Leap years exist to compensate for the difference between the calendar year (365 days) and the astronomical year (approximately 365.25 days). By adding an extra day to the month of February every four years, we keep the alignment of the seasons with the calendar.
One year, you think it's exactly 365 days, but in reality, the Earth takes exactly 365.2422 days to complete one full orbit around the Sun (this is called a solar year). This somewhat unclear figure causes a slight shift of about a quarter of a day each year. A quarter of a day is about 6 hours. At first glance, it doesn't seem like much, but after four years, it adds up to almost a full day of accumulated delay. Without adjustments, the seasons would eventually become completely misaligned with our calendar. To prevent summers from ending in February and Christmas falling in the middle of summer, this delay needs to be corrected regularly. This is the whole principle of leap years.
The Julian calendar was introduced by Julius Caesar in 46 BC because the Roman calendar of the time had become completely chaotic: the months were no longer aligned with the seasons, and no one could really keep track of it. With the help of a Greek astronomer named Sosigenes of Alexandria, Caesar decided to restore order. The year of the Julian calendar lasted precisely 365.25 days, or 365 days plus a quarter of a day per year. To compensate for that extra quarter of a day, an additional day was added every four years. It seems simple today, but at the time it was a big novelty and a huge step toward aligning dates with the seasons. This system worked quite well, but it still had a small flaw: the Julian year was still slightly too long compared to the actual solar year—this tiny discrepancy would eventually cause problems a few centuries later.
At the time of Julius Caesar, the Julian calendar simply added an extra day every 4 years, but it remained too imprecise. As a result, after several centuries, a discrepancy of about ten days had accumulated compared to the solar year. To prevent Christmas from eventually falling in autumn, Pope Gregory XIII launched a clever reform in 1582: directly skipping 10 days in the calendar! We went from Thursday, October 4, straight to Friday, October 15. Additionally, to refine this further, he established a more precise rule: a year is a leap year if it is divisible by 4, but not if it is divisible by 100, unless it is divisible by 400. A concrete example, the year 2000 was a leap year (divisible by 400), but the year 1900 was not. Today, it is this system, called the Gregorian calendar, that keeps our calendar well aligned with the seasons, with a discrepancy of only one day every 3000 years or so. Not bad, huh?
The Earth revolves around the Sun in about 365.25 days, not exactly 365. This little quarter of a day each year accumulates gradually. As a result, after four years, we end up with almost a full extra day, or about 24 hours, which we add to the calendar. Without this, our seasons would eventually shift, and we would find ourselves celebrating Christmas in the middle of summer. So, every four years, we simply add a February 29 to the year to keep everything in line with the solar year. Simple and effective, right?
Leap years add an extra day to our calendar: the famous February 29th. That means an additional day in the year to work, rest, or even celebrate a special birthday for those born on that day. It also shifts the calendar: the year following a leap year will start on a day of the week one day later than usual (if your birthday was on a Saturday, it will move to Monday two years later). In practical terms, without leap years, after a while, we would find ourselves wearing swimsuits at Christmas or skiing in August. Not very convenient. Leap years thus prevent the seasons from gradually shifting away from our calendar. They ensure that our daily lives remain aligned with the natural rhythms of the sun and the seasons.
The tradition of proposing marriage on February 29th is popular in some Anglo-Saxon countries. On this day, women can ask men to marry them. This is called the 'Leap Day Proposal'!
Without leap years, our calendar would drift by about 6 hours each year, which would cause the seasons to be completely out of sync after just a few centuries!
The Romans once celebrated the festival of Terminalia on February 23. Julius Caesar introduced an additional day right after this date, and the Romans initially referred to this day, now known as February 29, as 'bis sextus' (twice sixth), from which the term 'leap year' (bissextile) is derived.
Although a leap year generally occurs every 4 years, it must be omitted in years divisible by 100, unless they are also divisible by 400. Thus, 2000 was a leap year, but 1900 and 2100 were not!
Yes, the Gregorian calendar is the one adopted internationally, but other calendars exist and have different adjustment mechanisms. For example, the Islamic lunar calendar has only 354 or 355 days, and its months shift in relation to the seasons. In contrast, the Hebrew calendar combines lunar months with a system of embolismic years (adding an extra month approximately every three years) to keep its holidays aligned with the seasons.
No, a leap year only affects the total duration of the year. A day still lasts approximately 24 hours. The year simply has one extra day, changing from 365 to 366 days to correct the discrepancy with the solar year.
In non-leap years, people born on February 29 typically celebrate their birthday on either February 28 or March 1. For administrative reasons (such as reaching the legal age), February 28 is often the date chosen.
Without leap years, our calendar would fall behind the solar year, accumulating about six hours of delay each year. Thus, after a century, the seasons would be shifted by approximately 24 days. Eventually, this would result in a complete shift of the seasons in relation to the current months of the calendar.
Originally, in the ancient Roman calendar, February was the last month of the year. Thus, to correct the accumulated discrepancy with the solar year, it was naturally this last month that was adjusted. This tradition continued with the Julian calendar and was subsequently firmly established in the Gregorian calendar.
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