Why The Year 2100 Will Not Be A Leap Year?

The next leap year immediately following 2096 falls only after 8 years, on 2104, why is 2100 not a leap year?

Usually we believe that a leap year falls once every four years, in that case why would 2100, 2200 and 2300 will not be considered a leap year.

Get to know the reason in this article.

Leap year is divisible by 4:

A general thumb rule applicable for any year to be a “leap year” is: it needs to be divisible by 4.

In case a year is divisible by 100 but not by 4 then it is NOT considered as a LEAP YEAR.

For a century, the year needs to be divisible by 100 as well as 400 and only then it is considered as a Leap Year.

Rule 1: If the Year is divisible by 100 and the Year is not divisible by 400 then it is not a leap year.
(Thus 1900 2100 2200 2300 2500 are not leap years)

Rule 2: Years divisible by 4 are leap years; except where specified by Rule 1.
(Thus 1992 1996 2000 2004 2008 …
2092 2096 2104 2108 …
2192 2196 2204 2208 …
2292 2296 2304 2308 …
2392 2396 2400 2404 2408 are leap years)

From the perspective of mathematical logic:

When Earth spins around its own axis this is termed as Earth’s Rotation and one rotation takes 24 hours which makes up for day and night.

Earth’s Revolution is when Earth moves around the Sun and one revolution makes up for one year.

The time taken by Earth for one revolution is around 365.2425 days which means 365 days and an additional 1/4 day(0.2425).

So a normal year is made of 365 days and the remaining 1/4 days which is left behind at each revolution needs to be compensated once every four years in the form of 1 additional day making the leap year of 366 days.

Now, when one day is added in a leap year it becomes 146100 days in 400 years; however in reality is should be only 146097 days in 400 years. So, ultimately now 3 extra days have been added which needs to be fixed and hence it was decided that in a span of 400 years these 3 leap years would be made “common years” comprising of 365 days instead of 366.

So to sort this confusion it was decided that a leap year needs to be divisible by 100 as well as 400 and if a century is divisible by 100 but not by 400 then it is NOT a leap year.

Mathematicians give this reason as to why 2100 will not be a leap year.

Need to get the clocks and days in sync: 

We consider one Earth’s revolution equal to 365 days as a method of correction and carry the quarter day ahead which finally results in a leap year.

However, one Earth year is not exactly 365 days and it is 11 minutes less. So when we keep adding one full day to a leap year, it means we would end up adding 3 extra days in a span of 4 years.

So after a few years, the clock and days would not be in sync with each other.

For instance, the clock would show 12.00 noon and Sun would not be seen at its highest point and it would already have been past.

Maybe after some years the 12 noon on clock would match at the Sunrise! And it would take roughly 133 years for the clock and the days to sync in together once again.

Thus, to get this sync issue rectified, it is essential that in a span of 400 years the 3 days are adjusted and what better than getting it done at the century. That is the reason as to why 2100, 2200, 2300 will not be leap years.

Some historical aspect:

We saw the mathematical aspects above which people have been using to measure the leap years. But just imagine how people might have sorted this sync issues some centuries back?

Earth would have taken same time for rotation and revolution, isn’t it? So lets check some interesting historical aspect related with leap year.

Julian Calendar: 

It seems ancient Chinese and Greeks knew that one Earth year equals to 365 days, 5 hours, 48 minutes and 46 seconds and hence their calendars were too complicated with extra months added after every 19 years.

In 45 AD, astronomer Sosigenes recommended that the year should be prolonged to 445 days instead of regular 365 days because he found that the calendar was not in sync either with the Roman festivals nor with the seasons of the year. Thus, he developed a calendar which is named as Julian Calendar wherein one calendar year had 365 days and once every 4 years there would be a leap year with 366 days as a compensation for the lost quarter day.

Gregorian calendar: 

In 730 AD, it was a great Anglo-Saxon scholar who actually pointed out that one Julian year is 11 minutes and 14 seconds longer and this adds up very slowly which results in the calendar falling out of sync with the sun by a day in a span of 128 years. However, there was no remedy for this till the year 1582.

Pope Gregory XIII found that in 1582, the spring equinox fell on March 11 instead of March 21 and hence the remedy was to erase 10 days from that calendar due to which after Oct 4, 1582 the next day was directly Oct 15 1982. To get the Gregorian calendar in sync with that of Julian calendar, Pope used his abacus and made the necessary changes to a leap year. Pope declared that “a leap year needs to be divisible by four. Further if a year is just divisible by 100 then it cannot be considered as a leap year unless the year is also evenly divisible by 400.”

The Gregorian calendar was accepted by Roman Catholic nations such as Italy, Poland, Portugal, Spain however it was not easily accepted by the protestant nations.

It was in 1712 when Sweden and Finland accepted this calendar however they had to add two “double leap year” in 1712.

Finally it was in September 1752 when the Gregorian calendar was accepted by  Britain and America after much revolt from public and also by erasing 11 days from their calendar.

The Gregorian calendar too is not perfect. It was seen that after every 10,000 years this calendar has an error of two days, 14 hours and 24 minutes which means even this requires some correction.

Calendars in Asian countries:

It has been observed that Muslim community have their own calendar wherein a year is based on 12 lunar months.

Similarly, India and Africa have about 17 different calendars which have their own permutation combination methods based on which things fall in place.

These were some of the interesting historical aspects to the leap year adjustments.

So readers, do you have any other explanation as to why 2100 is not a leap year. If yes, do add your suggestions in the comments session below for our readers and future generations!!!