 6 8 10 122 4 5 7 9 11
1851boar1 2 4 7 9 113 5 6 8 10 12
1852rat1 2 3 4 7 9 115 6 8 10 122
1853ox1 3 4 6 8 10 122 5 7 9 11
1854tiger2 4 6 7 8 10 121 3 5 9 117
1855hare2 5 6 8 9 111 3 4 7 10 12
1856dragon1 4 6 8 9 10 122 3 5 7 11
1857snake2 5 6 8 9 11 121 3 4 7 105
1858horse2 5 8 9 11 121 3 4 6 7 10
1859goat1 3 6 9 11 122 4 5 7 8 10
1860monkey1 3 6 9 11 122 4 5 7 8 103
1861cock2 3 5 7 9 121 4 6 8 10 11
1862dog1 3 4 6 8 9 112 5 7 10 128
1863boar2 3 5 7 8 10 121 4 6 9 11
1864rat3 5 7 8 9 111 2 4 6 10 12
1865ox1 4 7 8 10 112 3 5 6 9 125
1866tiger1 3 7 8 10 11 122 4 5 6 9
1867hare2 4 8 10 11 121 3 5 6 7 9
1868dragon2 3 5 8 10 111 4 6 7 9 124
1869snake1 2 3 6 9 11 124 5 7 8 10
1870horse2 3 5 7 9 111 4 6 8 10 1210
1871goat1 2 4 5 7 9 123 6 8 10 11
1872monkey2 4 5 7 8 10 121 3 6 9 11

Acknowledgements

Thanks to (in alphabetical order) Peter Chiappa, who helped with the entry and checking of the data on intercalary months.
Back to JNC's home page
© Copyright 2009, 2014 by J. Noel Chiappa 

Last updated: 1/April/2014