It only happens once every 4 years
Aziz Inan
Issue date: 2/28/08 Section: Opinions
Friday, Feb. 29, 2008, the second leap day of the 21st century, is at our doorstep reminding us that not only four years passed since the last leap day, but we will have to wait four more years for the next one to arrive. We will use this occasion to cover some facts related to leap days.
Question: Unlike the last century, which had 25 leap days (the 25th being Tuesday, Feb. 29, 2000), the 21st century has only 24 leap days. Why?
Answer: In the Gregorian calendar (which is the current standard calendar adapted by a large percentage of the countries in the world), the duration of one year approximately corresponds to the duration of a solar year. A solar year is the time it takes for the Earth to complete one full orbit around the Sun, which is nearly equal to 365ΒΌ days.
There are two types of years in the Gregorian calendar, a common year having 365 days total whereas the month of February has only 28 days, and a leap year having 366 days total where February has an extra leap day, February 29.
Most years that are divisible by four are leap years meaning that leap year occurs once every four years. Adding an extra leap day to the calendar once every four years compensates for the fact that a solar year is close to 365 1/4 days, that is, 1/4 day more than 365 days.
So, in four years, the extra 1/4 days add up to an extra day which is the reason why the leap year with 366 days was introduced into the Gregorian calendar. Having a leap day every four years synchronizes the calendar with the solar year.
However, some exceptions to this rule apply since the duration of a solar year is actually slightly less than 365 1/4 days. More accurately, the duration of a solar year is close to 365.2425 days (or 365 days, five hours, 49 minutes and 12 seconds). On this basis, even with a leap day every four years, the calendar still goes out of sync with the solar year, getting ahead of it by about 10 minutes and 48 seconds every year.
Question: Unlike the last century, which had 25 leap days (the 25th being Tuesday, Feb. 29, 2000), the 21st century has only 24 leap days. Why?
Answer: In the Gregorian calendar (which is the current standard calendar adapted by a large percentage of the countries in the world), the duration of one year approximately corresponds to the duration of a solar year. A solar year is the time it takes for the Earth to complete one full orbit around the Sun, which is nearly equal to 365ΒΌ days.
There are two types of years in the Gregorian calendar, a common year having 365 days total whereas the month of February has only 28 days, and a leap year having 366 days total where February has an extra leap day, February 29.
Most years that are divisible by four are leap years meaning that leap year occurs once every four years. Adding an extra leap day to the calendar once every four years compensates for the fact that a solar year is close to 365 1/4 days, that is, 1/4 day more than 365 days.
So, in four years, the extra 1/4 days add up to an extra day which is the reason why the leap year with 366 days was introduced into the Gregorian calendar. Having a leap day every four years synchronizes the calendar with the solar year.
However, some exceptions to this rule apply since the duration of a solar year is actually slightly less than 365 1/4 days. More accurately, the duration of a solar year is close to 365.2425 days (or 365 days, five hours, 49 minutes and 12 seconds). On this basis, even with a leap day every four years, the calendar still goes out of sync with the solar year, getting ahead of it by about 10 minutes and 48 seconds every year.