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When a calendar finally settles down
23 December 2006
From issue 2583 of New Scientist magazine
Chris Turney

In 1582, Christmas was cancelled in what is now Belgium. The day of 21 December came and went as normal, but when people woke up the next morning it was 1 January 1583. It might sound like a clever way to avoid the travails of the festive season, but that wasn't the reason: the unfortunate Belgians (then called the Southern Netherlanders and ruled by Spain) had to sacrifice their Christmas in the name of religion and science.

The lost Christmas was just one of many calendrical upheavals imposed over the years as humans gradually developed the scheme we take for granted today. Inventing a dating system that tallied with nature and could be used by the masses proved to be a serious conundrum, tying politicians, religious leaders and astronomers in knots for millennia.

Before the third millennium BC, nobody really needed a calendar. But with the rise of civilisation came a pressing need for a way to keep track of the passage of time and plan for the future. So how to do it?

For some of the earliest civilisations, the phases of the moon seemed a good place to start. The Babylonians certainly felt so, and were using this system as far back as 3500 years ago. Each day began at evening, with the first sighting of the crescent of a new moon signalling the start of a new "month". This is a dependably regular 29.5 days and so is extremely tempting to use as the basis of a calendar. The first Babylonian calendar was accordingly made up of 12 lunar months of 29 and 30 days.

The Romans later adapted the Babylonian scheme into a 10-month lunar calendar. Legend has it this was devised by mythical warrior king Romulus in the year he founded the Kingdom of Rome, in what we call 753 BC (though Roman historians would have seen it as year zero in their dating system, called Ab Urbe Condita, "since the founding of the city"). Unfortunately for the Romans, whoever did devise the calendar did not have a great grasp of mathematics or astronomy; a 10-month lunar calendar is hopelessly out of sync with the passing of the seasons. Two more months were added in around 40 AUC, but the basic problem remained: a lunar calendar is no way to divide up a year. Despite its failings, though, we continue to live with many of the Roman names for months - December marks the 10th month of the old year.

By 46 BC, the Roman calendar was running 90 days behind the seasons. Julius Caesar consulted astronomers, including Sosigenes of Alexandria, about what to do. They advised him to ditch the lunar system and instead define a year according to the sun - the solar or tropical year - which can be measured by the time between successive vernal equinoxes (the point during the northern hemisphere spring when the sun is directly above the equator and day and night are the same length). Greek astronomer Callippus had determined as early as 330 BC that the solar year was around 365.25 days long. Today, we know it is roughly 365 days, 5 hours and 49 minutes - around 11 days longer than one made up of 12 lunar months.

In 46 BC, the "Year of Confusion", Caesar made the changes necessary to switch to a solar calendar. He added two temporary months and extended the length of the existing 12 to make that year 445 days long. The jubilant public believed their lives had been extended by 90 days. More importantly, when 45 BC arrived it was back in phase with the seasons.

With 365 days, this "Julian" scheme was about 6 hours short of a true solar year, so Caesar's solution was to add an extra day every four years, the leap year, to correct for the missing quarter-day. The result was a reasonable first stab at a decent calendar, and was largely adopted across the Roman Empire. However, it was still 11 minutes longer than a real year. Although nobody would notice the difference over the course of one lifetime - it took 131 years to gain an extra day - over the longer term it did make a difference.

Years of confusion

The gradual drift had serious implications for the Christian calendar, particularly over the vexed question of Easter. In AD 325 a meeting of Christian leaders at Nicaea, now Iznik in Turkey, had agreed that Easter would be the first Sunday after the first full moon following the vernal equinox (confusing most of us ever since), which then fell on 21 March.

By the mid 16th century, however, 21 March and the vernal equinox were estranged to the tune of 10 days, making it difficult to work out when to celebrate Easter. The Catholic church finally agreed that the offset between the calendar and real time needed to be addressed, and appointed Pope Gregory XIII to reform it. Gregory followed Caesar's lead and took advice from astronomers.

In 1582, he proposed that the vernal equinox should be restored to 21 March. To achieve this, 10 days would be removed from October of that year. And to make sure the calendar was self-correcting and the whole palaver never had to be repeated, the leap years were to continue as before except at the end of each century: only those divisible by 400 were to have an extra day. Thus 1600 had a 29 February, but 1700, 1800 and 1900 did not. The revised scheme only gains half a minute over a year and so can go 2880 years before a day has to be added. At last the system matched real time. The Gregorian calendar had arrived.

Unfortunately for Gregory, it was not a great time to establish a new calendar. The Protestant Reformation had recently swept across Europe, and so while most Catholic countries introduced the changes immediately, Protestant countries were more wary. In England, Queen Elizabeth I was enthusiastic but was stalled by Protestant clergy. Where the changes were made it was often with comical results. Belgium's cancelled Christmas was one of them.

Great Britain and its colonies eventually adopted the calendar in September 1752, but by this time 11 rather than 10 days had to be removed. Wednesday 2 September was followed by Thursday 14 September. Many people were enraged at the loss of these days; "time riots" supposedly broke out.

Non-Christian countries and faiths felt even less urgency to adopt the system. The Islamic religious calendar is still based on a lunar scheme and moves through the seasons: the New Year drifts from winter to summer over the course of 17 years. China only accepted the Gregorian scheme in 1949.

Today, we no longer officially define a year by the Earth's orbit around the sun. Instead, using atomic clocks, we describe it as precisely the time it takes for a vaporised atom of caesium-133 to oscillate 290,091,200,500,000,000 times. In some ways we've almost become too precise; the Earth's orbital period does not keep that level of precision. Perversely, to make up for natural variability, we now have to continually adjust our atomic calendar to keep track of the seasons. Leap seconds are added almost every year to correct for the offset.

While it's fun to see how people have responded to the developments in the calendar over the years, we're not immune to misunderstanding how it works. How many of us celebrated the start of the new millennium in 2000 when there was never a year zero? For die-hard party-goers there is good news. Ethiopia continues to use a calendar similar to the Julian system and has one year of the 20th century left, so there's still time to celebrate the new millennium. Last one to Addis Ababa buys the champagne!

Chris Turney is a geologist at the University of Wollongong, Australia, and the author of Bones, Rocks and Stars: The science of when things happened (Palgrave Macmillan, 2006)

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