Why do we have leap years?

Leap Years


Leap years are calendar years that have one additional day, that is, they have 366 days. This particularity in our calendar is due to the fact that the length of calendar years – the ones that govern our daily life, formed by a integer number of days – is defined by the time it takes the Earth to complete its orbit around the Sun. In this way, a year is completed when the Earth has finished its orbit around the Sun, which takes place every 365 days, 6 hours and 9 minutes. This period of time called sidereal year implies that for every 365 days we have left 6 hours and 9 minutes that we have not included in that year’s calendar, so every four years we have lost a full day (4 years x 6 hours = 24 hours).

To compensate for this discrepancy, Emperor Julius Caesar commissioned the philosopher and astronomer Sosigenes of Alexandria to reform the calendar around 46 BCE. The new Julian calendar required an additional day every four years, specifically 24 February, since at that time February was the last month of the year. Pope Gregory XIII decided to replace the calendar designed by Sosigenes in 1582 by means of a papal bull. One of the modifications of the Gregorian calendar was that, instead of 24 February, the additional day would be 29 February, as it is now. In addition, a system of exceptions in leap years was established to ensure that the discrepancy would not occur again. The system postulated that years that were multiple of 100 would not be considered leap years, unless they were also multiple of 400. For this reason, neither 1800 nor 1900 were leap years, but 2000 was.

Leap years have a clear rationale: if we did not have them, the seasons would fall out of sync. The starting point of the seasons depends on when the Earth passes through certain points in its orbit around the Sun. If we determine that 21 September is the first day of autumn because the Earth passes through a particular point in its orbit and our calendar is out of date, then autumn would begin on a different day every year. Over the years, the northern hemisphere’s Christmas would fall in the middle of summer and vice versa in the southern hemisphere. Astronomers would have to be alert every year to the moment when the Earth passed those specific points in its orbit that determine the seasons, and this would make it very difficult to set fixed hours to serve as a reference for society. Tasks such as compiling the dates of historical or everyday events, such as a birth date, would be quite difficult, and the results would be extremely unreliable.

Amelia Ortiz-Gil has a PhD in astrophysics and is a member of the news service of the Astronomical Observatory of the University of Valencia.

Question submitted by Bruna Devi.

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