# Leap year babies celebrate birthdays at last

Today is a ‘leap day’ – an extra day inserted into the calendar in an effort to keep the seasons from slipping out of sync. Even today, keeping dates in line with nature takes constant adjustment.

Today, James Clarke is celebrating his 11th birthday. Why is he in the news? Because James was born in 1968, is a successful car salesman, and already has an 11-year-old son.

The secret, of course, is that Clarke was born on a leap year, on the 29th of February – a month which, usually, is only 28 days long. Only 11 Februaries have had a ‘29th’ in the last 44 years.

A complicated rule dictates when February gets its extra 24 hours. To get a leap day, the number of the year must be divisible by four, but not by one hundred – unless (like the year 2000) it is divisible by four hundred, in which case the extra day is put in again.

Why such mathematical complications? The adding of leap days goes back to 45BC, when Julius Caesar introduced the reformed Julian Calendar to Ancient Rome in order to keep the calendar months in sync with the passing seasons.

It takes approximately 365.242 days for the Earth to complete a full circuit around the Sun. Astronomers call this a ‘tropical year’ – the precise length of time between the arrival of Spring on one year and the next.

For human purposes, however, there would be little use in a calendar that ended each year just under a quarter of the way through its final day – not least because the timing of midnight, for example, would soon get badly out of sync with the rising and setting of the actual Sun.

Instead, Caesar arranged that the year should be 365 days long, with an extra day every four years making an overall average year length of 365.25 days. This, he hoped, would be close enough.

But, over the centuries, even the tiny difference between 365.25 and 365.242 days began to add up. By the 1500s, Spring was arriving ten full days earlier each year, causing havoc for priests who used the seasons to calculate the dates of Christian festivals. In 1582, Pope Gregory XIII reformed the calendar again, cutting days off the year to push Spring back and setting up the complicated leap year formula that still applies today.

Even today, the tinkering is not over. With the development of extremely precise atomic clocks in the 1960s, it was discovered that the length of a day can vary randomly by as much as four milliseconds – and is slowly growing. Now, ‘leap seconds’ have to be added around every 18 months to compensate for the drift.

Counting seconds

There are now many scientists who think all this constant adjustment is not worth the effort. Humans, they say, have evolved past the point where we need to tie our measurement of time to the imprecise, variable, inconvenient motion of our planet around its star.

More romantic types disagree. The constant need to adjust time, they say, just shows up the inadequacy of human attempts to describe and codify a universe that is really beyond description.

## You Decide

1. What would the world be like if nobody ever knew the date or the time?
2. Can humans ever really hope to understand and describe the universe?

## Activities

1. Civilisations have been building, writing and drawing elaborate calendars for thousands of years. Research some ancient examples and then design your own.
2. Maths challenge: taking leap years into account, work out what day of the week it was on today’s date exactly one century ago. An internet search should reveal the answer if you get stuck (and see Become an Expert).

## Some People Say...

“We should forget about times and dates; then no one would have to hurry.”

What do you think?

## Q & A

All this worry over a few dates?
Knowing the exact date and time is vitally important for things like communications technology or GPS navigation. But there are more personal reasons for taking leap years seriously.
Oh really?
Yes. By longstanding tradition, the 29th of February is the only date on which women are supposed to propose marriage to men, rather than the other way around. And, if the man says no, he has to give the rejected woman a silk gown and a kiss – or so the story goes.

## Word Watch

Ancient Rome
Before the Julian reforms, the Roman calendar had a short year, but often added an entire ‘leap month’. This system was so confusing that many Romans never knew what day it was at all. Julius Caesar’s calendar was a huge improvement, and the month of July still bears his name today.
Arrival of Spring
Spring officially begins at the ‘vernal equinox’: the moment when the Sun circles directly around the equator and night and day are exactly the same length.
Cutting days off the year
Different countries adopted Pope Gregory’s reforms at different times, which meant that for years different parts of Europe had different dates. It was possible, for a while, to sail from one side of the English Channel to the other and arrive six days earlier than you set off.
Atomic clocks
Atomic clocks count the passage of time by measuring the activity of atoms of the element caesium. They are accurate to within a billionth of a second.