1) A year that is divisible by 4 (no remainder when divided by 4) is a leap year:
1666 is not divisible by 4 (there is a remainder when divided by 4).
We could have stopped at this step.
The year 1666 is not a leap year but a common one.
It does not meet this condition.
2) In addition to step 1 above: If the year is divisible by 100 (no remainder when divided by 100) then it isn't a leap year:
1666 is not divisible by 100 (there is a remainder when divided by 100).
3) In addition to step 2 above: If the year is also divisible by 400 (no remainder when divided by 400) then it's a leap year:
1666 is not divisible by 400 (there is a remainder when divided by 400).
1666 is not a leap year, it's a common year.
The last leap year before 1666: 1664.
The next leap year after 1666: 1668.
What is a leap year?
- A leap year has 366 days. 1666, being a common year, has 365.
- In a leap year the month of February has 29 days. February 29th is a valid date.
- In a common year the month of February has 28 days. February 29th, 1666 does not exist.
The leap years and the Gregorian calendar (the modern calendar, civil)
Calendar adoption
- The first year in the actual calendar (also called modern, civil) was 1582. Before this year another calendar (Julian) was in use, with slightly different rules.
- Not all the countries adopted the Gregorian calendar at the same time, in 1582. The adoption process took hundreds of years and is not even now complete.
How accurate is the Gregorian calendar?
- The actual Gregorian calendar will get out of sync by 1 day with the astronomical calendar around the year of 4818, after ≈ 3236 years from its creation in 1582.
How often do the leap years occur?
- Nearly once every four years is a leap year. More exactly, leap years occur 97 times in every cycle of 400 years.
Why do we need leap years in our calendar?
- The leap years keep the calendar year in sync with the astronomical year, preventing us, for example, from celebrating Christmas in the month of November.
- The reason being, the astronomical year is neither 365 nor 366 days in lenght, but 365.242216 days, or around 365 days, 5 hours and 48 minutes. Since in a calendar we cannot have chunks of days (for example that chunk of only 0.242216 of a day), but whole days only, the leap years compensate for the lack of exact match with the astronomical year.
More operations with Leap Years:
The last leap year before 1666: 1664.
The next leap year after 1666: 1668.