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History of Leap Years

2024 is going to be a leap year, that occasional phenomena where an extra day is added to the month of February. This represents an attempt to ensure that the calendar year remains aligned with the 'astronomical year' which relates to the time it takes for the earth to do an entire orbit around the sun. Given that the length of this orbit is approximately 365.242 days, an extra day must occasionally be added - typically every four years. Today's blog delves into how this extra day came to be, the result of human ingenuity in a quest to synchronise our calendars with the celestial dance of the Earth around the sun.

When did the Concept of Leap Years Originate?

Roman bust of Julius Caesar

Statue of Julius Caesar by Andrea di Pietro di Marco Ferrucci, c. 1512–14 in the Metropolitan Museum of Art

Many different ancient cultures and civilisations have implemented their own calendar systems throughout history. However, one of the most famous early calendars incorporating the concept of the 'leap year' was the 'Julian Calendar'. This system was introduced by Julius Caesar, the famous Roman dictator who ruled immediately prior to the nomination of Augustus as the first emperor of Rome. Caesar consulted with the astronomer Sosigenes to establish this new calendar which was actually closely based on the ancient Egyptian calendar, most likely owing to the influence of his association with Cleopatra. He implemented his calendar in the year 45 BCE, in what is often referred to as the 'Julian Calendar Reforms'.

He famously established the fixed year length corresponding with the solar year to be 365.25 days. Therefore, by his calendar, there became 365 days in a common year and 366 days in a leap year. Every fourth year became a leap year, with an extra day added in the month of February. The structure and naming of the months, however, were largely retained from the earlier Roman calendar. Why Julius Caesar chose the fixed month of February for the additional leap year day is uncertain. However, several historians have suggested possible reasons for this occurrence. Unlike our calendar system today, the earlier Roman calendar began in March and ended in February. Therefore, the end of the calendar year may have appeared as a logical place for the additional calendar day.

Why was the Julian Calendar Adopted?

Fasti Antiates Maiores - One of the earliest known calendars

Fasti Antiates Maiores - One of the earliest known calendars

The Julian calendar was adopted for more reasons than simply astronomy. Unusually, politics played a large role. The calendar that came before it is known as the 'Roman calendar'. It was originally a lunar-based system whereby the months aligned with the different phases of the moon. This calendar consisted of ten months totalling 304 days and, as such, failed to account for the entirety of the earth's orbit around the sun. Although the Julian Calendar was primarily introduced to correct for the irregularities between the Roman calendar and the solar year, it also strove to protect the calendar system from political abuse surrounding the concept of the 'intercalary month'. This represented an additional 11th month introduced by the Romans which was named Mercedonius or Intercalaris, and represented early attempts to correct the misalignment of the Roman calendar from the solar year.

However, this extra month was inserted irregularly between the existing calendar months by the pontifices who oversaw the Roman calendar - unlike in the Julian Calendar which specified that the additional month must be placed in February. Consequently, it allowed high-ranking individuals to extend favourable conditions or shorten others, alongside manipulating the timings of Roman elections and legal proceedings. Therefore, the calendar system became extremely irregular due to the use of the month by Roman politicians to:

  • Time local elections in their favour

  • Extend a high-ranking individual's year in office

  • Delay a rival's assumption of office

  • Postpone court proceedings to extend preparation time

  • Timing militaristic actions away from 'problematic' days due to superstitions and / or religious beliefs

  • Allow government manipulation of agricultural planting and harvesting schedules

  • To extend or reduce the tax year

What Calendar is Used Today?

Although the Julian calendar vastly improved the alignment of the solar year with the calendar year, it still contained slight overestimations of the length of the solar year. This resulted in the misalignment between the calendar and the equinoxes over time. Mathematicians and astronomers played key roles in identifying the errors of the reformed Julian Calendar. One of the key figures in this discovery was the German mathematician and astronomer Johann Müller - or, as he was better known, Regiomontanus. He made detailed observations of the equinoxes during the 15th century which highlighted the misalignment of the calendar months.

This theory was better proven during the Renaissance due to the increased accuracy of observational astronomy during this period. As such, by the 16th century, famous astronomers like Christopher Clavius and Aloysius Lilius had collected sufficient data to prove the need for a second calendar reform. In response, Pope Gregory XIII collated together a selection of trusted mathematicians and astronomers to advise this reform during the 16th century. The new system was announced in the year 1582 and became known as the Gregorian calendar.

Under this new calendar, a year divisible by 4 became a leap year, except for years divisible by 100 but not by 400. The switch to the Gregorian calendar also involved adjusting the current calendar date at the time of the reform to correct the accumulated discrepancy. It was decreed that ten days should be skipped to allow the vernal equinox to occur around March 21 which was a more accurate reflection of its astronomical timing. As such, the day following Thursday, October 4, 1582, was designated as Friday, October 15, 1582. Various Catholic countries quickly adopted the new reform, with Protestant and Eastern Orthodox countries following second. The delay in the adoption of the new system elsewhere was largely attributed to religious differences - with different groups becoming reluctant to adopt systems directly associated with the Catholic Church.


As 2024 is now in full swing, we hope you'll embrace the extra day it offers, not just as a historic adjustment to our calendars, but as a symbol of the endless possibilities that time affords us. It is, after all, our most precious resource.

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