Is it a Leap Year Right Now?
Right now is 2016. Is it a leap year?
I wrote an article yesterday called “Useful Math: Divisibility Signs” where I mentioned briefly about how to determine if given year is a leap year. And because not everyone usually thinks about it but the topic is somewhat interesting I decided to elaborate on it a little bit.
So the easiest way to spot a leap year is to check if it is divisible by 4 and if it is then it’s a leap year.
No calculators please.
Lets take the divisibility sign from my “Useful Math: Divisibility Signs” article and apply it here: if the last two digits of the given number are zeros or they form a number that is divisible by 4 then given number is divisible by 4.
For example, lets take the number 52100. Is it divisible by 4? For those unfamiliar with divisibility signs it would be quite hard to find out without using a calculator. But we know that if a number ends with 2 zeros then it is divisible by 4, so 52100 is divisible.
For another example lets take number 14676. We are looking at the last 2 digits: 76. 76 is divisible by four (2*7+6=20, 20 is divisible by 4, see my previous article for this sign)
Now back to our 2016 year. Is 16 divisible by four? Easy and absolutely! 2016 is a leap year.
Easy! Right? But not quite, now lets complicate things a little bit.
Pope Gregory removed 10 days — October 5 to October 14 — from the year 1582 and proclaimed that from that time onwards 3 days would be dropped from the calendar every 400 years. Thursday, October 4, 1582 (Julian) was followed immediately by Friday, October 15, 1582 (Gregorian).
Every year that is exactly divisible by four is a leap year, except for years that are divisible by 100. Centurial years that are divisible by 400 are still leap years. For example, the year 1900 is not a leap year; the year 2000 is a leap year, meaning if this is a centurial year, then you need to look for divisibility by 400.
Checking divisibility of the centurial years by 400 also quite simple:
2000/400 = 20/4, 20 is definitely and absolutely divisible by 4.
Lets be optimistic and pretend that we all will live till the year 2100 and when somebody asks you if it’s a leap year right now, you will remember this article and quickly calculate in your mind 2100/400 = 21/4, which isn’t divisible and quickly reply, no, right now is not a leap year.
