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This year, the month of February has 29 days instead of its usual 28. Years having this extra day are called "leap years," and the reason for the extra or "leap day," is that Earth rotates roughly 365 and a quarter times on its axis before it completes one full circle around the Sun.
Earth has three significant motions: it revolves around the Sun, it rotates about its axis, and that axis precesses, which means that it wobbles like a top spinning down.
Calculating which years are leap years
A solar, or tropical year, is the time between two successive occurrences of the vernal equinox when the Sun crosses the celestial equator moving north. Earth actually rotates 365.242188931 times during a solar year, which means that leap year has to be skipped every once in a while in order to keep the calendar correct.
Years that are not leap years are called common years, and the actual formula for determining which years are leap years can be explained with the following: Every year that is divisible by 4 is a leap year, unless the year is also divisible by 100, but not 400, in which case it's not a leap year.
This is actually more understandable if written as an algorithm:
if (year is not divisible by 4) then (it is a common year)
else if (year is not divisible by 100) then (it is a leap year)
else if (year is not divisible by 400) then (it is a common year)
else (it is a leap year).
Accordingly, the years 1700, 1800, and 1900 were not leap years, while the years 1600 and 2000 were leap years. 2004, 2008, 2012, 2016, and 2020 are leap years because they're divisible by 4, but century dates are only a leap year if they're also divisible by 400. That means 2100, 2200, and 2300 will not be leap years, but 2400 will be a leap year.
Where did the term "leap" come from?
The calendar we currently use is called the Gregorian calendar, and it is named after Pope Gregory XIII, who introduced it in 1582. It was an improvement over the Julian calendar, which was named after Julius Caesar.
By the time the Gregorian calendar was adopted, the Julian calendar was off by 10 days with respect to the equinoxes. During the switchover in 1582, Italy, Spain, Portugal, and Poland actually eliminated the dates from October 5th through October 14th entirely. That meant you went to sleep on October 4th and woke up the next day on October 15th.
If a date on the Gregorian calendar falls on, say, a Monday, during one year, that date will fall on a Tuesday the next year, and on a Wednesday the following year. However, when an extra day is added during a leap year, it causes a calendar date to "leap over" its expected day of the week, say, a Thursday, and to land one day farther out, on a Friday.
As an example, in 2012, Christmas Day (December 25th) fell on a Tuesday, it fell on a Wednesday in 2013, on a Thursday in 2014, and on a Friday in 2015. However, due to the leap year in 2016, Christmas Day "leaped over" Saturday and fell on a Sunday.
Odd leap year traditions
People born on February 29th are called "leapers," and during non-leap years, they usually celebrate their birthday on February 28th.
There is an odd tradition of women being able to propose marriage to men during leap years, which dates back to the 5th century. It involves St. Patrick, who was said to have driven all the snakes out of Ireland.
Legend has it that St. Patrick was approached by St. Bridget, who wanted to discuss the unfairness of only men being able to propose marriage. The two agreed that every four years, women could propose marriage, and being a leap year at the time, St. Bridget got down on one knee and proposed marriage to St. Patrick. While she didn't get a marriage, she did get a kiss from St. Patrick and a new dress. A small consolation, we would guess...
The lunisolar Chinese calendar, which indicates both the Moon phase and the time of the solar year, adds an extra month to its leap year according to a rule that ensures that month 11 is always the month that contains the northern winter solstice.
The Iranian Solar Hijri calendar adds 8 leap days every 33 years. This calendar more closely aligns with astronomical observations than the Gregorian calendar.
The Islamic calendar, which is made up of lunar months that contain either 29 or 30 days, does not include regular leap days. However, the tabular Islamic calendar that was used by Islamic astronomers during the Middle Ages adds a leap day to the last month of the lunar year every 11 years of a 30-year cycle.
The lunisolar Hebrew calendar adds an extra month 7 times every 19 years to ensure that Passover always occurs during the spring. Other oddities of the Hebrew calendar include that the first day of the year can never fall on a Sunday, Wednesday, or Friday, and the first day of Passover can never fall on a Monday, Wednesday, or Friday.
The solar Bahá'í Calendar is comprised of 19 months of 19 days each, which adds up to a year containing 361 days. This calendar adds a leap day when needed to ensure that the new year always begins on the March equinox.
Whatever calendar you use, enjoy an extra day this year.