(KWTX) 2020 is a leap year, which calls for adding an extra day to the calendar, but what would happen if we didn’t leap at all?
If we didn't add a leap day on Feb. 29 every four years, the calendar would lose almost six hours every single year. (Roger Higgins, via Library of Congress)
If we didn't add a leap day on Feb. 29 every four years, the calendar would lose almost six hours every single year, so “After only 100 years, our calendar would be off by around 24 days,” the group Time and Date (T&D), at timeanddate.com says.
A leap year (also known as an intercalary year or bissextile year) “is a calendar year containing an additional day added to keep the calendar year synchronized with the astronomical or seasonal year,” Jean Meeus wrote in 1998 in her book Astronomical Algorithyms.
Off by 24 days equals June beginning on July 1, which means after 300 years September would begin on Jan. 1.
We need leap years to keep our modern-day Gregorian calendar in alignment with Earth's revolutions around the sun.
Technically, “It takes Earth approximately 365.242189 days, or 365 days, 5 hours, 48 minutes, and 45 seconds, to circle once around the sun -- a year.
“This is called a tropical year, and astronomers measure this from the March equinox,” T&D says.
The March equinox, also called the vernal equinox, marks the first day of spring, when the sun's path is directly over the equator, thus the 24-hour day has an equal amount of daytime and nighttime, which typically happens each March 20 but occasionally on March 21.
The Egyptian Great Sphinx points directly at the sun on this day when the sun crosses the equator into the Northern Hemisphere, which has the effect of making the days longer in the Northern Hemisphere until the Summer Solstice occurs, T&D says.
Since the calendar we use has only 365 days in a year, if a day wasn’t added every four years on February 29, we would lose almost six hours off our calendar every single year.
The length of a day is also changed occasionally by adding “leap seconds” into Coordinated Universal Time because of the variability of Earth's rotational period, but unlike leap days, “leap seconds are not introduced on a regular schedule, since the variability in the length of the day is not entirely predictable,” T&D says.
The name "leap year" probably originated because while a fixed date in the Gregorian calendar normally advances one day of the week from one year to the next, “the day of the week in the 12 months following the leap day will advance two days due to the extra day (thus "leaping over" one of the days in the week),” wrote Douglas Harper, in "leap year", for the Online Etymology Dictionary.
For example, Harper points out, “Christmas Day (December 25) fell on a Tuesday in 2012, Wednesday in 2013, Thursday in 2014, and Friday in 2015 but then "leapt" over Saturday to fall on a Sunday in 2016.”
On it’s face it seems kind of simple but underlying all that things get really complicated.
“Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400,” the manual “Introduction to Calendars,” (10 August 2017) from the United States Naval Observatory, says.
It goes on the explain: “For example, the years 1700, 1800, and 1900 are not leap years, but the years 1600 and 2000 are.”
Over a period of 400 years, the accumulated error of adding a leap day every four years amounts to about three extra days, so the Gregorian calendar drops three leap days every 400 years, which is the length of its leap cycle.
That’s accomplished by dropping February 29 in the three century years (multiples of 100) that cannot be exactly divided by 400, says “Leap Years”, June 14, 2011, a manual of the United States Naval Observatory.
Technically the manual says “The years 1600, 2000 and 2400 are leap years, while 1700, 1800, 1900, 2100, 2200 and 2300 are not leap years.
“By this rule, the average number of days per year is 365 + 1⁄4 − 1⁄100 + 1⁄400 = 365.2425.”
The Gregorian calendar is the standard calendar in most of the world, but there are others, and each one must
account for the impreciseness of time that results in leap days and years.
In the lunisolar Hebrew calendar, Adar Aleph, a 13th lunar month, is added seven times every 19 years to the twelve lunar months in its common years to keep its calendar year from drifting through the seasons and the Bahá'í Calendar adds a leap day “as needed” to ensure that the following year begins on the vernal equinox.
The Julian calendar, which was developed in 46 BC by Julius Caesar, and became effective in 45 BC, distributed an extra ten days among the months of the Roman Republican calendar.
Caesar also replaced the intercalary month by a single intercalary day, located where the intercalary month used to be.
In 1582, Pope Gregory XIII developed and introduced a new calendar, the Gregorian calendar, to repair and replace Cesar’s.
“The calendar was developed as a correction to the Julian calendar, shortening the average year by 0.0075 days to stop the drift of the calendar with respect to the equinoxes,” the manual “Introduction to Calendars,” (10 August 2017) from the United States Naval Observatory, says.
“To deal with the 10 days' difference (between calendar and reality) that this drift had already reached, the date was advanced so that 4 October 1582 was followed by 15 October 1582,” the manual notes.
The reform was adopted initially by the Catholic countries of Europe and their overseas possessions and now it is the primary system used around the world.
Leap year and leap day come with some unique traditions, as well.
Anthony, Texas, in February 1988, declared itself "leap year capital of the world,” and an international leapling birthday club was started there for anyone who has a Feb. 29 birthday.
Leapers, or leaplings, those born on Feb 29, usually celebrate their birthdays on February 28.
Technically, a leapling will have fewer birthday anniversaries than their age in years, but for legal purposes, leapling’s birthdays depend on how local laws count time intervals.
“In Ireland and Britain, it is a tradition that women may propose marriage only in leap years.
Supposedly, a 1288 law by Queen Margaret of Scotland (then age five and living in Norway), required that fines be levied if a marriage proposal was refused by the man; compensation was deemed to be a pair of leather gloves, a single rose, £1 and a kiss.
In Finland, the tradition is that if a man refuses a woman's proposal on leap day, he should buy her the fabrics for a skirt.
In France, since 1980, a satirical newspaper entitled La Bougie du Sapeur is published only on leap year, on February 29.
In Greece, marriage in a leap year is considered so unlucky that statistics show one in five engaged couples in Greece will plan to avoid getting married in a leap year.