The extraordinary history of the leap year – the reason Christmas stays in the winter

First introduced by Julius Caesar, leap years remain crucial to how we operate

Astronomers introduced the leap year after recognising how long it takes the Earth to orbit the Sun in a year. PA

March 1 is 24 hours late this year, meaning February has a 29th day. Known as a leap year, this happens every four years.

This is done for administrative reasons. While the calendar year is 365 days, the Earth actually takes 365 days and a quarter to complete a full orbit of the Sun.

Here, The National takes a look at the history of the leap year and why it is still so important today.

Why we have a leap year

Without a leap year, the world would pretty soon descend into administrative chaos.

The Earth takes 365 days, 5 hours, 48 minutes and 46 seconds to complete a full orbit of the Sun. So, without the extra day every four years, the calendar would be increasingly out of synch with the real world.

Over a century, this would add up to 24 days. It would mean Christmas, for example, would eventually be celebrated in what we know as the middle of July.

It would seriously disrupt everything from holiday planning to computers, which continue to use a 365-day calendar unless prompted to add the extra day.

History of the leap year

The Romans were the first to address the problem, with Julius Caesar introducing the Julian calendar in 45 CE, adding an extra day to the month of Februarius, named after the God of purification.

More than 500 years later, this was replaced by the Gregorian calendar, introduced by Pope Gregory XIII in 1582.

Renaissance mathematicians realised there was a problem with the Julian calendar, which added the extra day based on the assumption that the Earth's real rotation around the Sun was exactly five hours longer each year.

By not taking account of the extra 48 minutes and 46 seconds, it meant the calendar was still slipping, less than before, but creating a problem when calculating Easter, the holiest day in the Christian calendar.

The Gregorian calendar solves this by adding February 29 every four years, but only when that year that is divisible by 100 but also by 400. So the year 2000 was a leap year but 1900 was not and neither will 2100. Losing that extra day more or less keeps the calendar in line.

At first, the Gregorian calendar was not for everyone, notably in Protestant countries.

Britain and her colonies, which included America, adopted it only in 1750, by which time the calendar was seriously out of alignment.

To bring it into line, 11 days were removed from September that year, causing reports of unrest among British citizens who believed their lives were being shortened by the change.

Problems with the leap year

The issue has not been completely solved by the Gregorian calendar, though.

Minute differences between the solar year and the calendar year can still be detected, meaning that every now and again a leap second is introduced, the last being on December 31, 2016.

But even this causes problems with everything from satellite communication to telecoms and computer software, which require constant exact time, because the Earth’s rotation is not at a constant speed.

The last leap second will be added some time before 2035, after which the time difference will be allowed to accumulate until a time and date still to be determined, when perhaps a full minute will be added.

The Gregorian calendar also means days of the week move forward by 24 hours each year, or 48 in a leap year. Christmas Day 2023 fell on a Sunday, then moved to Monday last year but will be on a Wednesday in 2025. That is why we need a new calendar each year.

Alternatives

Other calendars are available, of course.

The Islamic or Hijri calendar, which begins with the Islamic New Year, has 12 months divided by lunar cycles of 28 days, meaning a year is measured as 354.37 days.

This means religious festivals such as Ramadan and Eid move back by about 11 days each year.

Ramadan in 2024 is expected to begin at sunset on March 11 but 10 years ago it started on June 28. The difference between the solar year and the lunar year also means there will be two Ramadans in 2030.

While the lunar calendar is used for religious occasions in the Islamic world, to make it easier for global business, Muslim countries also use the Gregorian calendar for everyday activities.

The UAE also made Friday a working day in 2022.

Leap year trivia

For those born on February 29, a leap year birthday creates the amusing fiction that they are younger than they really are.

It is estimated to affect about five million people, meaning someone born in 1960 celebrates their 16th actual birthday in 2024.

Some businesses adjust their profits by adding an extra week to their fiscal year as opposed to calendar quarters, in other words by dividing the year by four.

The practice can have benefits and downfalls. When Apple did this in 2012, it was able to report stronger profits, the result of an extra week. The following year, without the extra week, first-quarter profits naturally fell, causing the Apple share price to drop.