We have a leap year every four years. During a leap year, an extra day is added to the calendar, so instead of the usual 365 days in a year, there are 366. This extra day is added to the month of February, meaning that during a leap year there are 29 days in that month.
The Gregorian calendar is the most widely used calendar in the world. It is named after Pope Gregory XIII, who introduced it in October 1582. This calendar is based on the Earth’s orbit around the Sun. The Earth takes 365 days to make one complete circumsolar orbit.
Well, it actually takes 365.24 days. In other words, it takes the Earth 365 days and six hours to travel around the Sun. These six hours add up, and every four years, they total an extra day. That is why we add another day to the calendar every four years. This addition is important to keep the calendar synchronized with the movement of the Sun, the solar seasons, and the patterns of the Moon and tides.
In the 8th century BC, the Romans used a 10-month, 304 day calendar that began with the spring equinox in March, and ended in December. Back then, calendars were used primarily by farmers, who regarded winter as a useless time period, so the season was not even counted.
King Numa Pompilius thought this was weird. So in 713 BC, he created a new calendar using the year’s 12 lunar cycles (that means about 355 days). The months of January and February were introduced at the end of the calendar. The Romans were superstitious of even numbers in those days, so King Numa Pompilius tried to make every month have an odd number of days. However, it became clear that one month would need to have an even number to stick to the lunar cycle.
So February, being the last month got the even number of days. The months of the year had either 31 or 29 days and February had 28. Of course, with only 355 days, seasons soon fell out of sync, so the Romans began introducing leap months every now and then, and this confused everyone.
Julius Caesar would ultimately get rid of the leap month, and change the calendar altogether. He aligned his Julian calendar with the sun, and some days were added so each year would have a total of 365. Though his reason is not entirely clear, he decided to keep the 28 days in February.
Any year evenly divisible by four would be a leap year. This formula produced too many leap years, causing the Julian calendar to drift apart from the tropical year at a rate of 1 day per 128 years. This was not corrected until the introduction of the Gregorian calendar more than 1500 later, when a number of days were skipped to realign our calendar with the seasons.
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