This page is intended only for readers who are starting to study calendars and want to become familiar with the basic notions behind the main calendar types.
They will find it useful after first reading the page devoted to basic astronomy concepts. Specialists and already experienced readers do not need it. They would find it... simplistic.
Introduction
For those of us who have not read the page on the astronomy notions needed to understand calendar structure, here are a few essential figures:
Key figures to remember
- Earth's axial tilt relative to the ecliptic: 23°26'
- Tropical year: 365 d 5 h 48 min 48 s, i.e. 365.24221935 days
- Lunation: 29 d 12 h 44 min 2.8 s, i.e. 29.5305882 days
Now run an online search for “types of calendars”. On websites covering the subject, we often read that there are three calendar types: lunar calendars, solar calendars and lunisolar calendars.
If we only study present-day calendars, this three-part classification is usually enough. But if, as on this site, we also want to explore older calendars, we need to add a few more types to get a fuller picture.
Let's go through this in a little more detail.
If we define a calendar very quickly as a system for measuring time, we immediately understand that we need a unit of measurement.
And if we were stripped down in the middle of a field, what would that unit be?
The time between one sunrise and the next? Between one sunset and the next? That is still a bit short as a reference unit, even at human scale. So the day (daytime + night) cannot be the main reference unit of our calendar. The day will only be a subdivision of a larger unit.
Lunar calendar
With a little patience, we quickly notice that a nearby celestial body regularly shows easy-to-observe features. That body is the Moon. Those visible features are the New Moon and the Full Moon. A New Moon is when... we no longer see it. By contrast, a Full Moon is when we see it in full. Between two identical states, the average interval is 29.5305882 days. “Average” here matters, because in practice it may sometimes be 29 days, sometimes 30 days, or any value within that range.
No matter. We only need to take this lunation as our unit of measurement, and the job is done. We call that unit a month and group several of them (say... 12) to form a larger unit. We have just invented a lunar calendar.
A lunar calendar is therefore a calendar that uses lunar cycles (in days) as its unit of time.
The Muslim calendar is an example of a lunar calendar.
Solar calendar
If you are not Muslim yourself, ask Muslim friends whether Ramadan fasting always falls in the same season. The answer will be no.
It would be useful to find a time unit that keeps warm, rainy and cold seasons in the same place. With even more patience than for Moon observation, we notice that seasons are linked to the shadow cast by a stick planted in the ground. And what creates that shadow? The Sun.
In its apparent motion around the Earth (while in fact the Earth revolves around it - I did tell you to read the astronomy page), there is a position of the Sun in the sky for which the stick's shadow is shortest.
We then take the interval between two identical solar positions in the same season (for example, the spring equinox) to define our time unit: the tropical year. On average, it is 365.24221935 days. We call this unit a year and may divide it into arbitrary subdivisions (say... 12). We have just invented a solar calendar.
A solar calendar is therefore a calendar that uses solar cycles (in days) as its unit of time.
The Gregorian calendar (the one we use in France) is an example of a solar calendar.
The hardest part is making sure that the average number of days over several years matches the mean tropical year. The fewer years needed to reach that average value, the better the calendar.
Lunisolar calendar
An obvious question follows: why not try to reconcile both types and account for both lunar and solar cycles?
That is exactly the aim of lunisolar calendars. The main difficulty is reconciling the two cycles, because 12 lunar months are far from matching the length of a solar year. So from time to time, a lunar month must be added to a solar year to stop the two from drifting apart.
A lunisolar calendar is therefore a calendar that uses both lunar and solar cycles (in days) as units of time.
The Hebrew calendar is an example of a lunisolar calendar.
Other possible types
Wandering calendar
What happens if a solar calendar is “built wrong”, whether intentionally or not? For example, if you only use 365-day years and never make longer years from time to time to approach an average year of 365.25 days?
What happens is similar to what happens in lunar calendars: gradually, the calendar drifts over time, so supposedly fixed festivals move through the seasons. Christmas, for instance, first celebrated in winter, would later occur in spring, then in summer...
Because of this drift, such calendars are called wandering calendars.
The ancient Egyptian calendar is an example of a wandering calendar.
Horizon calendar
For some peoples or civilizations without writing, one way to orient themselves in time is to know that the Sun rises or sets at a given point on the horizon according to the day of the year. This observation-based “calendar” is called a horizon calendar.
The Hopi calendar is an example of a horizon calendar.
Conclusion
The Sun and the Moon are the two celestial bodies that underpin most calendars.
Because it is the easiest to establish, the lunar calendar is the one we most often find among ancient calendars.
The solar calendar, because it requires astronomy notions (equinoxes, solstices...), often appears later, and its accurate development is much more delicate. It still preserves traces of older lunar calendars, such as the number of months, even if those months no longer correspond to the lunar month as a time unit.
And now we can set off to discover calendars from here and elsewhere, present-day and ancient.
