Seize the day: The magic and mystery of leap years

It's the Roman Empire's enduring gift to the world – an extra day every four years. A source of mystery, intrigue and controversy, Christopher Hirst explores the most colourful date in the calendar

For most of us, the arrival of 29 February means one extra day at work, another day on the mortgage and 24 hours' delay in the arrival of the pay cheque. No wonder a leap year is universally regarded as unlucky. This is particularly the case for those unfortunates who can only celebrate their real birthday once every four years. For some reason, musicians tend to be born on 29 February – they include Rossini, the late avant-garde trombonist Paul Rutherford and the rap artist Jah Rule – but the most eminent UK leap-year baby is Joss Ackland, who will be 20 next Friday, though he has been on this Earth for 80 years.

Only in America is any attempt made to redress this gross injustice. Tomorrow, leap-year babies will be "honoured guests" at the Sixth Worldwide Leap Year Festival at Anthony, New Mexico. Celebrations are to include a chuckwagon breakfast, hot-air balloon rides and a huge birthday cake ("These people have been waiting for four years!"). At the Fourth Worldwide Leap Year Festival in 2000, musical entertainment was provided by Graham Nash, whose wife Susan is a leap-year baby, but he has not reappeared. Maybe once every four years is a little too frequent to hear Nash's maudlin hit "Teach Your Children".

But why do we need this calendrical hiccup every four years? What, exactly, is the point of it? "The leap year is basically down to humans trying to make sense of natural rhythms," explains David Rooney, curator of time keeping at the Royal Observatory. "If you're trying to run a calendar by the natural cycles of the Sun and the Moon, it doesn't work and you have to intervene. The technical term for this intervention is 'fudge factor'. The leap year is a fudge."

The ancient Egyptians recognised that the world does not revolve round the Sun every 365 days, but almost a quarter of the day more. The fact that this little bit extra is a smidgeon less than six hours – the Gregorian calendar year is 365 days, five hours, 49 minutes and 12 seconds – has, as we shall see, led to much tinkering over the millennia. Moreover, the natural year is changing in length due to predictable factors, such as changing orbit and gravitational drag, and the unpredictable effect of the Earth's liquid core hitting subterranean mountain ranges and ravines.

"When the National Physical Laboratory at Teddington invented the atomic clock in the Fifties, we discovered that timekeeping based on vibrating atoms was more accurate than the Earth," adds Rooney. "It was slightly embarrassing. When clocks diverge, it isn't good. By the Seventies, we needed another fudge factor. So, the leap second was introduced to push together Earth rotation time and atomic vibration time."

Intended to correct erratic internal friction, the leap second is not added every year. The decision to add or subtract a second (so far, it has always been added) is made by the International Earth Rotation Service in Paris. The last leap second occurred on 1 January 2006 – it caused a bit of a stir, and an extra pip was added to the BBC time signal. The one before that was in 1998.

Recent developments at the National Physical Laboratory mean that human time will become yet more implacably accurate compared with the slightly wonky rotation of the Earth around the Sun. "At Teddington, they're working on an even more accurate optical clock known as the ion-trap clock," says Rooney. "Lasers trap a single charged atom in a force-field and can hold it there for a period of time, months if necessary. The atom is then given a poke by another laser. It gets excited. Who wouldn't? As its excitement diminishes, it gives out a little shudder of energy with a very precise frequency."

"Frequency is just time in a very, very accurate form. The most pessimistic forecast suggests that if that clock were set running now and was still running at the predicted end of the universe, it would possibly be wrong by half a second. At present, the ion-trap clock only runs during the day. They switch it off at night to save energy."

Inevitably, the ordering of time is subject to political manipulation. A couple of years ago, the US suggested that world time should be switched entirely to the atomic clock, which would involve the dropping of leap seconds.

"If accepted, it would be the first time that time was not dependent on the rising and falling of the Sun," said Rooney. "Britain put up a fight against this idea. And, as I understand it, the jury is still out on this."

It isn't the first time that leap years have been at the centre of controversy. Adjustments of the leap year have consistently been entwined with politics. The first attempt to get it right, which laid the foundations for the modern calendar, had actually been by the Egyptian ruler Ptolemy II in 238BC. Since the year had been calculated, with near-as-dammit accuracy, as 365 days, he introduced the familiar cycle of three standard (or common) years with the fourth being a leap year.

The reform was soon abandoned. But two centuries later it provided a template for Julius Caesar, who wanted to sweep away Rome's hopelessly wonky calendar as a symbol of his new-broom administration. Advised by an Egyptian astronomer with the slightly unfortunate name of Sosigenes, he added an extra day to the shortest month of February once every four years.

In a radical move, Caesar added an extra 90 days to 46BC to realign the erratic administrative year with the agricultural seasons. The result was an epic 445-day year, named by Caesar as the "last year of confusion" though, inevitably, everyone else called it "the Year of Confusion". Contracts and shipping schedules were wildly disrupted, but at last people had a reliable guide for planting crops, organising business, planning holidays and booking holidays. The modern world was born.

In fact, Caesar would have been better off getting rid of the Ides (15th) of March. It was on this date in 44BC that he was stabbed 28 times while in the Senate. Among the less dramatic consequences was a misinterpretation of the Julian reforms, and Caesar's good intentions were interred with his bones. After the first leap year in 42BC, bureaucratic bungling led to an extra day being added once every three years, rather than the intended four years.

As a result, the Roman calendar again raced ahead of the seasons for 36 years. In 8BC, Augustus Caesar, Julius's reforming successor, rectified the error by skipping three leap years. Reinstituted in 8AD, the cycle of leap years continued unbroken until the 16th century. Along with this quadrennial adjustment, Julius and Augustus left another indelible mark on our calendar. July and August were named in their honour.

Over the centuries, the slight discrepancy between the Julian year and the actual time that the Earth takes to revolve around the Sun – 10 minutes, 48 seconds per year – began to add up. By the 16th century, the calendar was around 10 days slow. In 1514, Pope Leo X sent a letter to Henry VIII pointing out that "Jews and heretics" were laughing at the errors in the Christian calendar, but this was failed to elicit a response. Three follow-up reminders were also ignored. Since other rulers displayed the same indifference, the matter was dropped for another 60 years, until Gregory XIII assumed the papacy in 1572.

Like Julius Caesar, Pope Gregory XIII was determined to reassert the authority of Rome. Among the reforms of this dour zealot was the issuing of a new breviary, the book containing a service for every day of the year. If these requests for celestial intervention were to take place at the appropriate time, an updated calendar was required. This was drawn up by experts and enacted in a papal bull issued on 24 February 1582, which corrected the error of the Julian calendar by omitting leap years at the start of centuries unless divisible by 400.

As a result, 1900 was not a leap year but 2000 was, and anyone reading this who survives for another 92 years will be spared 29 February in the year 2100. In order to reset the calendar, Gregory ordered the excision of 10 days from October 1582. This was accepted by Roman Catholic countries (though sometimes grudgingly) and even by a few Protestant states. Peasants were annoyed that their weather proverbs no longer made sense and at the shifting of traditionally boozy holidays.

Being ardently Protestant by the 1580s, England regarded the Pope's amendments with suspicion, though Queen Elizabeth was by no means antagonistic. She asked the scientist, astrologer and mystic John Dee to look into the matter. He recognised that Rome's calendar reforms had merit and proposed dropping 10 days from 1583 in gentle stages over four months. Dee sent this proposal to Lord Burghley, head of the Queen's calendar-commission, accompanied by a poem that included the persuasive couplet: "the tyme untrew... Command anew."

There was, however, one hurdle to be overcome. The revisions had to be accepted by the Church of England. The archbishop of Canterbury William Grindal was as independent-minded as the current incumbent, though by no means as liberal. Dead set against Rome-ish time, Grindal reminded the Queen that she had been excommunicated by a papal bull in 1570.

In fact, it was the Spanish Armada of 1588, which was supported by Gregory's successor, that put paid to England accepting the Gregorian calendar for another 170 years. In familiar style, England refused to join in with the rest of western Europe. All other Protestant states, with the exception of Sweden, accepted calendar reform in the course of the 17th century. Voltaire scoffed: "The English mob preferred their calendar to disagree with the Sun than to agree with the Pope."

Eventually, the cause of calendar reform was successfully pursued by the Earl of Chesterfield, the same one whose inadequate patronage aroused the ire of Dr Johnson. In consequence, the dates of 3 to 13 September 1752 were omitted throughout the British Empire.

The extent of objections has been exaggerated. Some say the "calendar riots" were actually unpleasant anti-Jewish demonstrations, though there seems to have been a ruckus about the missing days in Bristol. In the following year, bankers in London refused to pay their tax on the customary date of 25 March and postponed payment by 11 days, which is why our tax year starts on 5 April. The loss of 11 days was, however, happily accepted in the go-ahead American colonies. "What indulgence is here," wrote Benjamin Franklin in Philadelphia, "for those who love their pillow to lie down on the second of this month and not perhaps wake until the morning of the 14th."

Pope Gregory's reforms have continued to hold sway, but there have been attempts to institute new systems. The most radical was after the French Revolution when – in a move that would have today's Eurosceptics seething – Gallic scientists divided the year into 12 newly named months consisting of three 10-day weeks (the name of one of the summer months survives in lobster thermidor) and the day was transformed into 10 hours, each consisting of 100 minutes, each in turn consisting of 100 seconds.

"Metric time only lasted a year because it was really silly," said Rooney. "But that was not the end of the story. As a quid pro quo for accepting Greenwich Mean Time in 1884, the French insisted that decimal time should be considered for international time-keeping. It came to nothing, though the idea still crops up. The irony is that the French are now running time. Coordinated Universal Time or UTC, chosen as an acronym because it means nothing in any language, is run from Paris."

Another attempt at a new calendar came with communism. Russian revolutionaries suggested the Soviet New Style Calendar based on 12 months of 30 days. It, again, was short lived. Man's attempts to make the heavens work to the precision of a human clock are almost always doomed. So what is to be done? The Oxford Companion to the Year points out that in 10,000 years the calendar will be three days 17 minutes and 33 seconds slow. Seeking a solution, the book proposes dropping the leap year in 3200 and, after the leap year in 4000, substituting 500 for 400 in the Gregorian division rule for centennial leap years, so they would occur in "4500, 5000, 5500, 6000 etc, until further adjustment should be needed". One possible alternative considered by the Companion is the adoption of the Iranian solar cycle, introduced in 1925. Its complex sequence of leap years results in an inaccuracy of just 1 minute 41.952 seconds every 2,830 years.

Despite the accuracy of modern clocks, however, Rooney insists time should always been based on nature. "We need to reconnect with the Sun and stars. If we start getting out of step with natural cycles, we risk getting into trouble. We need to remember that we're animals." Which is something to think about, next time you have to alter your watch.

On 29 February...1504

Christopher Columbus, stranded in Jamaica, needs to secure provisions for his crew from hostile islanders. He attempts to frighten them into submission by correctly predicting a lunar eclipse on 29 February. But, while shocked by Columbus's apparently miraculous powers, the natives fail to submit. Help arrives four months later, allowing Columbus and his men to return to Spain.

On 29 February...1940

Hattie McDaniel becomes the first black actor to win an Oscar for her supporting role as Mammy in Gone with the Wind. Making a memorable acceptance speech, she said: "This is one of the happiest moments of my life... I sincerely hope I shall always be a credit to my race and to the motion picture industry." McDaniel starred in dozens of films, appearing alongside Marlene Dietrich and Mae West, and was also the first black woman to sing on the radio.

On 29 February...1960

Hugh Hefner opens the first Playboy Club. Featuring scantily clad Bunny Girls, the Chicago night-spot quickly becomes a hit, spawning more than 35 clubs, including joints in London, Manchester and, er, Portsmouth. Membership becomes a status symbol – less than 20 per cent of members ever visited a club – and profits soar... for a while. Today, the Palms Hotel in Las Vegas is home to the last remaining Playboy Club.

On 29 February...1960

A devastating earthquake, measuring 7.6 on the Richter scale, kills 12,000 people in Morocco. Striking shortly before midnight, the 10-second quake causes a tidal wave and fires that ravage the city of Agadir. On the day, it is feared the death toll could top 20,000 and offers of assistance flood in from neighbouring states. Modern-day Agadir was rebuilt south of the old town and, boasting a long sandy beach, is now a tourist destination.

On 29 February...1792

The Italian composer Gioacchino Antonio Rossini is born in the town of Pesaro to a trumpeter and a singer. A precocious talent, he quickly immerses himself in opera and performs, as a boy, both in the pit and on stage. He goes on to pen more than 30 operas, most notably The Barber of Seville. The overture to one of his last operas, William Tell, based on Friedrich Schiller's play, Wilhelm Tell, becomes one of the most recognisable tunes in classical music.

On 29 February...1984

Canadian Prime Minister Pierre Trudeau announces his retirement, telling the House of Commons "today is the first day of the rest of my life". The notice confirms speculation that had been stoked by the appearance of a new swimming pool at Trudeau's home; he would now have more time to indulge his love of water. One of the most charismatic leaders of modern times, Trudeau's state funeral in 2000 was attended by President Jimmy Carter and Fidel Castro.

On 29 February...1996

The British actress, author, trouper and sex symbol Joan Collins, who played the ex-wife from hell, Alexis Carrington, in Dynasty, is awarded $1.3m from Random House, for breach of contract. The publisher had insisted Collins return the advance on a novel it claimed she never delivered. The jury agreed with Collins's defence that her contract required payment for unpublished manuscripts.

On 29 February...1964

The Queen's cousin Princess Alexandra gives birth to the only leap-year royal, James Ogilvy. Born to the business and charity man Angus Ogilvy, who married Princess Alexandra the year before, James Ogilvy has no title and is 35th in line to the throne. He is married to Julia Ogilvy, a former director at Lloyds TSB, with whom he has two daughters.

Ogilvy is 43, but tomorrow will celebrate only his 11th birthday.

On 29 February...1996

A Faucett Airlines Boeing 737, on a scheduled domestic night flight from Lima to Arequipa, Peru, crashes five miles short of the runway, killing all 117 passengers and six crew members. The crash, the worst in Peru's aviation history, causes the airliner to break into at least three sections, scattering burning bodies and debris across the rock-strewn mountain site. Founded in 1928, Faucett was one of the oldest airlines in Latin America. It went bust in 1999.

On 29 February...2004

Jean-Bertrand Aristide quits as President of Haiti and flees the Caribbean state following a rebel uprising. The former priest, who enjoyed three periods of rule between 1991 and 2004, later insists he had been ousted in a US-planned coup, a claim rubbished by then US Secretary of State, Colin Powell. In 2006, thousands demonstrated for the return of Aristide, but he remains in exile.