A lunar calendar is a calendar based upon cycles of the Moon's phases (synodic months), in contrast to solar calendars whose annual cycles are based only directly upon the solar year. The most commonly used calendar, the Gregorian calendar, is a solar calendar system which originally evolved out of a lunar calendar system. A purely lunar calendar is also distinguished from lunisolar calendars whose lunar months are brought into alignment with the solar year through some process of intercalation. The details of when months begin varies from calendar to calendar, with some using new, full, or crescent moons and others employing detailed calculations.
The Maya counted the number of days in the current lunation. They used two systems for the zero date of the lunar cycle: either the first night they could see the thin crescent moon or the first morning when they could not see the waning moon. The age of the moon was depicted by a set of glyphs that mayanists coined glyphs D and E:
A new moon glyph was used for day zero in the lunar cycle.
D glyphs were used for lunar ages for days 1 through 19, with the number of days that had passed from the new moon.
For lunar ages 20 to 30, an E glyph was used, with the number of days from 20.
Because each lunation is approximately 29.5 days (29 days,12 hours, 44 minutes, and 3 seconds, or 29.530588 days), it is common for the months of lunar calendars to vary between 29 and 30 days. Because the period of twelve such lunations—a lunar year—is approximately 354 days, 8 hours, 48 minutes, and 34 seconds (354.367056 days), purely lunar calendars lose around 11 days per year relative to the Gregorian calendar. In purely lunar calendars such as the Islamic calendar, the lack of intercalation causes the lunar months to cycle through all the seasons of the Gregorian year over the course of a 33-lunar-year cycle.
The cycle of the moon orbiting around the Earth is 28 days, an average of the synodic cycle (29.5 days between new moon to new moon) and the sidereal cycle (27.1 days until the moon reappears in the same place in the sky). 28 days also refers to the female biological cycle.
The original 13 moons x 28 days cycle was also used by the Egyptians (Thoth), Druids, Polynesians, Incas, some Native Americans and the Chinese.
Although the Gregorian calendar is in common and legal use in most countries, traditional lunar and lunisolar calendars continue to be used throughout the Old World to determine religious festivals and national holidays. Such holidays include Ramadan (Islamic calendar); the Chinese, Japanese, Korean, Vietnamese, and Mongolian New Year (Chinese, Japanese, Korean, Vietnamese, and Mongolian calendars); the Nepali New Year (Nepali calendar); the Mid-Autumn Festival and Chuseok (Chinese and Korean calendars); Loi Krathong (Thai calendar); Sunuwar calendar and Diwali (Hindu calendars).
In the modern version, each moon of the 13 Moon Calendar has the name and characteristics of the 13 Lunar Tones in the Tzolkin. Each moon is also named after an animal, based on the 13 major constellations rediscovered by Hugh Harleston.
The 13 Moon 28-day synchronometer is a harmonic timespace matrix. It takes the moon 28 days to orbit the Earth; it makes this orbit 13 times each year. The standard of measure is the 28-day cycle, called a moon, because it is the median between the 29.5-day synodic cycle of the moon (new moon to new moon) and the 27.1-day sidereal cycle of the moon. Hence, it is a measure of Earth’s solar orbit using the 28-day lunar standard. This creates a perfect orbital measure of 13 moons of 28 days, totaling 364 days, or 52 perfect weeks of 7 days each.
Most calendars referred to as "lunar" calendars are in fact lunisolar calendars. Their months are based on observations of the lunar cycle, with intercalation being used to bring them into general agreement with the solar year. The solar "civic calendar" that was used in ancient Egypt showed traces of its origin in the earlier lunar calendar, which continued to be used alongside it for religious and agricultural purposes. Present-day lunisolar calendars include the Chinese, Hindu, and Thai calendars.
Synodic months are 29 or 30 days in length, making a lunar year of 12 months about 11 days shorter than a solar year. Some lunar calendars do not use intercalation, such as most Islamic calendars. For those that do, such as the Hebrew calendar, the most common form of intercalation is to add an additional month every second or third year. Some lunisolar calendars are also calibrated by annual natural events which are affected by lunar cycles as well as the solar cycle. An example of this is the lunar calendar of the Banks Islands, which includes three months in which the edible palolo worm mass on the beaches. These events occur at the last quarter of the lunar month, as the reproductive cycle of the palolos is synchronized with the moon.
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