What Is a Lunar Calendar?

Understanding what a “lunar calendar” means is a great starting point. Simply put, a lunar calendar is a calendar based on the cycles of the Moon, rather than the solar year. The most widely used calendar today, the Gregorian calendar, is a solar calendar: it follows the Earth’s orbit around the Sun, roughly 365 days per year. By contrast, a pure lunar calendar measures months by the Moon’s orbit around Earth (one lunation is about 29.5 days), so 12 lunar months total only about 354 days.^[1] This is about 11 days shorter than a solar year. As a result, a strictly lunar calendar (such as the Islamic Hijri calendar) does not align with the seasons. Its dates “move” earlier by about 11 days each year in the solar calendar, cycling through all seasons over time. For example, the Islamic month of Ramadan can fall in spring one year and winter some years later, because the Islamic calendar has no mechanism to synchronize with the solar year.

Lunisolar Calendars

Many cultures, however, developed lunisolar calendars. These are hybrid systems that track lunar months but also periodically adjust to the solar year so that seasons stay in place. The traditional Chinese calendar is a lunisolar calendar (often loosely called the “Chinese lunar calendar”). It consists of 12 lunar months of 29 or 30 days each (totaling ~354 days), and adds an extra leap month in certain years to sync up with the ~365-day solar year.^[2] In other words, most years have 12 months, but every two to three years a 13th month is inserted. This keeps festivals and anniversaries (which are set by lunar dates) from drifting across seasons. Such intercalation is why Chinese New Year is always around late January to mid-February. Without leap months it would occur about 11 days earlier each year and eventually slip into autumn, which it does not. The Chinese lunisolar system ensures that the Lunar New Year always stays in winter/spring. (By contrast, the Islamic New Year can fall in any season, since that calendar is purely lunar with no seasonal adjustment.)

Because the Chinese calendar is lunisolar, many people refer to it as the “农历 nónglì” or agricultural calendar (in English, simply the Chinese lunar calendar), highlighting its traditional use in agrarian life to time planting and festivals. Importantly, the Chinese calendar influenced or was adopted by many neighboring cultures. Variants of this lunisolar calendar have been used in East Asia, including by Chinese, Koreans, Vietnamese, Japanese (historically), and others.^[3] These calendars share the same basic structure of lunar months and leap months, and often the same zodiac cycle of 12 animals to label years (more on that below). Because of this common basis, our Lunar Birthday Calculator can be used for birthdays in cultures that use the Chinese lunisolar system or a close variant. (For example, a person’s birthday on the Vietnamese or Korean traditional calendar will usually coincide with the date on the Chinese calendar.) However, it’s important to note that not all lunar calendars are the same. For instance, the Hebrew calendar or Hindu calendars have their own calculation rules, and the Islamic calendar is entirely different (pure lunar). Our calculator is designed for the Chinese lunisolar calendar system, so it will correctly find lunar birthdays in the context of the Chinese, Korean, Vietnamese (etc.) calendar. It would not apply to dates from unrelated lunar systems.

Next: History of the Chinese Lunar Calendar →