The lunar calendar tracks the length of time it takes the moon to complete 12 phases. The average cycle of a moon’s orbit is 29.5 days. The lunar calendar’s months, therefore, are shorter than the traditional 30 or 31 day month. Taking 12 cycles of moon at 29.5 days each, that brings the lunar calendar to 354 days.
The Lunar Calendar’s History and Cultural Differences
Lunar calendars have been used for centuries. The oldest known calendars were found painted on cave walls dating back to the Paleolithic times. They also found the calendars carved on animal bones, and it’s believed they were meant to be carried on hunting trips and seasonal migrations.
Not many calendars still stick solely to the lunar cycles. The Islamic Hijri Qamari is the only calendar that does follow the moon’s rotation. They do not follow seasons at all; they only calculate the cycle of the moon to track time. As a result, their years fall about a dozen years short to a Gregorian calendar, but things balance out every three decades.
The starting date on a lunar calendar differs from culture to culture. In some Asian cultures, China is one, they base the start of a month on the first day of the new moon. Many Hindu cultures begin the cycle on the first day of a full moon. Years ago, Hebrews used the crescent phases to start and end a month.
In addition to the lunar calendar, many cultures tie in the moon’s cycle with the Gregorian calendar. They use the lunar calendar to track important festivals, such as the Chinese New Year. In Vanuatu, there is a worm that bases its reproductive habits around the phases of the moon. As this culture eat this worm, they use the lunar calendar to calculate when the worms will be easiest to harvest.
Because the lunar calendar is shorter than the Gregorian calender, extra days must be added to even things out. Known as intercalary days, most cultures add one day to most of the months. Another option is to add an intercalary month every two or three years.
Phases of the Moon
During one full revolution of the moon, there are eight phases. The phase depends on the sun’s proximity to the moon. For the moon to be visible in the night sky, the sun must be hitting the moon’s surface.
1. New Moon – Moon is not visible in the night sky because of the sun’s position.
2. Waxing Crescent – Moon appears like a sliver of a fingernail with the left side of the moon dark, and the right side is illuminated.
3. First Quarter – Entire right side of the moon is lit up.
4. Waxing Gibbous – Close to three-quarters of the right side of the moon is illuminated.
5. Full Moon – The entire moon is lit up.
6. Waning Gibbous – The moon is starting to return to the New Moon stage. Three-quarters of the left side of the moon is illuminated.
7. Last Quarter – The entire left half of the moon is illuminated.
8. Waning Crescent – The fingernail-like sliver of the moon on the left side is the only illuminated section.
After the waning crescent, the moon reverts to the new moon and begins a new cycle. From time to time, a full moon falls twice in month. It’s not common; the full moon must start at the beginning of the month. When this happens, it’s called a blue moon. Blue moons only happen about once every two to three years.
To track the moon’s cycles, the Farmer’s Almanac offers a detailed lunar calendar that targets your exact location. This makes it easy to find when the moon will rise and set.