Why Leap Years Are Used

Why Leap Years Are Used: "

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Why Leap Years Are Used

A leap year in the Gregorian calendar includes an extra day in the
year. timeanddate.com provides the history of the leap year and its
purpose.

What is a leap year?

A leap year is a year in which one extra day has been inserted, or
intercalated, at the end of February. A leap year consists of 366 days,
whereas other years, called common years, have 365 days.

Which years are leap years?

In the Gregorian calendar, the calendar used by most modern
countries, the following three criteria determine which years will be
leap years:

1. Every year that is evenly divisible by four is a leap year;


2. of those years, if it can be evenly divided by 100, it is NOT a leap year, unless


3. the year is evenly divisible by 400. Then it is a leap year.

According to the above criteria, that means that years
1800,
1900,
2100,
2200,
2300 and
2500 are NOT leap years, while year 2000 and 2400 are leap years.

It is interesting to note that 2000 was somewhat special as it was the first instance when the third criterion was used in most parts of the world.

In the Julian calendar–introduced by Julius Caesar in 46 BC and
patterned after the Roman calendar–there was only one rule: any year
evenly divisible by four would be a leap year. This calendar was used
before the Gregorian calendar was adopted.

Why are leap years needed?

Leap years are needed to keep our calendar in alignment with the earth's revolutions around the sun.

Details

Note: The illustration is not to scale.

The vernal equinox is the time when the sun is directly above the
Earth's equator, moving from the southern to the northern hemisphere.

The mean time between two successive vernal equinoxes is called a
tropical year–also known as a solar year–and is about 365.2422 days
long.

Using a calendar with 365 days every year would result in a loss of
0.2422 days, or almost six hours per year. After 100 years, this
calendar would be more than 24 days ahead of the season (tropical
year), which is not desirable or accurate. It is desirable to align the
calendar with the seasons and to make any difference as insignificant
as possible.

By adding a leap year approximately every fourth year, the
difference between the calendar and the seasons can be reduced
significantly, and the calendar will align with the seasons much more
accurately.

(The term "day" is used to mean "solar day"–which is the mean time
between two transits of the sun across the meridian of the observer.)

Is there a perfect calendar?

No calendars used today are perfect; they are off by seconds,
minutes, hours or days every year. To make a calendar more accurate,
new leap year rules have to be introduced to the Gregorian calendar,
complicating the calculation of the calendar even more. It will,
however, need some modifications in a few thousand years. As for the
tropical year, it is approximately 365.242199 days, but varies from
year to year because of the influence of other planets.

Name of calendar	When introduced	Average year	Approximate error introduced
Gregorian calendar	AD 1582	365.2425 days	27 seconds (1 day every 3,236 years)
Julian calendar	45 BC	365.25 days	11 minutes (1 day every 128 years)
365-day calendar	-	365 days	6 hours (1 day every 4 years)
Lunar calendar	ancient	12-13 moon-months	variable

A calendar similar to the Julian calendar, with every fourth year
earmarked as a leap year, was first introduced by King Ptolemy III of
Egypt in 238 BC.

In ancient times, it was customary to have lunar (moon) calendars,
with 12 and/or 13 months every year. To align the calendar with the
seasons, the 13th month was inserted as a 'leap month' every two or
three years. Many countries, especially in Asia still use such
calendars. Read more about Leap Year in Other Calendars.

Note: Many other calendars have been and still are used throughout the world.

Why the change from the Julian to the Gregorian calendar?

The Julian calendar introduced too many leap days, thus increasing
the number of days between the vernal equinox of March 21, its
scheduled date as noted in AD 325 during the Council of Nicaea. The
introduction of the Gregorian calendar allowed for realignment with the
equinox; however, a number of days had to be dropped when the change
was made. Click on any one of the year links below for a better
explanation of the calendars and the days that were dropped in order to
make the switch to the Gregorian calendar.

• The Gregorian calendar was first adopted in Italy, Poland, Portugal and Spain in 1582. This was done by dropping 10 days in October of that year.


• .In Great Britain (and America), the Gregorian calendar would not be adopted until much later, in September 1752; 11 days were dropped.


• Sweden (and Finland) had a "double" leap year in 1712. Two days
  were added to February–creating a date of February 30, 1712. (This was
  done because the leap year in 1700 was dropped and Sweden's calendar
  was not synchronized with any other calendar. By adding an extra day in
  1712, they were back on the Julian calendar.) Read more about February 30


• .The Julian calendar is currently (between the years 1901 and 2099)
  13 days ahead of the Gregorian calendar (because too many leap years
  were added).

Other leap years facts

• The Gregorian calendar has a 400-year cycle until it repeats the
  same weekdays for every year–February 29, 2008, is a Friday and
  February 29, 2408, is a Friday.


• The Gregorian calendar has 97 leap years during those 400 years.


• The longest time between two leap years is eight years. The last time that occurred was between 1896 and 1904. The next time will be between 2096 and 2104.

Special Leap Years

Leap years that share the same day of week for each date repeat only every 28 years between the years 1904 and 2096. The year 2008
was a leap year. If you look at a 2008 calendar, you will see that
February has five Fridays–the month begins and ends on a Friday.
February comprised five Fridays in 1980 as well, and the next occurrence will be in 2036.

February 29, the leap day, has been associated with age-old traditions, superstitions and folklore.

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