The Why Series
Report by Preetinkar Jain & Priyanka.S
A Gregorian calendar (a standard calendar which is used all over the world) has both common years (365 days) and leap year (365 + 1 days). The extra day is designated as February 29th (as it is the shortest month) is also known as the intercalary day. A leap year occurs every 4 years to synchronise the calendar year with the solar year (the time earth takes to complete one orbit around the sun, i.e 365.25 days)
Earth takes 365.2422 days to complete a revolution around the sun. However, making a calendar with that figure would become quite tedious, hence, to ease the process, an year is made 365 days long and the balance is compensated by adding an extra day every 4 years .
Our calculation problem would have solved if we could assume that a solar year has 365.25 days. But since that’s not the case and 365.2422 is 11 minutes short of becoming 365.25. Even though it is such a small amount of time, it makes big difference when talking in terms of centuries.
4 x 365.25 = 1461 days
4 x 365.2422 = 1460.9688 days
1461 – 1460.9688 = .0312 days (45 minutes approx.)
So, the Gregorian calendar year gains 45 minutes every 4 years.
After 32 leap years (4 x 32 = 128 years), we gain 1 full day –
32 x .0312 = 0.9984 (approx. 1 day)
Again, for the ease of calculation and to make it easy for us to remember, we skip a leap year every 100 years to balance our calendar with the solar calendar. If you can see, we are still approximating and instead of skipping a day every 128 years, we skipped it every 100 years.
Confusing right?
Let me ease your life and give you a simple formula to decide if a year is leap or common:
We add a leap year every 4 years, except for every 100 years, except for every 400 years! Still confusing?
- If a year is divisible by 4, then it’s a leap year, unless
- it’s also divisible by 100, then it’s not a leap year, unless
- the year is divisible by 400, then it’s a leap year.
Phew! Who knew understanding a calendar was so much work!If these calculations and adjustments are not taken into consideration, then at the end of 2000 years we will have 7 extra days. What difference does that make! Seasons will not fall on their designated time. Knowing when to sow and when to harvest would become a nightmare for the farmers. Festivals will fall in the wrong season. Imagine celebrating Holi in summers! And even satellites will not work properly.