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A simple, accurate, solar calender for Earth

Calendar Types: Solar, Lunar andLunisolar Calendars

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

Months

Lunar and lunisolar calendars use lunationsas calendarmonths, whereas solar calendars define calendar months as a

fraction of the year that are only approximately equal to onelunation. A lunation is the average length of the synodic

month, the time between new moons, equal to about 29.53days. Months in lunar and lunisolar calendars therefore use

patterns of months 29 and 30 days in length to maintain thisaverage.

In a solar calendar with 12 months, the average month lengthis 365.24 12 = 30.44 days. Months in these types of

calendars, as most of us will be familiar with from theGregorian, 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 thatalmost all months are exactly equal to 4 7-day weeks (see

Months for more info).

Calendar Year Length

In a solarcalendar the intention is to organise the calendar

year to synchronise with the seasonal cycle. This is mostusually measured as the average time between successive

northern vernal equinoxes, equal to 365.2424 days (called thevernal equinox year). The northern vernal equinox is the most

commonly used seasonal marker to define a year's beginning

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because it marks the beginning of spring in the northernhemisphere. Spring is generally perceived as a time of new

beginnings, and most people (certainly most people who havebeen 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, solarcalendars typically define patterns of years 365 and 366 days

in length in order to produce an average length of about365.24 days.

In a lunarcalendar, such as the Islamic, the calendar year isdefined as 12 lunations, about 354.36 days on average. This

is about 11 days shorter than the vernal equinox year, andhence the seasonal markers tend to shift in the calendar,

occuring about 11 days later each year. This can be a problemin agriculture, tourism and other industries that revolve

around seasons.

Lunisolarcalendars solve this problem by introducing an

intercalary month (also called an "embolismic" month) every 2or 3 years, so that the average year length matches the

seasonal cycle. As 365.24 29.53 = 12.37 lunations, someyears 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 importantcalendars take this approach, including the Chinese, Hebrew

and Buddhist Calendars. However, this arrangement isgenerally 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 nationon 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 solarcalendar alongside the traditional lunisolar one. This second

calendar was aligned with the seasons, and divided the yearinto 24 equal parts called solar terms. It existed primarily for

the benefit of farmers, who need to be able to track progressthrough the seasonal cycle so they know when to sow, when

to reap, and so on.




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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 ofcertain plants, and those special evenings when werewolves

roam the land. So there is certainly some benefit to beingaware of the lunar cycle, which is why lunisolar calendars

persist, and why most printed solar calendars show lunarphases. However, they are not of critical importance to most

people living on Earth. (They would, however, be veryimportant 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 andother anniversaries using, not the day of the year, but themonth 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 dayand month occurs again. This is, of course, one of the mainuses of dates.

So, what happens if you're born on February 29? People bornon this date are called leap babies, because February 29 only

occurs in leap years. If you're a leap baby, what do you do foryour 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 issueaffecting just 0.07% of people, and it's no big deal to have

your birthday on the date before or after the actual date youwere born.

However, consider how much larger this problem becomeswhen using a lunisolar calendar. Some years have 12 months,

and others have 13. Imagine if you were born in that extra

13th month, as7/235 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 yearsyour 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 inlate 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 samesituation as in the Gregorian Calendar.

Clearly the leap baby problem is much worse in a lunisolarcalendar. Furthermore, because of the varying year lengths,

sometimes when your birthday occurs only 354 days have




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passed, and other times 383 have passed. This is obviously amuch less accurate way of measuring the passing of years.

Some calendars, such as the New Earth Calendar, use anintercalary week, in which case only 0.38% of people are

affected by the leap baby problem. This is obviously animprovement on a lunisolar calendar, but still not as good as a

solar calendar, plus there is not the benefit of being alignedwith the lunar cycle. (The primary benefit of this type of

calendar is that it is perpetual, yet does not disrupt the 7-daycycle.)

Moons

While our Moon is an important part of life on Earth, planetarysatellites and their orbital periods are not anywhere near as

important to a planetary civilization as the Sun and its orbitalperiod are. In fact, it's really just a fortunate coincidence that

Earth has a large natural satellite with an orbital period of auseful 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 hasno moon (e.g. Mercury or Venus), then there would be no

question of incorporating the cycles of planetary satellites intothe 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 haveorbital periods of about 0.3 and 1.3 sols, so there is really no

benefit to be gained from incorporating these cycles into amartian calendar. However, the seasonal cycle will be central

to Mars's primary industry, i.e. tourism, along with otherfacets 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 onlyoption. (Although, on many worlds, such as Luna and Venus,

the calendar is more likely to be based around the solar dayrather 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 numeroushuman settlements, and supporting a local economy built on

tourism, sport, energy, mining, materials and manufacturing.

What sort of calendar will theyuse?

For people living on Luna, the lunar cycle of 29.53 days will bethe most important cycle in their lives, as central to their time-

keeping systems as the day is to people on Earth. From the




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perspective of the Moon, a lunation is the time betweensunrises.

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 (123C), beforeplummeting to a mind-numbingly chilly 40K (-213C) during

the 2-week night. Much of the lunar equipment will likely besolar 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 researchwill be done during the lunar day.

So people living on the Moon will definitelyuse lunar monthsas the basis for their calendar. The lunar cycle will be much

more important to them than Earth's day, Earth's year, or anyother astronomical cycle. There are no seasons on Luna.

People living on the Moon will probably use artificial daysequal in length to Earth's days (that is, until we genetically

engineer humans that can stay awake or asleep for 2 weeksat a time). Thus, a calendar for the Moon will, naturally, be

lunar or lunisolar. People living on the Moon may simply adoptan 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 thetwo worlds will increase exponentially as we progress through

the space colonisation era. A thriving lunar community andeconomy 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 everyday. Within a few decades it will be the same between Earth

and the Moon.

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

However, the fact remains that, for Earth, a solar calendar isbetter, primarily because of the relative simplicity. The

population of the Moon will probably never exceed a tinyfraction of Earth's, and hence it is not super-critical to make

the calendars the same. It is much more important that thepeople of Earth have the best possible calendar. In any case,

even if we have to use two calendars for two worlds, insteadof just one, this would still be a massive improvement over

the multitude of calendars currently in use on Earth today.

Shaun Moss 2008

Shaun's Homepage | Email Shaun




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