What is a 'lunar' calendar?
One clear night, as I strolled through the neighborhood with my friend, a hushed excitement filled the air. "Look at the moon!" my friend exclaimed. "The crescent is so thin it looks like a nail clipping." I looked at the moon.
“Yes, it’s the first day of the lunar month.”
My friend, now puzzled, gazed at me.
"It's alright. Calendar definitions are kind of a lost art even among professional astronomers." I reassured my friend, realizing that not everyone delves into the intricate world of timekeeping.
Since time immemorial, the regular cycles of the sky have given humanity ingenious ways to measure the passage of time. The 24-hour alternation between light and dark, the waxing and waning cycle of the Moon, and the Sun’s return to the same point in the sky against the stars—we call these by the familiar names of day, month, and year.
In the West, we use a solar calendar, based on a Roman calendar, where the months are aligned with the seasons. We are all familiar with it – May, for instance, is perennially synonymous with spring in the northern hemisphere. However, the solar calendar has its quirks; the phases of the moon can unfold at any time of the month, leading to the occasional occurrence of two full moons within a single calendar month – a phenomenon known as a "blue moon."
That doesn’t happen in a lunar calendar, which shifts the spotlight to the Moon. In this type of calendar, the day of the month always corresponds to the same phase of the moon. The Hijri calendar, also known as the Muslim or Islamic calendar, is an example of a lunar calendar. Its months are Muharram, Safar, Rabea al-Awwal, Rabea al-Thani, Jumada al-Oula, Jumada al-Akhira, Rajab, Shaaban, Ramadan, Shawwal, Dhul-Qeada, and Dhu-Hijjah. In the Hijri calendar, the New Moon marks the conclusion of a month, and every month starts with the sighting of the slender crescent – that nail clipping my friend mentioned. The full moon graces the sky on the 15th day of each month, offering a celestial guidepost for those seeking temporal orientation. Can't remember what day of the month it is? Look at the moon.
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According to the Hijri calendar, we are currently in the month of Shaaban. The month will end on the 29th of Shaban with the new Moon, a day we will call March 10th. The slender crescent on March 11th will be the 1st day of Ramadan, the next Hijri month.
While having this advantage of the phase of the moon aligning with the day of the month, the lunar calendar comes with a cost: the months drift through the seasons. Shaaban, for instance, falls in the winter this year, but will happen in the autumn in 2030, and in the summer in 2040. This is simply a consequence of the fact that there is not an exact number of lunar months in the solar year.
But, I hear you say, can we have a calendar that has both advantages? Indeed we can, and it’s called a lunisolar calendar. A lunisolar calendar fuses the best of both worlds. In this type of calendar, the months still follow the phases of the moon, but every few years an extra month, a leap month is added, to realign with the seasons. The Hebrew calendar, or Jewish calendar, is among the most commonly used lunisolar calendar. Its months are Nisan, Iyar, Sivan, Tammuz, Av, Elul, Iishrei, Cheshvan, Kislev, Tevet, Shevat, and Adar, with a second Adar month added in leap years. The first month, Nisan, starts the day after the New Moon closest to the spring equinox. The holiday of Pesach, of Passover, starts on the 15th day of Nisan, the first full month of the year in the Jewish calendar. The Christian holiday of Easter happens on the Sunday after the full moon after the equinox. Another example of a lunisolar calendar is the Chinese calendar, whose new year we just celebrated. Also aligned with the seasons, in this calendar the winter solstice is always the 11th month of the year. The Chinese new year, usually called “lunar new year” (despite the calendar being lunisolar) thus falls in the second new moon after the winter solstice, between January 21st and February 20th.
That’s more poetic than the Gregorian calendar in use in the West. Have you ever wondered why the year starts on January 1st? The Roman calendar used to be lunisolar, the year starting in March. We see a reminder of it in the names of the months: September, October, November, December. They mean literally 7th, 8th, 9th, and 10th month. The month after June used to be called Quintilis (fifth), rename July after Julius Ceasar’s death. Likewise, the month before September used to be called Sextilis (sixth), before the Emperor Augustus also wanted a month named after him. The beginning of the year was moved to January for political reasons in the ancient Roman republic: it was when the consuls took office. Eventually the Romans decide to align the election year with the calendar year.
As we navigate the intricacies of these diverse calendars, we find ourselves caught between the seasonal predictability of the solar calendar and the poetic allure of the lunar calendar. When time is measured by the rhythmic dance of the heavens above, we glimpse in the infinite beauty of our universe a dance that transcends the boundaries of cultures and civilizations.
Wladimir Lyra is an associate professor of astronomy at New Mexico State University. He can be reached at @LyraKstellation (X).
This article originally appeared on Las Cruces Sun-News: What is a 'lunar' calendar?
