Pure Moon: The Parmigiani Fleurier Tonda ‘Hijiri’ Perpetual Lunar Calendar

The ubiquity of the Gregorian calendar in civil timekeeping has made the term “perpetual calendar” essentially synonymous with the Gregorian calendar. This is partly thanks to the practicality of the Gregorian calendar in organizing the affairs of daily life. The Gregorian calendar is based on the length of a solar year, and is constructed so that it stays in sync with the Equinoxes and therefore, with the seasons in general, with little need for correction (intercalary days are added once every four years, skipped every hundred years, and then added back in every four hundred years).

There are however, other calendars – many, many other calendars, since the dawn of human civilization. Most of these have fallen into disuse but some are still followed for the purpose of scheduling religious and other spiritual holidays and events. These calendars can be based on both the lunar and solar cycles – so-called lunisolar calendars, like the traditional Chinese calendar – or they can be pure lunar calendars, like the Hijiri, or Islamic calendar, which is the calendar on which the Parmigiani Fleurier Tonda Hijiri Perpetual Calendar is based.

The Hijiri calendar is so-called after the Prophet Mohammed’s Hijirah, or “migration,” from Mecca to Medina in Saudi Arabia. The first year of the calendar corresponds to the Gregorian year 622 CE, and the Hijiri calendar is based on direct observations of the lunar cycles. Due to the fact that there can be significant variations in the observed start and end of days, and months, depending on location and observational conditions, Islamic astronomers developed so-called Tabular Islamic Calendar in the 8th century, which is based on mathematical rules and follows a predictable 30 year cycle. This allows astronomical observations and other events to be assigned predictable dates, and also aids in the conversion of a date in the Tabular calendar to a date in the Gregorian calendar.

The Tabular Islamic calendar, as a pure lunar calendar, drifts with respect to the Gregorian calendar and is therefore generally used only for establishing the dates of critical events like the month of Ramadan, which occurs on different dates in the Gregorian calendar every year.

The classic problem that every calendar has to grapple with, is that the orbits of heavenly bodies, like the orbit of the Earth around the Sun, and of the Moon around the Earth, do not fit into a whole number of days. The tropical or solar year, for instance, is not 365 days exactly; instead, it’s about 365.2422 days, which is why the Gregorian calendar adds an extra day every four years. The lunar month is approximately 29 days, but the true mean length of a synodic month is closer to 29 days, 12 hours, 44 minutes and 2.9 seconds. This means that if you want your calendar to be reckoned in whole days, you have to alternate between 29 and 30 day months according to some set of rules that keeps the calendar in sync with the actual phases of the Moon. As an example of the problem, a year of only 29 day long lunar months would have only 348 days in a 12 month year, and would fall out of synchronization with the actual phases of the Moon very quickly.

The Hijiri Calendar follows a 30 year cycle of dates and months. Each 30 year cycle consists of 19 354 day-long years, called “common” years, and 11 years, called “abundant” years, in which there are 355 days. Each month is either 29 or 30 days in length. In the observational Hijiri calendar, the number of days in each month is determined by observing the Moon directly. In the rule-based Tabular calendar, odd numbered months have 30 days, and even numbered months have 29 days, except in the case of an “abundant” or Leap Year, in which case the final, or 12th month, has an intercalary 30th day added.

The entire cycle is a way of providing a stable time base which still reflects the lunar month accurately, and which is made up of whole numbered days, weeks, months, and years. Over the long term, the Tabular calendar is extremely accurate – it takes about 2,500 solar years before a one day error has accumulated.

The Tonda PF is the second Islamic calendar watch created by Parmigiani Fleurier – the first version won the GPHG Innovation Prize in 2020.

If you understand the basic structure of the Tabular Hajiri calendar, the indications of the watch are easy to understand.

Starting at 12:00, we have an indicator for the years. The hand rotates once around the dial every 30 years, and common years are shown by white lines, with abundant years in beige. At 3:00, is the month indicator; the month of Ramadan is indicated in red. At 9:00 is the day indicator, with odd numbered days indicated by white lines, and even numbered days by their ordinal number in Arabic. The small round window is colored gold if the month is 30 days in length.

Finally, at 6:00, we have the phases of the Moon as seen from the Northern and Southern hemispheres.

The entire system is easy to read and eminently logical, once you understand the rationale behind the various month and year lengths and the interposition of intercalary days. The robustness of the mechanical system and the Tabular calendar are also both remarkable – there are Gregorian secular perpetual calendar watches which account for the 100 year rule in the Gregorian calendar, but any calendar and its mechanism capable of running, theoretically, for 2,500 years before requiring a correction, is an attractive proposition both aesthetically and intellectually. Parmigiani Fleurier is the only company of which I’m aware which simultaneously offers a solar (Gregorian) annual calendar, a lunisolar (Chinese traditional) perpetual calendar, and a pure lunar perpetual calendar, in their Tonda Cultural Calendars collection, and as an exercise in cultivating a broader perspective in both astronomical cycles and human civilization, these timepieces really do stand alone.

The Parmigiani Fleurier Tonda PF Hijiri Perpetual Calendar: case, stainless steel with platinum knurled bezel, 42mm x 11.2mm, water resitance 100M. Dial, viridian green, hand-guilloché “grain d’orge” pattern, 18k gold rhodium plated markers. Movement, PF009 Hijiri perpetual calendar moonphase, 48 hour power reserve, running at 28,800 vph in 32 jewels; movement 33.8mm x 5.7mm with platinum oscillating weight. Hours, minutes, center seconds, Hijiri perpetual calendar and precision moonphase. Price, $67,200; for more, see it at Parmigiani.com.