A year by the Moon alone

A calendar has to bring together two celestial bodies whose motions do not divide neatly into one another: the Sun, which sets the beat of the seasons, and the Moon, which counts the months. Out of the ways of handling this discrepancy, three calendar types emerged. A solar calendar follows the Sun alone; a lunisolar calendar keeps both.

The lunar calendar takes the third path. It follows the Moon's phases exclusively and leaves the Sun out of the division of the year. Its basic unit is the synodic month, the time from one new moon to the next. The Moon thus sets the beat entirely. The author Andreas E. Zautner calls this type the "unbound" lunar calendar, because nothing ties it to the solar year.

Twelve months, eleven days short

To see how a lunar calendar behaves, a little arithmetic helps. A synodic month lasts on average 29.53 days. In practice a lunar calendar therefore alternates "hollow" months of 29 days with "full" months of 30 days. Twelve such months add up to about 354 days, more precisely 354.37 days.

But the solar year runs a little over 365 days. Between the two remains a gap of about eleven days. A lunisolar calendar fills this gap every few years with a leap month. A lunar calendar deliberately does not. It lets the shortfall stand, so each month begins about eleven days earlier than the year before. Little by little the whole lunar year drifts backward through the seasons.

The Islamic calendar

The only pure lunar calendar still in wide use today is the Islamic one. It is strictly lunar and has no leap months. The common year counts twelve months of alternately 30 and 29 days, 354 in all; in a leap year the last month gains a day, so the year runs to 355. These leap days follow a fixed pattern: in a cycle of 30 years, eleven years are leap years. That keeps the start of the calendar month close to the mean astronomical new moon.

Because the Islamic year is about eleven days shorter than the solar year, its start drifts backward through the solar year and takes about 32 solar years to do so, that is roughly 33 Islamic years. E. G. Richards compares this with the Egyptian wandering year, only it runs considerably faster. The fasting month of Ramadan, the ninth month, and the pilgrimage to Mecca in the twelfth month therefore shift from year to year and pass once through all the seasons over roughly three decades.

For religious practice most Muslims go by observation. The month begins with the first sighting of the thin crescent of the new moon, and because that moment depends on the observer's location, the start of the month can differ by up to two days from one region to another.

How Islam arrived at the pure lunar year

The Islamic calendar was not always purely lunar. Before the time of the Prophet Muhammad, part of the Arab tribes used a lunisolar calendar and occasionally inserted a thirteenth month, the nasī'. Responsibility lay with the Kinana tribe, who delegated the intercalation to their own calendar officials. This intercalation also fixed the position of the protected months, in which feuds and raids were meant to rest. Some tribes prevailed on the officials to shift the intercalation to their advantage and so evade the times of peace.

The Prophet turned against this abuse. In an address shortly before his death in AD 632 he forbade any further intercalation. The Koran holds the number of months fixed: "The number of months with God is twelve … intercalating a month is adding to unbelief" (Sura 9:36–37). What remained was a pure lunar year of twelve months. Its fixed, calculable form was set up about a decade later by Caliph Umar I, who took Muhammad's migration from Mecca to Medina in the year 622 as the starting point of the Islamic era.

What the pure lunar calendar keeps and gives up

A lunar calendar keeps the Moon in its purest form. The start of its month can be read directly from the night sky, without a table and without a reckoning master. The price is the tie to the seasons: a festival bound to a lunar month falls once in summer and, decades later, in winter.

This choice was no astronomical naivety. Islam knew the lunisolar solution with the leap month and deliberately rejected it on theological grounds. The Moon is enough for it as a timekeeper. Binding it to the solar year, by contrast, gains fidelity to the seasons and pays for it with more computation and an authority to keep that computation. Both paths are a considered answer to the same question.

The bridge: Moon and Sun at once

If you want to keep the seasonal fidelity of the Sun and still hold on to the living beat of the Moon, you arrive at the lunisolar calendar, the bound moon calendar. It keeps the lunar months and binds them to the solar year through a leap month. So the Ramadan of the one system keeps drifting, while the festivals of the other stay anchored in their season. Which cultures around the world reckon by the Moon is set out in the overview on the history of lunar calendars.

The Germanic-speaking peoples reckoned lunisolar. How their calendar brings together the first crescent, the winter solstice as its solar anchor, and the leap month is set out in the article on the Germanic lunar calendar. That is precisely the computation the Ártala app takes off your hands. It calculates the moon phases astronomically correctly and even without an internet connection, shows the months with the inserted leap month, and reminds you of the festival days of the Germanic lunisolar calendar – source-based and ad-free. You can find it for web, Android, and iOS on the Ártala home page.

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