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Time Span

Calendars and our Time

From the day of our birth until the atoms that form us return to the cosmos, everything we do has a time to do it and a time to move on to something else. We each have a history, a passing of our time, a time to be aware of ourself. Taken together with others, the history of civilization is humanity’s time to be aware of itself. In our time of evolution from primate to Homo sapiens to modern human we have accomplished a great deal, and keeping track of the times of these accomplishments is important to us.

One of those important times in our history was the advent of agriculture around 11,000 years ago. When it became important to determine the best times for planting and harvesting crops, most cultures around the world came up with some sort of time reckoning system. One of these, for example, was at Bru na Boinne in Ireland, a large stone structure that was set up to track the rising of the sun on the winter solstice. The neolithic Irish knew that after that annual astronomical phenomenon the hours of daylight  would gradually begin to get longer and the time to plant the crops for the upcoming season, usually wheat and barley, was coming.

Even before agriculture had spread around the world, various Stone Age groups came up with ways to keep track of days, months, and years. One of the earliest found so far that is believed to date back about 10,000 years was in Scotland. Their ‘calendar’ consisted of 12 pits containing large rocks and is believed to have been used to track the phases of the moon, apparently to help the hunter-gatherer group mark the times of animal migrations or when fish such as salmon came upstream to spawn.

Today we keep track of everything from our birthdays to the split-second difference between when one Olympic runner hits the tape before another one, or the 2.2 micro second lifespan of a subatomic particle called a muon compared with the 230 million years it takes our solar system to orbit the black hole in the center of the Milky Way galaxy. We have learned to live with seconds, minutes, hours, days, months, years, decades, and centuries. In our urban time-driven world it would be difficult to get through the day without our clocks and our calendars.

It would be easy to make a calendar if the solar year, the time it takes Earth to orbit the sun, was exactly 365 days and the time it takes the moon to orbit Earth exactly 30 days. But they are not, and so it becomes complicated to figure out how to make calendars accurate year after year. The actual time it takes Earth to orbit the sun — called the tropical year after the Greek word tropikos, meaning turn — is 365.242199 days as we currently measure it. The moon’s orbit is measured 2 different ways: the synodic orbit from one phase to the same phase, full to full, for example, takes 29.53 days. The sidereal orbit when the moon becomes lined up with the same background stars takes only 27.32 days. These fractions of solar and lunar orbit times have caused calendar makers headaches for thousands of years, and different cultures have coped with them in a variety of ways.

Archaeologist believe the basis for our modern calendar was developed by the Sumerians around 6000 years ago in the area we call Mesopotamia between the Tigress and Euphrates rivers. Their lunisolar calendar, based on the orbit of Earth around the sun and the moon around Earth, set the standard for others to follow right down to today.

The Sumerians set their months up on the 29.53-day synodic lunar orbit from new moon to new moon. Each year had 12 months, each one beginning at the first sign of the waxing crescent moon following a dark new moon. Six of the months had 30 days and 6 had 29 days. This gave them a year of 354 days, which was 11and ¼ days short of the orbit time of the Earth as they measured it then. To keep the years up to date, an extra or intercalary month was added every 3 or 4 years to bring the calendar more in line with actual solar time. This seems like a cumbersome way to keep track of months and years, but it no doubt helped the Sumerians know when to plant crops and to have harvest festivals.

It is interesting to note that it was the Sumerians who not only gave us our calendar, but they also gave us our base 60 system for measuring time, which is believed to have been based on the human heartbeat of around 60 beats per minute and a rounded off year of 360 days. And so today we live with 60-second minutes and 60-minute hours, even though nearly everything else we measure is base 10, a system originated by the ancient Greeks. The Sumerians also noticed that 60 is divisible by 12 numbers: 1,2,3,4,5,6,10,12,15,20, 30, and 60, and that 60 multiplied by 6 was a convenient number for measuring around a circle. So full circles today are 360 degrees.


The Sumerians did not divide their month into weeks, but the seven-day week shows up a few years later on the Babylonian calendar. It was based on the phases of the moon as it crossed the sky, changing from new moon to first quarter in 7 days, then first quarter to full moon in another 7 days, and so on, making the month an even 28 days long. But again, calendar makers had to cope with fractions of days. It would be convenient to say that moon phases went from new to full in an even 14 days or two weeks and on to the next new moon in another 14 days. But 28 days is 7 tenths of a day or 16.8 hours too long for the sidereal orbit and 1.5 days or 36 hours too short of the synodic orbit time. In order to get the months to work out to match the Earth’s solar orbit, the Babylonians, like the Sumerians, inserted an intercalary month as decreed by the king based on advice for his astronomers. It was not easy getting a perfect month from the moon.

Even today, our calendars must cope with lunar orbit times that are not exactly in line with Earth’s solar orbit times. It is these fractions of lunar orbit times that causes the number of days between moon phases on our current calendar to be 7 days, or 8 days, and sometimes even 6 days.

The names of the days of the week changed from culture to culture. The English names we are familiar with today, for example, come from three different sources: Latin, German, and Nordic. Sunday is sun’s day, based on the Latin word sol. Monday is moon’s day and comes from the Proto-Germanic word menon. Tuesday comes from the Norse god Tyr. Wednesday is named after Wodan, another Norse god. A third Norse god gave us Thor’s day for Thursday. Frigg, the wife of Wodan, is the basis of Friday. Saturday is based on the Roman god Saturnus who also got a planet named after him. It is interesting to note that the German and Norse names probably came from the German Anglo-Saxon people who invaded Britian about 500AD and the Vikings from Scandinavia who came around 800AD.

After the Sumerian and the Babylonian calendars, came the lunisolar calendar devised by the Egyptians that had 12 months of 30 days plus an additional 5 days added at the end of the year, which gave them a 365-day year. Their year had three seasons: the Nile flooding season, the cultivating season, and the harvest season. Over the centuries, the Egyptians developed three different calendars, the most accurate being the sidereal calendar which started the new year at the rising of the star Sirius – the brightest star in the night sky – as it came over the eastern horizon. Modern astronomers have determined that the actual sidereal year – from star rise to star rise – is 365 days 6 hours, 9 minutes and 9.6 seconds. However, is highly doubtful the Egyptian calendar was that accurate.

It was the Egyptians who first came up with the 24-hour day, but the hours were not the same as our hours today. In a rather complicated system of star grouping, the Egyptians divided the stars into 36 groups, and each night the passing of 10 of the groups, called decans, was the length of the night. Each decan rose over the eastern horizon about every 40 minutes, making their night hours 40 minutes long.

The day, on the other hand, was divided into 12 hours that were longer than the decan hours, plus they added one hour each for dawn and twilight. From midday to midday was 24 hours, but the day and night hours were different lengths.

A few years later, the Greek astronomer, Hipparchus, worked out a system to make each of the 24 hours the same length. His system used the vernal and autumnal equinoxes, each of which had 12 hours of daylight and 12 hours of night of equal lengths. He had solved the problem of the unequal hours.


In a 24-hour day, the Earth revolves counterclockwise approximately 15 degrees of longitude per hour in respect to the sun, which gives us our time zones, and in 24 hours it revolves a full 360 degrees. That seems straightforward enough and should make it easy to keep track of time around the Earth. But the problem was, where on Earth do you start measuring…Paris, Moscow, Washington DC. Many cities around the world wanted to be the starting point for time.

It was not until 1884 that global time became standardized. At a meeting called the International Meridian Conference held in Washington DC, it was decided that the Prime Meridian or 0 degrees longitude, would be fixed at the Royal Observatory at Greenwich, England, near London. Time beginning from that point is called Universal Coordinated Time or UTC and for uniformity, all of the world’s clocks are set in respect to that time. For example, going west there are 5 times zones between London and Washington DC. So, when it is 10pm in London, it is 5 pm in Washington DC. Going east is the opposite. 10pm Saturday night in London is 6am Sunday morning in Tokyo.

In the last few years, there has been some discussion about having one time for the entire world. So, if it is 10pm Saturday in London it is also 10pm Saturday in Washington DC and Tokyo. Although this system could simplify keeping global time, the details as to how it might work have not been ironed out.

Although we think of the ancient Greeks as the brilliant originators of democracy and most of western math, geometry, and philosophy, their methods for keeping track of days and years were somewhat arbitrary. Most large cities had their own calendar based on the Egyptian lunisolar calendar, with the Athenian calendar being the most popular. In Athens and some other areas of Greece, the new year started at the first new moon after the summer solstice, which actually caused the new year to start on a different day each year.

In spite of the arbitrariness of their calendar systems, an unknown Greek mechanical genius came up with a device that has been determined to be one of the most detailed mechanisms for keeping track of time ever devised in the ancient world.

Found in 1901 in a ship that sank around 70 BC near the island of Antikythera, the device, called simply the Antikythera Mechanism, is considered one of the most sophisticated calculating devices of its time, and possibly the first analog computer in human history. It is believed to have been made between 200 BC and 80 BC possibly on the Island of Rhodes. With a set of intricate gears and dials, it could indicate astronomical data such as the exact position of the sun and moon and the phase of the moon for any day, as well as eclipses and much more. It even gave the dates of the Olympic games begun in 776 BC which were held every four years until 393 AD. It would be over 1,200 years before another device as complex would be invented.


The calendar considered the predecessor of the calendar in use in most of the world today is the Julian calendar that was officially put into use in 45 BC by Julius Caesar to replace the old 10-month Roman calendar that had been in use for several years. Again, it is a lunisolar calendar based on the orbits of Earth and moon. As with any calendar, a starting date must be chosen, and Caesar chose the supposed year of the founding of Rome, 753 BC. Thus, in 45 BC, that would have made the first day in the Julian calendar January 1, 708 Ab urbe Condita, or 708 years after the founding of the city.

The names given to the months on our current calendar are merely Anglicized versions of the Latin names on the Julian calendar. In fact, our word calendar comes from the Latin word Kalends, the name for the first day of each month. The names are from gods the Romans worshipped, to crop planting times, to political leaders, to their placement on the calendar.

Janus, for example, was chosen for the first month since he was the god of endings and beginnings. He is depicted as having two faces, one facing forward, the other facing backward, very appropriate for the month of January when the old year ends and the new one starts. Februarius was a time for religious purification. Maritus was named after the god Mars. Aprilis was dedicated to farming and rural life since it was the month for planting crops. Maius was named after Maia, the goddess of fertility and growth. In Roman mythology she is one of the Pleiades visible in our winter night sky. Junius was named after Juno, wife of Jupiter and mother of Mars. July was named for Julius Caesar. August was named for Augustus Caesar. September, number 7, was the seventh month in the pre-Julian calendar 10- month calendar and the name stuck even though it is now our ninth month. It is the same with October for 8, November for 9, and December for 10, pointing out that in the Julian calendar using traditional names took precedence over accuracy.

The Julian calendar began the practice of adding a leap day every four years in an attempt to keep the days in line with the Earth’s orbit time around the sun. It was an ingenious idea, but the problem was that the orbit time calculated by the Roman astronomers, such as Sosigenes of Alexandria, was off by a few minutes per year. The Julian calendar was based on one Earth orbit around the sun of 365.25 days, close to the actual time of 365.242199 days, but far enough off to cause the year to be 11 minutes and 14 seconds too long, which over several centuries, became problematic. For example, 1000 years after 45 BC, the calendar was off an entire 7-day week.

A few centuries later as the Catholic church was spreading throughout Europe, it became important to accurately determine the dates of holy days, especially Easter. In order to solve the date problem as well as other ecclesiastical issues, Roman emperor Constantine arranged the Council of Nicaea in 325AD, a gathering of Catholic bishops from various countries. They decreed, after a great deal of holy haggling, that Easter would fall on the first Sunday following the first full moon after the vernal equinox, which the Nicaean delegates set to be March 21 on the Julian calendar. That seemed to solve the problem for the next few years.

Yet, by the mid-1500s, some 1200 years after the Council of Nicaea, those fractions of lunar and solar orbit times unaccounted for in the Julian calendar, had caused the vernal equinox to fall on March 10, a full 11 days ahead of where it should be. And since Easter is one of the most important holy days in the Christian year, the Roman Catholic curia, the governing board of the church, felt that something should be done to correct the 11-day error.

Pope Gregory XIII who was elected pope in 1572, decided to take on the problem and sought help from mathematician Christopher Clavius and astronomer Aloysius Lilio to get the vernal equinox back to or near March 21. Their research and observations determined the tropical year to be 365.2422 days, which was closer to the actual tropical year than the Julian calendar.

The two scholars continued the Julian leap-year system adding an extra day every four years to the month of February to ensure that the Earth is in the same point of its solar orbit at the same time every year. When the calendar went into use in October 1582, 11 days were dropped to keep the 365-day count correct. Thursday October 4 was followed by Friday October 15. This upset a number of people, and since the Gregorian calendar was considered to be a Roman Catholic calendar, it was several years before some Protestant and Muslim countries adopted it. In fact, it was 1752 before Great Britain adopted the calendar, and Sweden adopted it in 1753. The latest was Saudi Arabia which continued using its Islamic calendar until 2016. Some Orthodox Christian Churches, which do not consider themselves affiliated with the Roman Catholic Church, still use the Julian calendar.

The Gregorian calendar kept the use of the BC, Before Christ, and AD, Anno Domini, system set up in 525AD by Dionysius Exiguus, a Sythian monk. He used bible scriptures and his knowledge of cosmology to determine the date of Jesus’ birth and promoted the idea that this date should be the dividing point for the calendar. The system was adopted and is still in use today. But starting in the 18th century the letters CE and BCE for ‘common era’ and ‘before common era’ began to be used to denote the same years but making the dates more religiously neutral.

The result of all of those years of observing and calculating orbit times by different cultures at different times has produced the calendar that is currently in use as the global standard for trade and politics. But it is still not perfect. For example, in 2023 the vernal equinox falls on March 20 at 3:25pm EDT, not quite on March 21. The 12-month Gregorian calendar requires months of different lengths and leap years. Would a 13- month calendar be any better?


For centuries there had been interest in establishing a thirteen-month calendar. Sumerians, Egyptians, and other cultures tried it for a while but it never proved to have as much long-term appeal as the twelve-month calendar. The most recent attempt in America was as recent as 1928 when George Eastman, inventor of the Kodak camera, brought a group of government officials together at the National Academy of Sciences in Washington to discuss the matter. In attendance with Eastman were such dignitaries as D.C. Marvin, head of the National Weather Bureau, Ethelbert Stewart, head of the Bureau of Labor, G.K. Burgess, director of the Bureau of Standards, and others. The calendar was to be called The International Fixed Calendar.

The calendar that George Eastman advocated had originally been worked out in the early 1900s by Moses Cotsworth, a North Eastern Railroad advisor. But Cotsworth lacked the money and government connections to get his idea going. Eastman, who had both the money and political connections, liked the calendar and put it to use in his company. In fact, The Eastman Company used the calendar until 1989. The calendar had 13 months of 28 days each making a total of 364 days. The 13th month called Sol was inserted between June and July and every month started on a Sunday. To get the correct 365 days of the solar year, an extra ‘year day’ was added at the end of year on December 29.

Many people in America and around the world liked the idea of the simplified calendar, but felt as if it would create more stress and upheaval than it was worth to replace the old calendar. When Eastman died in 1932, the calendar proposal slowly faded away.

Cotsworth’s and Eastman’s calendar was one of many attempts to bring order to the phenomena that effect our time. But most of these effects are beyond the control of humans, such as the fact that Earth’s rotation is gradually slowing down due to the pull of the moon and sun’s gravity, making even the length of the day hard to keep up with. For example, it has been determined that some 4 billion years ago, Earth was spinning faster than today and completing one revolution in around 5 hours. Gravitational friction between Earth and the moon has slowed it to the current 24 hours.

Also, to add to the confusion, selenologists — scientists who study the moon — have noticed that the moon rises a few minutes later each 24-hour period, sometimes only about 20 minutes later, sometimes over an hour – averaging about 50 minutes later each day. This happens because the Earth rotates slightly faster than the moon orbits it. This gives us 12.368 lunar months in a year instead of a nice even 12.


Do you think you are sitting still while you read this? You are not, and your movements affect how time is measured. The Earth spins on its axis at about 1,000 miles per hour giving us day and night. Also, we are orbiting the sun at roughly 66,700 miles per hour giving us seasons. But an even faster movement, and the one that affects how we see the stars, is that in one 24-hour day, Earth, along with our entire solar system, travels some 2.5 million kilometers or 1.5 million miles through space as it orbits around the center of the Milky Way galaxy. This galactic orbital movement puts Earth in a different position each day relative to the fixed stars, a phenomenon that causes Earthlings to see stars rise about 3 minutes and 56 seconds earlier every night. For example, if you see Sirius rise above the horizon at 9pm, the following night you will see it rise at about 8:56pm.

We humans have added to our confusion by creating two different ways to measure the hours of the day. 9pm in countries that use a 24-hour clock is 21:00 and 8:56pm is 20:56. The practice of dividing the day at midday when the sun is directly overhead is believed to have originated around 2500 years ago in Egypt. The Romans also used the system and gave it the Latin names ante meridiem, am, and post meridiem, pm, that are in use today in some countries, especially English-speaking countries. Much of the rest of the world, as well as most military units, uses the 24-hour clock. Midnight is 00:00, 1:15am is 01:15. 11:25am is 11:25, etc.  After midday, adding 12 hours to the time will give you 24-hour time. For example, 2pm plus 12 is 14:00 on the 24-hour clock.

Some countries have instituted Daylight Saving Time which moves clocks up one hour in the spring and back to standard time in autumn. The idea is to give citizens an extra hour of daylight in the evening during the warm months when there are more hours of sunlight. It was first proposed by New Zealander George Hudson in 1895 and in the next few years was adopted by several nations. The United States began using it in 1918. Although it has become popular with most people, farmers and others who go to work early in the morning, complain that because of the time change, they have to start work while it is still dark. The debate continues.

Keeping track of time is important to us, and we see that a number of factors must be taken into account to make sure our clocks and calendars are always correct. Today, a number of organizations are dedicated to keeping our clocks and calendars as accurate as possible. One of these, for example, is the International Earth Rotation and Reference Systems Service, which keeps track of variations in time caused by irregularities in Earth’s rotation. Another group called the International Organization for Standardization sets international standards for each nation to be able to communicate correct times and dates to other countries given the many language and cultural differences in the world.

Other phenomena, such as Earth’s elliptical rather than round orbit around the sun, contribute to the difficulty in developing calendars and clocks that would be perfectly accurate over long periods of time. The Gregorian calendar with its 365.2425-day year is still a bit off the actual tropical year of 365.242199. And clocks based on the current 24-hour day may not be exactly accurate years into the future as Earth’s rotation time varies. But these methods of keeping track of time are the most accurate we have come up with to date. No wonder that after thousands of years, we are still working on ways to make our calendars and clocks accurate. So, in light of of all of this complexity and imperfection, are there more accurate calendars and clocks to be developed in our future? As we adjust our atomic clocks and measure lunar and solar orbit times to ever greater precision, the answer is probably yes.

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