Gregorian-Lunar Calendar Mapping: Sexagenary Cycle Rules

The Chinese calendar is often treated as if it were simply the lunar counterpart of the Gregorian calendar, but the two systems are not parallel translations of one another. A date in one calendar cannot be converted to the other by a fixed formula that remains valid across years. The practice of keeping a 國曆農曆對照 table exists precisely because the correspondence changes every year, and because the Chinese calendar contains structural features, such as intercalary months and solar terms, that have no direct equivalent in the Gregorian system.

國曆與農曆量的不是同一件事

The Gregorian calendar is a solar calendar. Its months are conventional divisions of the year and are not tied to the phases of the Moon. The Chinese calendar, by contrast, is a lunisolar calendar. Its months are lunar months: each month begins on the day of the new moon and is either 29 or 30 days long, following the cycle of lunar phases. The year, however, is kept aligned with the solar year through the insertion of intercalary months.

A purely lunar calendar, such as the Islamic calendar, does not add intercalary months. Its months therefore drift through the seasons over a cycle of about 33 years, so that a month that falls in summer in one period will eventually fall in winter. The Chinese calendar avoids this drift by adding a leap month in certain years, so that the first month does not migrate into summer. This is the basic sense in which the calendar is lunisolar: the month follows the Moon, while the year is corrected to follow the Sun.

That correction has a consequence. Because the length of a Chinese calendar year is not fixed, the Gregorian date corresponding to a given lunar date is not fixed either. A Chinese year may be as short as 353 days or as long as 385 days, depending on the number and distribution of long and short months and on whether an intercalary month is present. The same lunar date therefore falls on a different Gregorian date each year. A conversion table is not a convenience; it is the only practical way to make the correspondence.

對照為什麼每年都要重查

The variability of the Chinese year means that a rule such as “the first day of the first month is always near early February” is only approximate. In some years the lunar new year falls in late January; in others it falls in late February. The same is true for every other lunar date. Because the lunar year is shorter than the solar year in ordinary years, the lunar calendar runs ahead of the Gregorian calendar by about 11 days each year until an intercalary month pulls it back.

For example, a date expressed as 正月初一 in one year may correspond to 10 February, while in another year it may correspond to 22 January or 19 February. There is no simple offset that can be applied year after year. Each year’s table must be consulted anew. This is also why the same Chinese date can appear twice in relation to different Gregorian years only in a superficial sense: the actual identity of the day is determined by the lunar month and the solar year in which it occurs, and that relationship is reset annually.

The need for yearly tables is particularly acute in record-keeping. A person born on a date recorded only as 四月初十 in a year with an intercalary fourth month has an ambiguous record unless the table for that specific year is checked and the status of the month is known. The Gregorian date alone removes that ambiguity, but only if it was recorded at the time. Once the original record is lost, reconstructing it from a Chinese date requires the table for the correct year, and the correct year itself may be in doubt.

閏月那一格怎麼讀

In a leap year, one month of the Chinese calendar is repeated. The repeated month is called an intercalary month, or 閏月, and it takes the name of the month it follows. In a year with an intercalary fourth month, for example, the sequence runs: 三月，四月，閏四月，五月. In that year, the month name 四月 appears twice, once as an ordinary month and once as an intercalary month.

This has a direct effect on reading a conversion table. The entry for 四月初十 in such a year is not unique. There is a 四月初十 in the ordinary fourth month and another in the intercalary fourth month. If the 閏 character is omitted, the reader will select the ordinary month and arrive at a Gregorian date that is one month earlier than the intended date. The two dates are separated by a full lunar month, not by a day or two.

The same caution applies to any date in the intercalary month. A table may list both 四月初十 and 閏四月初十 as separate rows or as a single row with a clear mark. The reader must identify which row is meant. In many printed almanacs, the intercalary month is marked with the character 閏 before the month number, and the days within it are numbered in the same way as the days of the ordinary month. There is no separate numbering system; the only distinction is the 閏 prefix.

節氣落在國曆上幾乎固定

The 24 solar terms are defined by the position of the Sun on the ecliptic, not by the Moon. Each term begins when the Sun reaches a multiple of 15 degrees of celestial longitude. Because the Gregorian calendar is also solar, the solar terms fall on nearly the same Gregorian dates every year. 立春, for example, always falls between 3 February and 5 February. Other terms show a similar range of one or two days.

This near-fixity in the Gregorian calendar is often misunderstood. A solar term is not a lunar event. It does not mark a phase of the Moon, and it is not tied to the first day of any lunar month. The Chinese calendar places the solar terms within its lunar months, but the underlying event is solar. This is why the terms correspond more closely to the Gregorian calendar than to the Chinese lunar months.

The converse is also true. The lunar date of a given solar term varies considerably from year to year. The same term may fall on a lunar date in one year and on a lunar date nearly a month later in another year. The solar terms are printed in Chinese almanacs and calendars, but their Gregorian positions are the stable ones. The lunar positions shift with the intercalary structure of the year.

報生日時用哪一種最不會出錯

For recording a birth date, the Gregorian date plus the time of birth is the least ambiguous form. A Gregorian date is unique within the calendar; it does not require a statement of whether the year was a leap year, whether the month was long or short, or whether the month was intercalary. The time of birth is added because the Chinese hour system divides the day into 12 double-hours, and the boundary between two Chinese hours does not fall at a whole hour in the Gregorian system. A time such as 23:40 may already belong to the next Chinese day in some conventions. Stating the Gregorian date and time allows the correct Chinese hour and day to be derived later.

The use of the Chinese calendar for birth records is traditional, but it carries more conditions. A date given as 四月初十 is incomplete if the year had an intercalary fourth month. Even when the year is known, the date may be misread if the 閏 mark is lost. A date given only as a solar term is also incomplete for civil purposes, because the term identifies a moment, not a whole day, and the day on which that moment falls must be determined from an astronomical table.

The practical recommendation is therefore to record the Gregorian date and the clock time. That record has a single interpretation. The Chinese date, the lunar month, the solar term, and the sexagenary day can all be computed from it by reference to the year’s table. The reverse process, starting from a Chinese date without the year and without the intercalary status, can produce multiple candidates, and the correct one may be recoverable only through additional evidence such as season, historical context, or family records.

In summary, 國曆農曆對照 is not a simple one-to-one mapping but an annual table that encodes the shifting relationship between a solar calendar and a lunisolar calendar. The Gregorian date is stable and unique; the Chinese date is stable only within the structure of a particular year, and that structure includes features such as intercalary months, long and short months, and solar terms that are solar rather than lunar. A correct reading of any conversion table requires attention to the year, the month, the presence or absence of 閏, and, where relevant, the time of day.