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Leap Year |
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It takes the earth one year to make one complete orbit around the sun. We think of one year as being a nominal 365 days long, but we all know that it takes the earth approximately 1/4 of a day more than 365 to complete its orbit around the sun. So, every four years we have what we call a leap year to compensate for the extra day in the earth’s orbit. Thus, every four years, we have 29 days in February rather than the usual 28 days. The 29th day is called a leap day or an intercalary day. Even so, the extra 1/4 day (6 hours) every year is only an approximation of the time it takes for one complete orbit of the earth around the sun. The actual extra time each year is slightly less than 1/4 of a day (5 hours, 48 minutes, and 46 seconds, according to NASA). Thus, every so many years we must adjust for the extra time less than the 1/4 day per year that it takes for the earth’s complete orbit. We do this by not observing leap year in century years that are not evenly divisible by 400. ("Evenly divisible" means that when we divide one number by another, we get a result that is a whole (or integer) number; that is, the result of the division has no fractional parts, or it has no remainder.) So, century years (such as 100, 1200, 2000, etc.) that are not evenly divisible by 400 are not leap years.
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Our most recent century year was 2000 which was a leap year because 2000 is evenly divisible by both 4 and 400. The next century year we will encounter will be the year 2100 which, though evenly divisible by 4, will not be a leap year because it is not evenly divisible by 400. Similarly, the century years 2200 and 2300, although evenly divisible by 4, will not be leap years because they are not (evenly) divisible by 400. The century year 2400, however, will be a leap year because it is divisible by both 4 and 400. (I know! I know! It’s likely that most people reading this in 2024 will not even be around in those years, so who cares...right??) Nevertheless, the general rule for leap years is that any year evenly divisible by 4 is a leap year with February of that year having 29 days instead of the usual 28 days. The one exception to the general rule is that any century year that is not evenly divisible by 400 is not a leap year and February will have its usual 28 day in its calendar for that year.
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