⌛🪶 Story of Calendar 📅
I make history, possible! How and when will you celebrate "New Year's Day", without me?
Questions:
Why are the years 1800, 1900, and 2100 not leap years, despite being divisible by 4?
Furthermore, why do we even have to take a leap every few years?
Why is September not the seventh month, October not the 8th, November not the 9th and December not the 10th month?
When does the calendar “starts”? Which year is the zero year?
We will get the answer to all these questions and more, in today’s article.
Let’s start the journey of the evolution of the calendar. A journey that will involve astronomy, astrology, religion, politics, science, economics and a lot of common sense.
Need of having a calendar
In current times it is hard to imagine life without a calendar. We anxiously wait for Mondays to go and Fridays to come. Holidays, birthdays, festivals and important events of the past and future — all these are possible because of the calendar.
Several thousands of years ago, the need for a calendar was primarily limited to three things. Maintaining cycles of agricultural activities, handling livestock and offering prayers to deities.
For example, the month of November was earlier referred to as “Blod-monath” in the Anglo-Saxon culture. It was the month of immolation, the month of sacrifice. It was in this month that the cattle were slaughtered and dedicated to the gods.
Some basic stuff
Before we jump into the details, it is important to get some important concepts clear. It will help you in understanding and enjoy this journey of the calendar’s evolution.
What is a day?
Many of us believe that “day” refers to the time taken for the earth to complete one rotation about its axis. Well, that time is referred to as what is called “Sidereal day”.
The day that we use in our daily lives is technically called “Solar day”. Let’s understand the difference in simple words.
Put a stick on the ground. On a sunny day, when the sun is exactly overhead, at the highest point in the sky, the stick will form the shortest shadow or no shadow at all. This is what we call “noon”. The time between two consecutive noon is what is referred to as “solar day”, or simply a “day” in general.
The mean of all such solar days throughout the year is called “Mean solar day” and is equal to 24 hours or 86400 seconds, exact.
So how is this different from the time taken to complete one rotation (the sidereal day)?
The difference occurs because when the earth rotates around its axis, it also revolves around the sun. Refer to the figure above. After exactly one rotation, the stick (the red arrow), is still not perfectly below the sun. The earth has to rotate a bit more. For about 4 minutes extra and reach position 3.
From position 1 to position 2, is the sidereal day. And from position 1 to position 3 is the solar day.
Solar Year (or Tropical Year)
Solar Year is equal to 365.24217 mean solar days. This is our Target! This is what humanity has been trying to track and keep our lives in sync with.
Lunar month (or Synodic month)
The synodic month refers to the average period required by the moon to orbit the earth, with respect to the line joining the earth and the sun. This is slightly longer than the lunar month, which refers to the time required by the moon to orbit the earth with respect to the faraway fixed stars. This difference occurs because while the moon is revolving around the earth, the earth also moves ahead revolving around the sun.
The Moon completes one orbit around Earth every 27.3 days — a sidereal month. Due to Earth's orbital motion around the Sun, the Moon takes ~29.5 days — a synodic month, also called Lunar month in general.
Intercalation
This refers to the insertion of “leap” days, weeks, or months to a calendar year, in order to match the calendar year with the “target”— usually the Solar year.
Equinox
The moment when the Sun appears directly above the equator.
This results in the day and night being approximately equal all over the planet. This occurs twice each year, around 20 March and 23 September.
Solstice
The moment when the Sun appears at the extreme far away — either north or south of the equator.
This results in the longest (or shortest) day and shortest (or longest) night in either the northern hemisphere or the southern hemisphere. This also occurs twice each year, around 21 June and 21 December.
Equinoxes and Solstices were often used as reference points to compare the accuracy of the calendar with the solar cycle.
Let’s start tracking the time
Now that we have covered the basics, let’s see how humanity developed means and systems to track time and its repetitive nature.
Prehistoric Era
So far, historians have identified the presence of megalith structures that are believed to be developed for the purpose of timekeeping. (Megalith refers to a large stone or group of stones cut or placed for some purpose, in prehistoric times). Some examples:
Wurdi Youang
This is the name given to a stone arrangement of around 100 basalt stones, found in Australia. It is estimated to be 11,000 years old and considered by some historians the oldest astronomical observatory in the world.
As shown in the figure above, the alignment of stones marks the positions of the setting sun at the equinoxes and solstices. The alignment is accurate to within a few degrees.
Warren Field
Another structure built around 8000 BC was discovered in Scotland. It contains 12 pits that are believed to correlate with the 12 phases of the moon. It is considered to be the oldest Lunar calendar yet found.
Ancient Calendars
Sumerian
The Sumerian calendar was an ancient lunar calendar based on the lunar cycles and used in Sumer, the first civilization in the world. Believed to have been created around 2100 BC, the calendar divided the year into 12 lunar months of 29 or 30 days. This made the Sumerian year to be of 354 days (=12 x 29.5).
This also created a difference of 11 days from the solar year of 365 days.
To account for this difference intercalation of a leap month was added irregularly every 2-3 years. Every new month began with the sighting of the new moon. There was no concept of weeks in this calendar.
Babylonian
However, by the 5th century BC, it was realized by Sumerians and Babylonians that the lunar cycles and solar cycles can be combined as a lunisolar cycle of 19 solar years being almost equivalent to 235 lunar months.
For math lovers: 19 x 365 = 6935 and 29 x 29.5 = 6932.5
This cycle of 235 lunar months or 19 solar years was called as Metonic Cycle, named after Meton of Athens, a Greek astronomer and mathematician. This cycle formed the basis of the Greek and Hebrew calendars.
This led to the usage of 12 short years of 12 months and 7 long years of 13 months, making total of 12 x 12 + 13 x 7 = 235 months.
The years 3, 6, 8, 11, 14, 17, and 19 were taken as long years of 13 months, with rest years being of 12 months The complete period of 19 solar year being referred as “The Great Year”.
Roman
The Romans in the first century BC used a 10-month calendar in which a month was 30 or 31 days, making a total of 304 days. In this, about 50 days of unorganized winter were added every year.
This calendar is attributed to Romulus, the legendary founder and first king of Rome.
The word "calendar" is derived from the word “calends” which is the first day of every month in the Roman calendar.
Naming of Months: Here comes the interesting part and the answer to one of the questions asked at the beginning of the article —
In the calendar of Romulus, the numbering of months was as follows:
March (1), April (2), May (3), June (4), July (5), August (6), September (7), October (8), November (9), and December (10).
March was named after Mars, the God of war; April after Aphrodite, the Goddess of Love; May after Maia, the mother of the Greek God Hermes; June after Juno, another Roman Goddess; July, after Roman General Julius Caesar; and August (after Roman emperor Caesar Augustus)
The last 4 months from September to December were indeed named after the Latin names of seven (septem), eight (octo), nine (novem), and ten (decem).
Later, January (after Janus, the God of beginnings) and February (after februum, Latin for purification) were added above March, taking the year to 354 days.
Julian Calendar: The big leap!
The Roman Calendar soon evolved into a very complex 24-year calendar. This had a year of 355 days alternating with intercalary years, which itself alternated between 377 and 378 days long. Added to this complexity, the system had an 8-year period, which had only 3 intercalary years of 377 days each.
Thus, 13 normal years of 355 days, 7 intercalary years of 377 days, and 4 intercalary ears of 378 days.
While the system was complex, it gave a very accurate average of 365.25 days over the period of 24 years.
So, what was the problem?
Politics!
The task (and authority) of adding the intercalary years was of the highest priests and politicians, who often (similar to current times), abused their power. They would often lengthen or shorten the year in order to increase their time in office (or reduce the same of their opponents!)
Julius Caesar intended to solve this problem permanently, by creating a calendar that remained aligned to the sun without any human intervention. The new calendar, by the name Julian calendar, came into being on 1 January 45 BC.
The Julian calendar added 10 days to the previous 355-day calendar. Making it a total of 365 days. These 10 days were distributed across the 12 months, with some months getting 2 extra days and some getting one extra day. Every 3 years, 1 day was added to the month of February, giving a leap year of 366 days.
This resulted in the Julian calendar being 365.25 days on average. But remember the solar year is of 365.24217 days. So, although very accurate and simple, the Julian calendar drifted by 3 days over every 400 years, compared to the observed equinox times.
Math lovers: 365.25 days minus 365.2417 days = +0.00783 days/year drift
0.00783 days/year x 400 years = ~3 days drift.
This was ignored in those times, as 400 years was a big period but will have to be eventually taken into account soon. Before we get into that, we have to jump into AD.
From BC to AD (year zero?)
An important question to think about while making a calendar, is where to fix the starting point. What should be considered as the “Year zero”?
And since it is practically impossible to reach the beginning of time and place all the events scientifically on a timeline, we found a simpler route. Take a particular event and use it as a reference point for all forward and backward dates.
Dionysius Exiguus, a Roman monk, was the person credited for inventing the AD-BC system. The conception or birth of Jesus was chosen to be the landmark event and the AD counting years starting from this event and BC for the years before this. He invented this system in AD 525.
AD stands for “anno Domini” (and not After Death, which some people incorrectly assume). Anno Domini is Latin for "in the year of the Lord".
BC stands for “Before Christ”. There is no concept of zero. AD 1 is immediately preceded by 1 BC, with nothing in between them.
Further, to make the system more neutral and inclusive of non-Christian people, the terminology CE (Common Era) and BCE (Before Common Era) are also used. Both systems are numerically equivalent, that is AD 2000 = 2000 CE.
Gregorian
The Julian calendar was immensely successful and lasted for more than 1600 years until 1582 when Pope Gregory XIII introduced the new Gregorian calendar. The new calendar had a minor modification in order to reduce the average length of the year from 365.25 days to 365.2425 days. This made it closer to the actual 365.2417 days.
As discussed previously, Julian calendar drifted ahead by 3 days, every 4 centuries. By 1600, the accumulated error had resulted in a drift of about 10 days.
Aloysius Lilius, an Italian doctor, astronomer, philosopher and chronologist, came up with the ingenious proposal of reducing the number of leap years in four centuries from 100 to 97 by making three out of four centurial years common instead of leap years.
Aloysius’s proposal was accepted, and the new Gregorian calendar was prepared by first deleting 10 extra days from the calendar of 1582. The Julian calendar day Thursday, 4 October 1582 was followed by the first day of the Gregorian calendar, Friday, 15 October 1582.
Final rules of Leap year
Every year that is exactly divisible by four is a leap year, except century years.
The centurial years are leap years if they are exactly divisible by 400.
Hence, the years 1700, 1800, and 1900 are not leap years.
Proposal for increasing accuracy
The difference between the average Gregorian year and the Solar year is 0.00033 days which will result in a drift of ~1 day over a period of 4000 years.
365.2425 — 365.24217 = 0.00033 days/year
0.00033 x 4000 = 1.32 day over a period of 4000 years
To account for this small error, in the 19th century, Sir John Herschel proposed a modification to the Gregorian calendar. The proposal is to make the year 4000, and multiples thereof, a common year instead of a leap year. This would reduce the average year to 365.24225 days, reduce the error to 0.00008 days and make it accurate for ~12,500 years.
Probably because the first usage of this will occur in the far future, in the year 4000, this proposal has not been officially accepted.
Concept of Week and different weekdays
The concept of the week can be traced back to the Sumerians and Babylonians. Babylonians celebrated the 7th, 14th, 21st, and 28th as "holy days", also called "evil days" (meaning inauspicious for certain activities). On these days, officials were prohibited from various activities and common men were forbidden to "make a wish", and at least the 28th was known as a "rest day".
The names of different days of weeks and their origin are as follows: Sunday, day of the Sun; Monday, day of the Moon; Tuesday, day of Tiw (a Proto-Germanic God); Wednesday, day of Woden (a German God); Thursday, God Thor; Friday, German Goddess Frigg; Saturday, Roman God Saturn.
Things to ponder
So, that was the brief history of Calendar.
The development of the calendar brought in the usage of many inventions from other fields. Lenses, telescopes, writing instruments, mathematics, trigonometry, geometry, geology and knowledge of many other subjects were needed along with a lot of patience and perseverance, for tracking the Solar year accurately!
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