The history of lunar calendars: why a month is still a moon

A lunar calendar counts time in months that run from one new moon to the next, about 29.53 days each. Twelve of those months add up to 354 days, roughly 11 days short of a year of seasons, and every calendar in history is an answer to that gap. Lunar calendars accept it, lunisolar calendars patch it with an extra month, and solar calendars, like the Gregorian one on your wall, drop the Moon altogether.

Yet the Moon never left. It hides in the word month, in the date of Easter, and in the calendars that set Ramadan, Lunar New Year, Passover and Diwali. This article covers how people first used the Moon as a clock, how different cultures kept it in step with the Sun, how Rome cut it out, and which lunar rules still decide dates in 2027.

Why is a month called a month?

Because a month used to be a moon. The English word goes back to Old English monað and a Germanic root tied to the word for moon, and further back to an Indo-European form that also produced Sanskrit masa, Greek mēn and Latin mensis. The deepest root most likely meant “to measure”. The Moon was the measurer.

A thin crescent low in the west after sunset meant a new month had begun. A round moon rising at dusk meant you were halfway through. Count crescents and you have a calendar. Traditional names such as Wolf Moon and Harvest Moon come from the same habit of naming each moon after its season, as you can read in full moon names explained.

The trouble starts when you set that count next to the seasons. After one year of twelve moons you are 11 days ahead of the Sun. After three years you are more than a month off, and your spring festival is sliding toward winter. There have only ever been three ways out:

• Lunar: keep the moons and let the year drift through the seasons. The Islamic calendar does this.
• Lunisolar: keep the moons, but add a 13th month every two or three years. Babylonian, Hebrew, Chinese and Hindu calendars work this way.
• Solar: forget the moons and make months of fixed length that follow the Sun. That is what Rome did, and what most of the world uses for civil dates today.

What is the oldest lunar calendar?

The earliest candidates are notched bones, and they are hard to prove. The Ishango bone from the Democratic Republic of the Congo, more than 20,000 years old, led the researcher Alexander Marshack to suggest a six-month lunar count; other scholars reply that a row of notches can just as well be a tally or a decoration.

Much stronger is Warren Field in Aberdeenshire, Scotland. On the grounds of Crathes Castle, archaeologists excavated an arc of twelve pits about 50 meters long, dug by hunter-gatherers around 8000 BC. In 2013 a team led by Vince Gaffney of the University of Birmingham published their reading: the pits mimic the phases of the Moon and let people track lunar months through the year. The clever detail is the alignment. The arc points to the spot on the horizon where the Sun rises at midwinter, which gave its users a yearly moment to reset the moon count against the Sun. That is the lunisolar problem, solved with a shovel. If the interpretation holds, Warren Field is about 5,000 years older than the first formal calendars of Mesopotamia.

How does a lunisolar calendar work?

Babylon and the 19-year cycle

In Babylon each month began on the evening the new crescent was first seen, and lasted 29 or 30 days. At first the king added an extra month whenever the calendar drifted. From around 500 BC Babylonian astronomers fixed this into a rule: in every 19 years, seven years get a 13th month. It works because 19 solar years and 235 lunar months are almost exactly the same length, about 6,940 days, with an error of only around two hours per cycle. The Greek astronomer Meton introduced the cycle in Athens around 432 BC, which is why it is called the Metonic cycle, although the Babylonians had it first.

The Hebrew calendar

The Hebrew calendar uses the same cycle, and its month names, Nisan and Tishrei among them, match the Babylonian ones. Since around AD 359, traditionally under Hillel II, the calendar has been calculated rather than observed, so dates are known centuries ahead. Years 3, 6, 8, 11, 14, 17 and 19 of each cycle get an extra month, Adar I.

That leap month shapes 2027. The Hebrew year 5787 is a leap year, so Passover does not begin until sunset on April 21, 2027, right after the April full moon (April 20 in the Americas and the UK, April 21 in India and Australia), nearly four weeks after Western Easter. Rosh Hashanah, the Jewish New Year, begins at sunset on October 1, just after the new moon of September 30.

China

The Chinese calendar starts each month on the day of the new moon. The month that contains the winter solstice is always the 11th, and a leap month is inserted whenever a month passes without one of the twelve principal solar terms. Lunar New Year therefore usually falls on the second new moon after the December solstice, always between January 21 and February 20. In 2027 that is Saturday, February 6, the start of the Year of the Goat, on the new moon of February 6 (15:56 UTC, still February 6 in Beijing).

India: tithis and the panchang

Hindu calendars are lunisolar too, and the traditional almanac, the panchang, runs on the tithi: the time it takes the Moon to gain 12 degrees on the Sun. Thirty tithis make a lunar month, fifteen in the waxing half and fifteen in the waning half. A tithi lasts about 20 to 27 hours, so it rarely matches a civil day. Much of northern India ends the month at full moon (purnimanta), much of the south and west at new moon (amanta). Festivals land on the same day either way; only the month’s name in the waning fortnight differs. An extra month, adhika masa, is added about once every 32.5 months.

Why does the Islamic calendar move through the seasons?

One major calendar refuses the patch on purpose. The Islamic, or Hijri, calendar has twelve lunar months and no leap month, so its year lasts 354 or 355 days. Each month traditionally begins when the new crescent, the hilal, is seen after sunset. The Quran condemns nasi’, the pre-Islamic practice of inserting and postponing months, and the calendar has stayed purely lunar ever since, counting its years from the Prophet’s move to Medina in 622.

The effect is that every Islamic date moves about 11 days earlier each Gregorian year and passes through all seasons in roughly 33 years. A Ramadan that falls in June means very long fasting days in London or Toronto and short ones in Sydney or Cape Town; a Ramadan in December flips that. In 2027 Ramadan is expected to begin around February 8, after the new moon of February 6, with Eid al-Fitr around March 9 or 10, depending on the crescent sighting. The full list is in holidays set by the Moon in 2027.

Every calendar ever made answers the same sum: twelve moons fall eleven days short of one Sun. Babylon added a month, Islam let the year drift, and Rome threw the Moon out.

How did Rome take the Moon out of the calendar?

The Roman calendar began lunar, and you can still see it in the vocabulary. The first day of each month was the Kalends, from the Latin calare, to call out, because priests announced the new moon. Our word calendar comes straight from it. The Republic’s 355-day year needed an extra month, Mercedonius, which priests inserted irregularly and sometimes for political reasons, so the calendar wandered away from the seasons.

Julius Caesar ended that. The year 46 BC was stretched to 445 days to catch up, and from then on the year had 365 days with a leap day every four years. The months kept their names but lost their moons. Caesar’s year was still about 11 minutes too long, so in 1582 Pope Gregory XIII removed ten days: Thursday, October 4 was followed by Friday, October 15. Britain and its colonies, including those in America, waited until 1752, when September 2 was followed by September 14.

How is the date of Easter calculated?

Easter is the one place where the Moon survived Rome. It grew out of Passover, which falls at the full moon of the first spring month. The Council of Nicaea in 325 asked all churches to celebrate Easter on the same Sunday, and over the following centuries the rule settled into its familiar form: Easter is the first Sunday after the first full moon on or after March 21.

The church uses March 21 instead of the real equinox, and a table moon built on the 19-year Metonic cycle instead of the astronomical one. Usually they agree; when they do not, Easter can land a month away from what the sky suggests. Easter always falls between March 22 and April 25. Orthodox churches run the calculation on the Julian calendar, so their Easter often comes later.

In 2027 the sky and the tables agree. The March equinox is on March 20 (20:24 UTC), the full moon of March 22 follows at 10:44 UTC, and Easter Sunday is March 28. Orthodox Easter falls on May 2. In Australia, New Zealand and South Africa that March full moon is the Harvest Moon, so Easter there is an autumn feast.

Where lunar calendars show up in 2027

In 2027 you can watch the calendar’s lunar past at work. The new moons of 2027 mark Lunar New Year on February 6, the expected start of Ramadan around February 8, Rosh Hashanah at sunset on October 1 and Diwali on October 29. The full moon of March 22 sets Easter on March 28, and the full moon of April 20 sits right before Passover.

The mismatch even creates its own events. Twelve months cannot hold thirteen new moons neatly, so August has two in the Americas, Europe, Africa and India, giving the black moon of August 31, and the season between the March equinox and the June solstice holds four full moons, giving a seasonal blue moon on May 20. Every full and new moon, with exact times, is in the full moon calendar 2027, or on your phone through the calendar subscription: a small remnant of the time when people looked at the evening sky to know what day it was.