We have just entered a leap year, meaning we will get an extra day this year. However, why are these extended years necessary, and how did they come?
What is a leap year?
Leap years have 366 days on the calendar instead of the standard 365. According to the Gregorian calendar, the one most people use worldwide, it takes place every fourth year. In leap years, there is one additional day, February 29, referred to as a leap day. All divisible by four years, like 2020 and 2024, are leap years, except a few centenary years ending in 00, like 1900.
The term "leap" refers to the fact that starting in March, every date in a leap year is one day ahead of the previous year. March 1, 2023, for instance, fell on a Wednesday, but in 2024, it will fall on a Friday. Usually, the same date advances by just one day across two successive years.
Leap years are also observed in other calendars, such as the Hebrew, Islamic, Chinese, and Ethiopian calendars; however, they only sometimes occur every four years and frequently fall in years other than those seen in the Gregorian calendar. Additionally, some calendars feature several leap days or even shorter leap months.
The Gregorian calendar contains leap seconds in addition to leap days and years. The most recent additions of leap seconds to specific years were in 2012, 2015, and 2016. But starting in 2035, leap seconds will no longer be used by the International Bureau of Weights and Measures (IBWM), which maintains timekeeping worldwide.
Why do we need leap years?
All of this "leaping" may appear foolish at first glance. However, leap years are crucial; without them, our years would appear radically different.
In addition to rotating on its axis, the Earth undergoes revolution—a counterclockwise movement in its orbit around the sun. One revolution of the Earth takes 365 and a quarter days, or one solar year. While a solar year is approximately 365.24 days long—365 days, five hours, 48 minutes, and 56 seconds—we assume it to be 365 days and refer to it as a year or calendar year.
The time difference between the beginning of a solar year and a calendar year would increase by 5 hours, 48 minutes, and 56 seconds for every year that went by if we did not consider this discrepancy. This roughly six-hour annual shortfall for four years equates to almost a full day. This would cause the seasons' timing to change throughout time. According to the National Air and Space Museum, for instance, the summer season in the Northern Hemisphere would start in December rather than June in about 700 years if leap years were not considered.
This issue is mainly resolved by adding leap days every fourth year, as an extra day equals the discrepancy that accrues over this period.
The system is not flawless, though: every four years, or every 129 years, we gain about 44 extra minutes. To solve this, we ignore leap years every hundred years—that is, years like 1600 and 2000—that are not divisible by 400. There is still a very slight discrepancy between solar and calendar years, which is why the IBWM has tested leap seconds.
But overall, leap years mean that the Gregorian calendar stays in sync with the Earth's journey around the sun.
The history of leap years
The idea of leap years dates back to 45 B.C.E. The Ancient Roman ruler Julius Caesar prepared the Julian calendar, which is composed of 365 days divided into the 12 months currently used in the Gregorian calendar. Quintilis and Sextilis were the original names of July and August; their names were eventually changed in honor of Julius Caesar and Augustus, his successor. Leap years are commonly attributed to Sosigenes, Caesar's math and astronomy advisor. Given that the Romans traditionally observed March 1 as the start of the New Year, the end of February seemed natural to add an extra day.
The "final year of confusion" in 46 B.C., which consisted of 15 months totaling 445 days, you were allowed the Julian calendar to incorporate leap years every four years without fail and to be in line with Earth's seasons, according to the University of Houston.
The Julian calendar worked perfectly for centuries. However, significant holidays like Easter did not match specific dates like the vernal, spring, or equinox. By the mid-1600s, astronomers had realized that the seasons were commencing around ten days earlier than predicted.
Pope Gregory XIII corrected this anomaly by introducing the Gregorian calendar in 1582. It is identical to the Julian calendar but does not include leap years for most centennial years to reflect a more accurate understanding of the Earth's orbit.
For many years, Catholic nations like Italy and Spain only utilized the Gregorian calendar. However, in 1752, Protestant nations like Great Britain adopted it when their years started to diverge significantly from those of Catholic countries.
Because of the discrepancy, countries that eventually adopted the Gregorian calendar had to omit days to synchronize with the rest of the world. For instance, when Britain swapped calendars in 1752, September 2 was followed by September 14, according to the Royal Museums Greenwich.
Pope Gregory XIII's lone measure was insufficient to address the issue. Thus, to accommodate the extra day, it was decided to eliminate a few leap years, or roughly one leap year every century. The years that ended in 00 were the ones selected for this.
But removing the leap year from all years that finish in 00 would cause the computation to become erroneous again. Ultimately, the 00 years divisible by 400 became leap years in the Gregorian calendar. Consequently, 2000 was a leap year, but 1900 was not.
