Why Leap Years Are UsedFebruary 29, 2008
The year 2008 is 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. Between the
years 1904 and 2096, leap years that share the same day of week for each date
repeat only every 28 years. The most recent year in which February comprised
five Fridays was in 1980, and the next occurrence will be in 2036.
February 29, the leap day, has been associated with age-old traditions, superstitions
and folklore. 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:
Every year that is divisible by four is a leap year;
of those years, if it can be divided by 100, it is NOT a leap year, unless
the year is 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 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.
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.

