Why Do We Have Leap Year?
Even though the standard calendar year is 365 days, Earth actually takes 365 days 5 hours 48 minutes and 46 seconds to go completely around the sun. (This is called a solar year.) In order to keep the calendar cycle synchronized with the seasons, one extra day is (usually) added every four years as February 29.
The Julian calendar (established by Julius Caesar in 46 BCE) introduced the Egyptian solar calendar to the Roman world, standardized the 365-day year, and was the first attempt at using a leap year to match the calendar year to the solar year. However, this first version of a leap year was different than the one we know today: the Julian calendar didn’t have a February 29, rather February 23 was repeated every four years.
But this didn’t fix the problem. There were still 11 minutes and 14 seconds unaccounted for. This doesn’t sound like much, but the seasons had shifted 10 days by the 16th century. In 1582, Pope Gregory XIII created a slightly modified calendar to try to make the calendar match the solar year as closely as possible so the seasons wouldn’t shift again in the future. In order to accomplish this, the Pope’s new calendar had to have some oddly specific rules to account for all of the math going on.
Called the Gregorian calendar, this new calendar system said that no century year (like 1900) would be a leap year except for centuries divisible by 400 (like 2000). In order to put the seasons back to where they should be, the Pope eliminated October 5 through October 14, 1582.
The calendar moved directly from the fourth to the fifteenth that year to align the dates with the seasons again. It feels almost like science fiction to think that 10 full days were removed from the calendar in the year 1582.
But where does the phrase leap year come from?
In 365-day years, known as common years, fixed dates advance one day in the week per year. For example, Christmas fell on a Sunday in 2022 and on a Monday in 2023. With the insertion of a leap day, dates (following February) advance two days instead of one. In 2024, Christmas will leap over Tuesday and fell on a Wednesday.
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