Children's Health & Safety Association

Issue 43: July 2018

Tuesday, 31 December 2013 11:55

Things You Wanted to Know about Calendars…but were afraid to ask

Written by 

Thirty days hath September,

April, June, and November

All the rest have thirty-one,

Except for February—and that's no fun!

December 15, 2013 - Why do we have different calendars? Have you ever wondered why some months have 30 days while others have 31? And why does February have 28 or 29 days? Why do we need leap years?

Canada is very much a multicultural country comprised of approximately 208 ethnicities and in 2006, international migration accounted for two-thirds of Canadian population growth. Our families celebrate events and special occasions by honouring our ancestral traditions and rather than just let these blessed days go by without a hint of curiosity or acquired knowledge - it is good to understand why the diversities within our calendars exist.

Ukrainian Christmas takes place on January 7th. Christmas is celebrated in the western world on December 25th, so to most of us in North America it seems unusual to have Christmas in January. Why does this difference exist? The answer is because different calendars are being used. Most Orthodox religions use the Julian calendar while North Americans use the Gregorian calendar. On both calendars, the birth of Christ is celebrated on December 25th – but the two calendars, even though they have the same months and the same numbers of days per month, are not in synch. These two calendars differ by thirteen days: December 25th on the Julian calendar coincides with January 7th on the Gregorian calendar.


Let’s discover some of the interesting history behind these calendars, and why and how they differ.

Lunar and Solar Cycles

Human civilizations have tried to keep track of time for many reasons. When is it time to plant crops? When is it time to move to a new location in order to avoid the onset of harsh weather? When will migrating animals return? Ancient people used nature’s cues: changing weather, the behaviour of birds and animals, and alterations in plants, to name a few.

cal-005The sky provided better indicators for marking time. In early ages, people did not know that the earth orbited the sun (one complete orbit is a solar year): it seemed that everything in the sky moved (stars, the sun, the moon) while the earth remained fixed. The movement of these “heavenly bodies” were studied and their motions were used as indicators of events, or as keys to mark the passage of time. The different seasons were known to be cyclic – and those seasons were related to the changing warmth of the sun. It was not easy to develop a system that accurately measured time for a solar year.

The Mayan civilization, for example, had a 365-day solar calendar called the Haab, which contained 18 months, each with 20 days, with 5 add-on “nameless days” at the end.  Each of these 19 unique sections of the calendar had its own name and a graphic image known as a “glyph.”

cal-006The moon, however, offered an easier alternative for recording the passage of time. The moon actually orbits the earth in just over 27 days (one complete orbit is called a sidereal month), and it is easily observed to go through a cyclic phase – changing its appearance as the days and nights progress, and returning again to its original appearance after a fixed period (called a lunar cycle or a synodic month) – a process that takes 29.53 days.

The difference between a sidereal month and a synodic month is caused by the fact that the earth is  moving through space in its orbit around the sun as  the moon orbits the earth, so our reference position  moves. To our point of view, it takes the moon just over 29 days to return to its original appearance. 

At one time twelve lunar cycles (12 X 29 days) were considered to be a year since the timing closely coincided with the seasonal cycle of a solar year.


The Observed Phases of the Moon

Unfortunately, the length of a solar year is not an exact multiple of the length of a synodic month. This means that a calendar based on a 12-month lunar year cannot coincide exactly with a solar year. However, 19 solar years is 234.997 synodic months, which is very close to the whole number 235. Every 19 years the phases of the moon fall on the same dates - nineteen years is called a Metonic cycle (after Meton, an astronomer from Athens in the 5th century BC). Very early calendar systems were based on nineteen-year periods because of this relationship.

cal-009Do we know when humans first began to use calendars to keep track of time? Prehistoric cave paintings on the walls of the Lascaux Caves in France, dating back 15,000 years, depict many of the animals that were hunted by Cro-magnon man. Other markings on the walls were once believed to represent stars and constellations.

Dr. Michael Rappenglueck from the University of Munich, Germany, believes that these patterns show the moon going through its different phases, and as well, is the oldest representation of a lunar calendar.

“…the groups of dots and squares painted among representations of bulls, antelope and horses depict the 29-day cycle of the Earth’s satellite."

cal-011Many civilizations have used lunar calendars as the basis for the timing of a year, including early Sumerians and Babylonians, Egyptians, Greeks, Muslims, Jews, Romans, and Chinese. The number of days in a 12-month lunar year total 354.36 (12 X 29.53), a difference of about eleven days from the (approximately) 365.25 days in a solar year. A recurring solar event that was recorded on such a lunar calendar in one year (such as the time of the summer solstice) would be 11 days off in the next calendar year. Frequent adjustments were needed in order to bring such lunar-based calendar systems into agreement with the actual time for events related to the annual orbit of the earth around the sun.

Intercalary Months

A year consisted of 12 months, each with exactly 30 days.  Each month was sub-divided into 3 weeks, each containing exactly 10 days.

The Egyptians developed one of the earliest scientific calendars to more accurately predict the annual flooding of the Nile River. A year consisted of 12 months, each with exactly 30 days. Each month was subdivided into 3 weeks, each containing exactly 10 days.

The Sumerians first settled ancient Babylonia (the region in what is now southern Iraq, from Baghdad to the Persian Gulf) around 6,000 years ago. By the time the Babylonian kingdom began to flourish under the reign of King Hammurabi (1792-1750 BC), the use of lunar reckoning had been in use for a few hundred years to help facilitate economic transactions. In 18th century BC, the Babylonian Empire standardized the year by adopting the lunar calendar of the Sumerian sacred city of Nippur.

The Sumerians had a calendar that divided the year into 30-day months, divided the day into 12 periods (each corresponding to 2 of our hours), and divided these periods into 30 parts (each like 4 of our minutes). Since it was necessary for this 360-day calendar to be brought into line with the solar (agricultural) year of approximately 365 days, an intercalated month was added from time to time (a 360-day calendar would be off by about 5 days in each solar year, so every six years or so an additional month would have to be added to the calendar in order to bring it into line with the actual seasons). Such months were added haphazardly, according to the whim of local city officials, so different Sumerian cities had different calendars. The adoption, by the prestigious Babylonians, of the method used in the city of Nippur assured that a move towards standardization would occur.

Egypt adopted a leap year system, with an extra day every four years, during the Greek rule of the Ptolemaic dynasty (305 to 30 BCE*).  It is said that Cleopatra, the last Ptolemaic ruler, was responsible for introducing the concept to Rome via her visiting lover, Julius Caesar.

The lunar cycle of 29.53 days was usually approximated to 30 days in lunar calendars, but as stated this resulted in annual discrepancies from the actual solar year. To eliminate the need for intercalary months, additional days were added to some of the months – and the concept of leap years was introduced. Egypt adopted a leap year system, with an extra day every four years, during the Greek rule of the Ptolemaic dynasty (305 to 30 BCE*). It is said that Cleopatra, the last Ptolemaic ruler, was responsible for introducing the concept to Rome via her visiting lover, Julius Caesar.

*Humans needed a system to record dates and events for many reasons (i.e. for historical, agricultural, religious and business purposes). There are many different calendars and dating systems, each using a different point of reference for the start of the system. The Gregorian calendar uses the birth of Christ as its reference point. The years following the birth are called AD (Anno Domini) and the years prior to the birth are called BC (Before Christ).  In the mid-1800s, Jewish academics introduced alternative notations: CE (Current Era, or Common Era, or Christian Era) could be used instead of AD, and BCE (Before Current /Common/Christian Era) could be used instead of BC. Today, this notation is popular in scientific and academic communities, and is often used by publishers wishing to remain secular in their treatment of materials.

Early Roman Calendars

In ancient Rome, a priest would observe the sky and announce the start of a new lunar cycle to the king. Romans referred to the day of each full moon as the ides and the day of each new moon as calends (from their word calare, which means, "to proclaim”). The word calendar is derived from this tradition.

The Roman (or Pre-Julian) calendar, believed to be a lunar calendar, was created by Romulus, the founder and first king of Rome in what would be 753 BC. It was a ten-month calendar (which began the year in what we now call March) with four months of 31 days and six months of 30 days. This 304-day calendar didn’t work for long because it quickly came out of alignment with the seasons. King Numa Pompilius revised the calendar around 700 BC by adding two more months (January and February) at the end of the year, increasing the number of days in the calendar year to 354 or 355. Although this was an improvement, it proved imperfect causing the calendar to continually fall out of alignment with the seasons. Attempts were made to fix the calendar by adding an extra month to correct the misalignment.


The Julian Calendar

When Julius Caesar became Pontifex Maximus, he eliminated such intercalary months by adjusting the number of days in the months (to the system we use today) and introducing the concept of a leap year. The extra day was added to the end of the year – which, at that time, was February (which helps to explain why February, which is now the second month of the year, continues to be the month that receives the extra day in leap years).

The Julian calendar eventually resulted (the year 45 BCE* is often cited as the year of its introduction). Caesar's reform only applied to the Roman calendar, but in the following decades many of the local civic and provincial calendars of the empire and neighbouring client kingdoms adopted the Julian calendar in order to have 365 days per year, with an extra day added every four years.

Roman historians used the date of the founding of Rome as the reference point for determining the number of years between events. Most Romans didn’t use this method (called ab urbe condita, or AUC); instead, the early Julian calendar identified a year for dating purposes by naming it after the two consuls who took office in that year (a so-named year was called a consular year). In 309 CE* and 310 CE*, and from time to time thereafter, no consuls were appointed, so a change was made.

During the 4th century the Romans used a method known as the Diocletian Era (Latin: Anno Diocletiani, shortened to AD), named for the Roman Emperor Diocletian who had severely persecuted Christians during his reign. This period, also known as the Era of Martyrs, began when Diocletian began his reign. AD 1 was equivalent to the year 284 AD on the Gregorian calendar. This new system of numbering was used by the Church of Alexandria to produce tables for Easter celebrations. A sixth-century Christian monk named Dionysius Exiguus did not wish to use a numbering system based on the memory of the tyrant Diocletian, so he created an alternate system based on the number of years that had passed since the birth of Christ. AD came to mean Anno Domini – the Year of Our Lord.

The Julian calendar remained in general use throughout Europe and northern Africa until the late 16th century.

The Gregorian Calendar

cal-018The Julian calendar marked the passage of a solar year, using the method with which we are all familiar. There were twelve months in a year; five months had 30 days, six had 31 days, and one had 28 days (which was extended to 29 days every four years in leap years – centuries that were divisible by 4). Although it was reasonably accurate, it erred by 11.5 minutes per year on average, despite the leap year adjustments, and by 1582 calendar dates such as Easter were ten days off where they should be according to the solstice. In that year, Pope Gregory XIII ordered a correction to the Julian calendar.

Although the Gregorian calendar is named after the Pope, it is an adaptation of a calendar designed by an Italian doctor, astronomer and philosopher Luigi Lilio (also known as Aloysius Lilius). Sadly, he died in 1576, six years before his calendar was officially introduced.

To make up for the discrepancy with the solar year, the calendar was advanced by ten days – and the concept of a “leap century” was introduced to eliminate the 11.5 minutes/year error in the Julian calendar. The concept of a leap year was maintained, but only century years (i.e. those years evenly divisible by 100) that were divisible by 400 remained as leap years. Century years such as 1600 and 2000 were leap years (i.e. leap centuries), but not century years such as 1700, 1800, and 1900.

This fine tinkering resulted in a calendar that differed from the solar year by only 26 seconds per year on average, a difference that adds up to a single day every 3,323 years. At last, a calendar system had been created that maintained a very close alignment with the seasons.

The Gregorian calendar was quickly adopted by most Catholic countries such as Spain, Portugal, Poland and parts of Italy. Protestant countries followed later, and the countries of Eastern Europe adopted it even later.  The British Empire and the American colonies converted to its use in 1752, Japan in 1873, Russia in 1918, Greece in 1923 and China in 1929.  Although all Eastern Orthodox countries (most of them in eastern or southeastern Europe) had adopted the Gregorian calendar by 1924, most of their national churches had not. The Orthodox Churches of Russia, Ukraine, Georgia, Serbia, Jerusalem, Macedonia, and other groups continue to use the Julian calendar, so they celebrate the Nativity of December 25th on the Julian calendar, which is January 7th on the Gregorian calendar.


This stamp was issued by Montenegro to commemorate the introduction of the calendar by Pope Gregory XIII in 1582. It features a watch fob engraved with a map of the Indian Ocean east to the South Pacific (including India, Indo-China and Australia). In the background is "1582" - the year the calendar was introduced, "425" -- the number of years between 1582 and 2007 when the stamp was issued, and "26" - the number of seconds the Gregorian Calendar differs from the solar calendar in a year.



The Curious History of the Gregorian Calendar – Info Please Database

Timeline of Interesting Calendar Facts – Calendars Through the Ages

Common Era - Wikipedia

Orbit of the Moon - Wikipedia

The Julian Calendar – Time and Date

The Julian Calendar – Wikipedia

Other Ancient Calendars – Calendar Through the Ages – extensive information!

Leap Year: How the World Makes Up for Lost Time – National Geographic

Thirty Days Hath September – Wikipedia

The Knuckle Mnemonic


Read 32362 times Last modified on Thursday, 03 December 2015 11:43


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