The lunar birthday and the solar birthday coincide every few years, why?
Generally, the lunar birthday and the solar birthday coincide once every 19 years. This mainly relates to leap months.To coordinate the contradiction between the tropical year and the lunar year, preventing the lunar year and month from being out of sync with the tropical year, i.e., the seasons, a leap year is added every 2-3 years. The lunar calendar adopted the method of adding 7 leap months within 19 years, known as the "19-year seven-leap-month method," which effectively coordinates the tropical year and the lunar year, keeping the lunar New Year (Spring Festival) always in late winter and early spring. The ancients referred to 235 synodic months as the "leap cycle."
What is the lunar calendar like?
The lunar calendar is a lunisolar calendar that combines elements of the lunar calendar (the Xia calendar) with solar calendar components. It takes the cycle of the moon's phases, i.e., the synodic month, as the length of a month, integrates the "24 solar terms" from the traditional Ganzhi calendar, and refers to the solar tropical year as the length of a year. By setting leap months, it ensures that the average lunar year aligns with the tropical year. Therefore, the lunar calendar contains both lunar and solar calendar elements.Unlike the solar calendar, where the year is fixed at 365 or 366 days, the lunar year can differ by about 10-20 days compared to the solar year. To coordinate the number of days between the tropical year and the synodic month, a lunisolar calendar, i.e., the lunar calendar, was created. The method is to use a reasonable leap year system, such as the "19-year seven-leap" method (adding 7 leap years within 19 lunar years). A common lunar year has an average of 354.3672 days, while a leap year has an additional leap month, averaging 383.8978 days. This makes the average number of days in a year match the tropical year. Therefore, this calendar aligns with both the moon's phases and the Earth's orbital motion around the sun.
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