Calendar Types: Solar, Lunar and Lunisolar Calendars

There are 3 basic calendar types: solar, lunar and lunisolar. This page discusses the difference and explains why a solar calendar is probably best for Earth.



Lunar and lunisolar calendars use lunations as calendar months, whereas solar calendars define calendar months as a fraction of the year that are only approximately equal to one lunation. A lunation is the average length of the synodic month, the time between new moons, equal to about 29.53 days. Months in lunar and lunisolar calendars therefore use patterns of months 29 and 30 days in length to maintain this average.

In a solar calendar with 12 months, the average month length is 365.24 ÷ 12 = 30.44 days. Months in these types of calendars, as most of us will be familiar with from the Gregorian, are either 30 or 31 days long (not counting February, the freak month).

If a solar calendar has 13 months (e.g. the Positivist Calendar) then these are usually 28 days long each, and occasionally 29, to maintain an average length of 365.24 ÷ 13 = 28.1 days. The main convenience of a 13-month solar calendar is that almost all months are exactly equal to 4 × 7-day weeks (see Months for more info).


Calendar Year Length

In a solar calendar the intention is to organise the calendar year to synchronise with the seasonal cycle. This is most usually measured as the average time between successive northern vernal equinoxes, equal to 365.2424 days (called the vernal equinox year). The northern vernal equinox is the most commonly used seasonal marker to define a year's beginning because it marks the beginning of spring in the northern hemisphere. Spring is generally perceived as a time of new beginnings, and most people (certainly most people who have been involved in calendar design) live in the northern hemisphere.

Since calendars are a way of tracking days, each calendar year naturally has a whole number of days. Thus, solar calendars typically define patterns of years 365 and 366 days in length in order to produce an average length of about 365.24 days.

In a lunar calendar, such as the Islamic, the calendar year is defined as 12 lunations, about 354.36 days on average. This is about 11 days shorter than the vernal equinox year, and hence the seasonal markers tend to shift in the calendar, occuring about 11 days later each year. This can be a problem in agriculture, tourism and other industries that revolve around seasons.

Lunisolar calendars solve this problem by introducing an intercalary month (also called an "embolismic" month) every 2 or 3 years, so that the average year length matches the seasonal cycle. As 365.24 ÷ 29.53 = 12.37 lunations, some years thus have 12 months (lunations) = 354.36 days (avg.), and others have 13 lunations = 383.39 days.

The result is a calendar that keeps in synch with both the Sun and the Moon. This would seem ideal, and several important calendars take this approach, including the Chinese, Hebrew and Buddhist Calendars. However, this arrangement is generally less convenient than a solar calendar, as will be explained below.


Which is Best?

If ever wondering if a lunisolar calendar is better than a solar calendar, think about this: why did the most populous nation on Earth, China, adopt the solar Gregorian Calendar in favour of its traditional lunisolar calendar?

There are several reasons why a solar calendar is superior to a lunar or lunisolar calendar.

Synchronicity with Seasons

Seasons are the most important cycle on Earth, since they are directly related to plant growth and hence food production, something that is of interest to almost all living creatures on the planet.

Before the Gregorian Calendar was adopted for official business in China in 1912, Chinese farmers used a solar calendar alongside the traditional lunisolar one. This second calendar was aligned with the seasons, and divided the year into 24 equal parts called solar terms. It existed primarily for the benefit of farmers, who need to be able to track progress through the seasonal cycle so they know when to sow, when to reap, and so on.

This illustrates that a lunisolar calendar is not enough by itself - a method for accurately tracking Earth's position in the seasonal cycle will always be required.

The lunar cycle does play a large part in life on Earth, being related to the female menstrual cycle, tides, behaviours of certain plants, and those special evenings when werewolves roam the land. So there is certainly some benefit to being aware of the lunar cycle, which is why lunisolar calendars persist, and why most printed solar calendars show lunar phases. However, they are not of critical importance to most people living on Earth. (They would, however, be very important to people living on the Moon - see "Humans on the Moon", below.)

Birthdates and Anniversaries - The Leap Baby Problem

In most Earth calendars we generally record birthdays and other anniversaries using, not the day of the year, but the month and the day of the month. Because each day of the year is identified by a day and month, we know a year (or thereabouts) has passed every time that combination of day and month occurs again. This is, of course, one of the main uses of dates.

So, what happens if you're born on February 29? People born on this date are called leap babies, because February 29 only occurs in leap years. If you're a leap baby, what do you do for your birthday on those years without a February 29? This is the leap baby problem.

Most leap babies celebrate their birthdays in non-leap years on either February 28 or March 1. This is a minor issue affecting just 0.07% of people, and it's no big deal to have your birthday on the date before or after the actual date you were born.

However, consider how much larger this problem becomes when using a lunisolar calendar. Some years have 12 months, and others have 13. Imagine if you were born in that extra 13th month, as 7/235 or 3% of people would be (assuming a constant birth rate). In a year with only 12 months, when would you celebrate your bithday?

In the Hebrew Calendar the problem is solved as follows: In a 12-month year there is a month called Adar, but in a 13-month year there are two Adars: Adar I and Adar II. Whether you are born in Adar, Adar I or Adar II, in 12-month years your birthday is celebrated in Adar. Hence, if you are born early in Adar II, in 12-month years you would actually celebrate your birthday before your friend who was born in late Adar I.

This is not the end of the leap baby problem in the Hebrew Calendar either. If you are born on 30 Adar I, then in a non-leap year you celebrate your birthday on 1 Nisan, because in a non-leap year Adar only has 29 days - much the same situation as in the Gregorian Calendar.

Clearly the leap baby problem is much worse in a lunisolar calendar. Furthermore, because of the varying year lengths, sometimes when your birthday occurs only 354 days have passed, and other times 383 have passed. This is obviously a much less accurate way of measuring the passing of years.

Some calendars, such as the New Earth Calendar, use an intercalary week, in which case only 0.38% of people are affected by the leap baby problem. This is obviously an improvement on a lunisolar calendar, but still not as good as a solar calendar, plus there is not the benefit of being aligned with the lunar cycle. (The primary benefit of this type of calendar is that it is perpetual, yet does not disrupt the 7-day cycle.)


While our Moon is an important part of life on Earth, planetary satellites and their orbital periods are not anywhere near as important to a planetary civilization as the Sun and its orbital period are. In fact, it's really just a fortunate coincidence that Earth has a large natural satellite with an orbital period of a useful duration that could potentially be incorporated into a calendar. Other planets don't have this feature.

Planets can have no moons, or many moons, but they all have one primary star that gives life to that world. If a planet has no moon (e.g. Mercury or Venus), then there would be no question of incorporating the cycles of planetary satellites into the calendar. If a planet has many moons (e.g. Jupiter or Saturn) then it becomes too complex.

For example, the next planet (not counting the Moon itself) to be inhabited by humans will be Mars. Mars's moons have orbital periods of about 0.3 and 1.3 sols, so there is really no benefit to be gained from incorporating these cycles into a martian calendar. However, the seasonal cycle will be central to Mars's primary industry, i.e. tourism, along with other facets of martian culture such as agriculture, energy production and scientific research. Martian calendars are always based around the martian solar year.

On most worlds other than Earth, a solar calendar is the only option. (Although, on many worlds, such as Luna and Venus, the calendar is more likely to be based around the solar day rather than the solar year.)


Humans on the Moon

Possibly one of the more compelling (yet less obvious) arguments for a lunisolar calendar for Earth is that in the not-too-distant future, people will be living on the Moon.

By the end of this century there is likely to be an established permanent human presence on the Moon, with numerous human settlements, and supporting a local economy built on tourism, sport, energy, mining, materials and manufacturing.

What sort of calendar will they use?

For people living on Luna, the lunar cycle of 29.53 days will be the most important cycle in their lives, as central to their time-keeping systems as the day is to people on Earth. From the perspective of the Moon, a lunation is the time between sunrises.

The lunar daytime is completely different from the lunar night. For one thing, the temperature during the 2-week lunar day becomes extremely hot, up to a boiling 396K (123°C), before plummeting to a mind-numbingly chilly 40K (-213°C) during the 2-week night. Much of the lunar equipment will likely be solar powered (since it is notoriously difficult to burn diesel fuel in a vacuum, and besides, there probably isn't any oil), and hence will only function during the day. Most tourism, exploration, mining and engineering and scientific research will be done during the lunar day.

So people living on the Moon will definitely use lunar months as the basis for their calendar. The lunar cycle will be much more important to them than Earth's day, Earth's year, or any other astronomical cycle. There are no seasons on Luna.

People living on the Moon will probably use artificial days equal in length to Earth's days (that is, until we genetically engineer humans that can stay awake or asleep for 2 weeks at a time). Thus, a calendar for the Moon will, naturally, be lunar or lunisolar. People living on the Moon may simply adopt an existing lunisolar calendar like the traditional Chinese or Hebrew.

Luna's culture will be highly integrated with Earth's, and the degree of communications, trade and transport between the two worlds will increase exponentially as we progress through the space colonisation era. A thriving lunar community and economy on the Moon means a continuous flow of people travelling to and from Earth. We can compare the future Earth-Moon system to Earth today - once it took months to travel between continents, yet now thousands of people do it every day. Within a few decades it will be the same between Earth and the Moon.

Consider this high degree of interaction, wouldn't it be convenient if we all used the same calendar?

However, the fact remains that, for Earth, a solar calendar is better, primarily because of the relative simplicity. The population of the Moon will probably never exceed a tiny fraction of Earth's, and hence it is not super-critical to make the calendars the same. It is much more important that the people of Earth have the best possible calendar. In any case, even if we have to use two calendars for two worlds, instead of just one, this would still be a massive improvement over the multitude of calendars currently in use on Earth today.